Earth from Space
This photo-like view is based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASAs Terra satellite.
1 2 TH E D I T I O N
Frederick K. Lutgens Edward J. Tarbuck Illustrated by
Dennis Tasa
Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montréal Toronto Delhi Mexico City São Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo
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1 2 3 4 5 6 7 8 9 10—DOW—15 14 13 12 11 www.pearsonhighered.com
ISBN-10: 0-321-75631-2; ISBN-13: 978-0-321-75631-2 (Student Edition) ISBN-10: 0-321-78035-3; ISBN-13: 978-0-321-78035-5 (Instructor’s Review Copy)
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Brief Contents 1 2 3 4 5 6 7 8 9 10 11 12 13
vi
Introduction to the Atmosphere
2
Heating Earth’s Surface and Atmosphere
34
Temperature
64
Moisture and Atmospheric Stability
96
Forms of Condensation and Precipitation
128
Air Pressure and Winds
160
Circulation of the Atmosphere
188
Air Masses
220
Midlatitude Cyclones
238
Thunderstorms and Tornadoes
270
Hurricanes
302
Weather Analysis and Forecasting
326
Air Pollution
356
14 15 16
The Changing Climate
378
World Climates
408
Optical Phenomena of the Atmosphere
448
Appendix A Metric Units
465
Appendix B Explanation and Decoding of the Daily Weather Map
469
Appendix C Relative Humidity and Dew-Point Tables
476
Appendix D Laws Relating to Gases
478
Appendix E Newton’s Laws, Pressure– Gradient Force, and Coriolis Force
479
Appendix F Saffir–Simpson Hurricane Scale
481
Appendix G Climate Data
482
Glossary
488
Index
498
GEODe: ATMOSPHERE ATMOSPHERE
The GEODe: Atmosphere interactive learning aid is accessed from the book’s Website (www.MyMeteorologyLab.com). This dynamic instructional tool reinforces atmospheric science concepts by using tutorials, animations, and interactive exercises. The GEODe chapter numbers relate to equivalent chapters in the text. The GEODe: Atmosphere icon appears throughout the book wherever a text discussion has a corresponding activity in GEODe.
1 Introduction to the Atmosphere
5 Forms of Condensation and Precipitation
1. The Importance of Weather
1. Classifying Clouds
2. Weather and Climate
2. Types of Fog
3. Composition of the Atmosphere
3. How Precipitation Forms
4. Extent of the Atmosphere
4. Forms of Precipitation
5. Temperature Structure of the Atmosphere 6. In the Lab: Reading Weather Maps
2 Heating Earth’s Surface and Atmosphere
6 Air Pressure and Winds 1. Measuring Air Pressure 2. Factors Affecting Wind 3. Highs and Lows
1. Understanding Seasons, Part 1 2. Understanding Seasons, Part 2 3. Solar Radiation
8 Air Masses
4. What Happens to Incoming Solar Radiation 5. The Greenhouse Effect 6. In the Lab: The Influence of Color on Albedo
9 Basic Weather Patterns 1. Fronts
3 Temperature Data and the Controls
2. Introducing Middle-Latitude Cyclones 3. In the Lab: Examining a Middle-Latitude Cyclone
of Temperature
1. Basic Temperature Data 2. Controls of Temperature
4 Moisture and Cloud Formation 1. Water’s Changes of State
2. Humidity: Water Vapor in the Air 3. The Basics of Cloud Formation: Adiabatic Cooling 4. Processes That Lift Air 5. The Critical Weathermaker: Atmospheric Stability 6. In the Lab: Atmospheric Stability
vii
Contents Preface xvi The Atmosphere and Media Walkthrough xx
to 1 Introduction the Atmosphere
2
Focus on Concepts 3 Focus on the Atmosphere 4 Weather in the United States 4 Meteorology, Weather, and Climate 4 Atmospheric Hazards: Assault by the Elements 7 Eye on the Atmosphere 7 The Nature of Scientific Inquiry 8 Hypothesis 9 Theory 9 BOX 1–1 Monitoring Earth from Space 10 Scientific Methods 10 Earth’s Spheres 12 The Geosphere 13 The Atmosphere 13 The Hydrosphere 13 The Biosphere 14 Earth as a System 15 Earth System Science 15 The Earth System 16 Composition of the Atmosphere 17 Major Components 17 Carbon Dioxide 17 BOX 1–2 The Carbon Cycle: One of Earth’s Subsystems 18 Variable Components 19 BOX 1–3 Origin and Evolution of Earth’s Atmosphere 20 Ozone Depletion—A Global Issue 22 The Antarctic Ozone Hole 23 Effects of Ozone Depletion 23 Montreal Protocol 23 Vertical Structure of the Atmosphere 24 Pressure Changes 24 Professional Profile: Kathy Orr, Broadcast Meteorologist 25 Temperature Changes 26 Eye on the Atmosphere 26 Vertical Variations in Composition 29 Ionosphere 29 Eye on the Atmosphere 29 The Auroras 30 Give It Some Thought 31 INTRODUCTION TO THE ATMOSPHERE IN REVIEW 32 VOCABULARY REVIEW 33 PROBLEMS 33
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WAVE RF/PHOTOLIBRARY
Earth’s Surface 2 Heating and Atmosphere 34
Focus on Concepts 35 Earth–Sun Relationships 36 Earth’s Motions 36 What Causes the Seasons? 36 Earth’s Orientation 38 Solstices and Equinoxes 39 BOX 2–1 When Are the Seasons? 40 Eye on the Atmosphere 43 Energy, Temperature, and Heat 43 Forms of Energy 43 Temperature 44 Heat 44 Mechanisms of Heat Transfer 44 Conduction 44 Convection 45 Radiation 46 Laws of Radiation 47 BOX 2–2 Radiation Laws 48 Severe and Hazardous Weather: The Ultraviolet Index 49 What Happens to Incoming Solar Radiation? 50 Reflection and Scattering 50 Absorption of Solar Radiation 52 Eye on the Atmosphere 53 The Role of Gases in the Atmosphere 53 Heating the Atmosphere 53 The Greenhouse Effect 54 Earth’s Heat Budget 56 Annual Energy Balance 56 Eye on the Atmosphere 57 BOX 2–3 Solar Power 58 Latitudinal Heat Balance 59
Contents
ix
Professional Profile: Captain Ryan J. Harris, Military Meteorologist 93 Give It Some Thought 94 TEMPERATURE IN REVIEW 95 VOCABULARY REVIEW 95 PROBLEMS 95
and 4 Moisture Atmospheric Stability
96
Focus on Concepts 97 MARTIN WOIKE/AGEFOTOSTOCK
Give It Some Thought 60 HEATING EARTH’S SURFACE AND ATMOSPHERE IN REVIEW 61 VOCABULARY REVIEW 62 PROBLEMS 62
3 Temperature
64
Focus on Concepts 65 For the Record: Air-Temperature Data 66 Basic Calculations 66 Isotherms 66 Why Temperatures Vary: The Controls of Temperature 67 Land and Water 68 BOX 3–1 North America’s Hottest and Coldest Places 69 Ocean Currents 71 Altitude 72 Eye on the Atmosphere 74 Geographic Position 74 Cloud Cover and Albedo 75 Severe and Hazardous Weather: Heat Waves 76 World Distribution of Temperatures 76 Eye on the Atmosphere 80 Cycles of Air Temperature 80 BOX 3–2 Latitude and Temperature Range 81 Daily Temperature Variations 81 Eye on the Atmosphere 83 Magnitude of Daily Temperature Changes 83 BOX 3–3 How Cities Influence Temperature: The Urban Heat Island 84 Annual Temperature Variations 86 Temperature Measurement 86 Mechanical Thermometers 86 Electrical Thermometers 88 Instrument Shelters 88 Temperature Scales 89 BOX 3–4 Applying Temperature Data 90 Heat Stress and Windchill: Indices of Human Discomfort 90 Heat Stress—High Temperatures Plus High Humidities 91 Windchill— The Cooling Power of Moving Air 92
Movement of Water Through the Atmosphere 98 Water: A Unique Substance 98 Water’s Changes of State 100 Ice, Liquid Water, and Water Vapor 100 Latent Heat 101 Humidity: Water Vapor in the Air 103 Vapor Pressure and Saturation 104 Eye on the Atmosphere 105 Relative Humidity 106 How Relative Humidity Changes 106 BOX 4–1 Dry Air at 100 Percent Relative Humidity? BOX 4–2 Humidifiers and Dehumidifiers 108 Natural Changes in Relative Humidity 108 Dew-Point Temperature 109 How Is Humidity Measured? 110 Adiabatic Temperature Changes 111 Adiabatic Cooling and Condensation 112 Processes That Lift Air 113 Orographic Lifting 113 Frontal Wedging 114 Convergence 114 BOX 4–3 Precipitation Records and Mountainous Terrain 115 Localized Convective Lifting 115 Eye on the Atmosphere 116 The Critical Weathermaker: Atmospheric Stability 116 Types of Stability 117
107
PAT & CHUCK BLACKLEY/ALAMY
x
Contents
PEDRO2009/DREAMSTIME
Stability and Daily Weather 119 How Stability Changes 121 Temperature Changes and Stability 121 Eye on the Atmosphere 121 Vertical Air Movement and Stability 121 BOX 4–4 Orographic Effects: Windward Precipitation and Leeward Rain Shadows 122 Give It Some Thought 124 MOISTURE AND ATMOSPHERIC STABILITY IN REVIEW 125 VOCABULARY REVIEW 126 PROBLEMS 126
of Condensation 5 Forms and Precipitation 128
Focus on Concepts 129 Cloud Formation 130 Condensation Aloft 130 Growth of Cloud Droplets 130 Cloud Classification 131 High Clouds 132 Middle Clouds 132 Low Clouds 133 Clouds of Vertical Development 133 Cloud Varieties 135 BOX 5–1 Aircraft Contrails and Cloudiness 136 Eye on the Atmosphere 137 Types of Fog 137 Fogs Formed by Cooling 137 Evaporation Fogs 138 How Precipitation Forms 140 BOX 5–2 Science and Serendipity 141 Precipitation from Cold Clouds: The Bergeron Process Eye on the Atmosphere 142 Precipitation from Warm Clouds: The Collision–Coalescence Process 143 Forms of Precipitation 145 Rain 145 Snow 146
Sleet and Freezing Rain or Glaze 146 Hail 147 Severe and Hazardous Weather: Worst Winter Weather 149 Rime 150 Precipitation Measurement 150 Standard Instruments 150 Measuring Snowfall 151 Eye on the Atmosphere 151 Precipitation Measurement by Weather Radar 152 Intentional Weather Modification 152 Snow and Rain Making 153 Fog and Cloud Dispersal 153 Hail Suppression 154 Frost Prevention 155 Give It Some Thought 157 FORMS OF CONDENSATION AND PRECIPITATION IN REVIEW 158 VOCABULARY REVIEW 158 PROBLEMS 158
Pressure 6 Air and Winds
160
Focus on Concepts 161 Wind and Air Pressure 162 Measuring Air Pressure 163 Pressure Changes with Altitude 164 Eye on the Atmosphere 165 Why Does Air Pressure Vary? 166 Influence of Temperature on Air Pressure 166 Influence of Water Vapor on Air Pressure 166 BOX 6–1 Air Pressure and Aviation 167 Airflow and Pressure 167 Factors Affecting Wind 168 Pressure Gradient Force 168 Coriolis Force 169 Friction 171 Winds Aloft 172 Geostrophic Flow 172 Curved Flow and the Gradient Wind 173
141
ED PRITCHARD/GETTY IMAGES
Contents
SAM PELLISSIER/SUPERSTOCK
BOX 6–2 Do Baseballs Really Fly Farther at Denver’s Coors Field? 175 Surface Winds 176 How Winds Generate Vertical Air Motion 177 Eye on the Atmosphere 177 Vertical Airflow Associated with Cyclones and Anticyclones 178 Factors That Promote Vertical Airflow 179 Eye on the Atmosphere 180 Wind Measurement 180 BOX 6–3 Wind Energy: An Alternative with Potential 182 Give It Some Thought 185 AIR PRESSURE AND WINDS IN REVIEW 186 VOCABULARY REVIEW 186 PROBLEMS 187
of the 7 Circulation Atmosphere 188
Focus on Concepts 189 Scales of Atmospheric Motion 190 Small- and Large-Scale Circulation 190 BOX 7–1 Dust Devils 192 Structure of Wind Patterns 192 Local Winds 193 Land and Sea Breezes 193 Mountain and Valley Breezes 193 Chinook (Foehn) Winds 194 Katabatic (Fall) Winds 195 Country Breezes 195 Global Circulation 195 Single-Cell Circulation Model 195 Severe and Hazardous Weather: Santa Ana Winds and Wildfires 196 Three-Cell Circulation Model 197 Pressure Zones Drive the Wind 198 Idealized Zonal Pressure Belts 198 Professional Profile: Sally Benson: Climate and Energy Scientist 198 Semipermanent Pressure Systems: The Real World 199
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Monsoons 201 The Asian Monsoon 201 Eye on the Atmosphere 202 The North American Monsoon 203 The Westerlies 204 Why Westerlies? 204 Waves in the Westerlies 205 Jet Streams 205 The Polar Jet Stream 206 Subtropical Jet Stream 207 Jet Streams and Earth’s Heat Budget 207 Eye on the Atmosphere 208 Global Winds and Ocean Currents 208 The Importance of Ocean Currents 209 Ocean Currents and Upwelling 209 El Niño and La Niña and the Southern Oscillation 210 Impact of El Niño 211 Impact of La Niña 212 Southern Oscillation 212 Global Distribution of Precipitation 213 Eye on the Atmosphere 214 Zonal Distribution of Precipitation 214 Distribution of Precipitation over the Continents 215 BOX 7–2 Precipitation Regimes on a Hypothetical Continent 216 Give It Some Thought 217 CIRCULATION OF THE ATMOSPHERE IN REVIEW 218 VOCABULARY REVIEW 219
8
Air Masses
220
Focus on Concepts 221 What Is an Air Mass? 222 Source Regions 223 Classifying Air Masses 223 Air-Mass Modification 224 Properties of North American Air Masses 225 Continental Polar (cP) and Continental Arctic (cA) Air Masses 225 Lake-Effect Snow: Cold Air Over Warm Water 226
MICHAEL COLLIER
xii
Contents Where Do Midlatitude Cyclones Form? 255 Patterns of Movement 255 Flow Aloft and Cyclone Migration 256 Eye on the Atmosphere 257 Anticyclonic Weather and Atmospheric Blocking 257 Case Study of a Midlatitude Cyclone 258 Severe and Hazardous Weather: The Midwest Floods of 2008 and 1993 262 A Modern View: The Conveyor Belt Model 264 Give it Some Thought 266 MIDLATITUDE CYCLONES IN REVIEW 266 VOCABULARY REVIEW 268 PROBLEMS 269 MIKE HILLINGSHEAD/PHOTO RESEARCHERS, INC.
Severe and Hazardous Weather: The Siberian Express 227 Maritime Polar (mP) Air Masses 228 Eye on the Atmosphere 229 Severe and Hazardous Weather An Extraordinary Lake-Effect Snowstorm 230 Maritime Tropical (mT) Air Masses 231 Eye on the Atmosphere 231 Severe and Hazardous Weather: January 12, 2011— Classic Nor’easter 232 Continental Tropical (cT) Air Masses 235 Give It Some Thought 236 AIR MASSES IN REVIEW 236 VOCABULARY REVIEW 237 PROBLEMS 237
9 Midlatitude Cyclones Focus on Concepts 239 Frontal Weather 240 Warm Fronts 241 Cold Fronts 243 Stationary Fronts 245 Occluded Fronts 245 Drylines 245 Eye on the Atmosphere 247 Midlatitude Cyclones and the Polar-Front Theory 248 Life Cycle of a Midlatitude Cyclone 248 Formation: The Clash of Two Air Masses 248 Development of Cyclonic Flow 249 Mature Stage of a Midlatitude Cyclone 249 Occlusion: The Beginning of the End 249 Eye on the Atmosphere 250 Idealized Weather of a Midlatitude Cyclone 251 Flow Aloft and Cyclone Formation 252 Cyclonic and Anticyclonic Circulation 252 Box 9–1 Winds as a Forecasting Tool 253 Divergence and Convergence Aloft 254
10 Thunderstorms and Tornadoes
270
Focus on Concepts 271
238
What’s in a Name? 272 Thunderstorms 272 Air-Mass Thunderstorms 274 Stages of Development 274 Occurrence 276 Severe Thunderstorms 276 Supercell Thunderstorms 277 Squall Lines 278 Severe and Hazardous Weather: Flash Floods— Thunderstorms’ Number-One Killer 279 Mesoscale Convective Complexes 281 Lightning and Thunder 281 Severe and Hazardous Weather: Downbursts 282 What Causes Lightning? 283 Lightning Strokes 284 Eye on the Atmosphere 285 Thunder 286 Tornadoes 286 The Development and Occurrence of Tornadoes 288 Tornado Development 288 Tornado Climatology 288
ALEXEY STIOP/ALAMY
Contents
xiii
Radar and Data Buoys 320 Hurricane Watches and Warnings 321 Hurricane Forecasting 321 Professional Profile: Daniel Brown: Senior Hurricane Specialist, National Hurricane Center 322 Give It Some Thought 323 HURRICANES IN REVIEW 324 VOCABULARY REVIEW 325 PROBLEMS 325
Analysis 12 Weather and Forecasting
326
Focus on Concepts 327 PUBLIC DOMAIN
Severe and Hazardous Weather: Surviving a Violent Tornado 290 Profile of a Tornado 291 Tornado Destruction 292 Tornado Intensity 293 Loss of Life 294 Tornado Forecasting 294 Professional Profile: Warren Faidley: Storm Chaser 295 Eye on the Atmosphere 296 Tornado Watches and Warnings 296 Doppler Radar 296 Give It Some Thought 298 THUNDERSTORMS AND TORNADOES IN REVIEW 299 VOCABULARY REVIEW 300 PROBLEMS 300
11 Hurricanes
302
Focus on Concepts 303 Profile of a Hurricane 304 BOX 11–1 The Conservation of Angular Momentum 306 Hurricane Formation and Decay 307 Hurricane Formation 307 BOX 11–2 Naming Tropical Storms and Hurricanes 309 Hurricane Decay 309 Hurricane Destruction 310 Eye on the Atmosphere 310 Saffir–Simpson Scale 311 Storm Surge 311 Wind Damage 313 Heavy Rains and Inland Flooding 313 Estimating the Intensity of a Hurricane 313 Severe and Hazardous Weather: Cyclone Nargis 314 Eye on the Atmosphere 315 Detecting, Tracking, and Monitoring Hurricanes 316 The Role of Satellites 316 Aircraft Reconnaissance 317 Severe and Hazardous Weather: Hurricane Katrina from Space 318
The Weather Business: A Brief Overview 328 Weather Analysis 329 Gathering Data 330 Weather Maps: Pictures of the Atmosphere 331 BOX 12–1 Constructing a Synoptic Weather Chart 332 Weather Forecasting Using Computers 332 Numerical Weather Prediction 333 Ensemble Forecasting 334 Role of the Forecaster 335 Other Forecasting Methods 335 Persistence Forecasting 335 Eye on the Atmosphere 336 Climatological Forecasting 336 BOX 12–2 Numerical Weather Prediction 337 Analog Method 338 Trend Forecasting 338 Upper Airflow and Weather Forecasting 338 Upper-Level Maps 338 The Connection Between Upper-Level Flow and Surface Weather 341 Eye on the Atmosphere 342 Long-Range Forecasts 344 Forecast Accuracy 345 Satellites in Weather Forecasting 346
SMILEY N. POOL/RAPPORT PRESS/NEWCOM
xiv
Contents
14 The Changing Climate
378
Focus on Concepts 379
DOABLE/AMANA IMAGES/GLOW IMAGES
Professional Profile: Harold Brooks: Research Meteorologist 347 What Type of Images Do Weather Satellites Provide? Eye on the Atmosphere 350 Other Satellite Measurements 350 Give It Some Thought 351 WEATHER ANALYSIS AND FORECASTING IN REVIEW 353 VOCABULARY REVIEW 353 PROBLEMS 354
13 Air Pollution
347
356
Focus on Concepts 357 The Threat of Air Pollution 358 Sources and Types of Air Pollution 360 Primary Pollutants 360 Severe and Hazardous Weather: The Great Smog of1952 361 Eye on the Atmosphere 362 BOX 13–1 Air Pollution Changing the Climate of Cities 363 Secondary Pollutants 364 Trends in Air Quality 366 Establishing Standards 366 Air Quality Index 367 Meteorological Factors Affecting Air Pollution 368 Wind As a Factor 368 Severe and Hazardous Weather: Viewing an Air Pollution Episode from Space 369 The Role of Atmospheric Stability 369 Acid Precipitation 371 Extent and Potency of Acid Precipitation 371 Eye on the Atmosphere 373 Effects of Acid Precipitation 373 Eye on the Atmosphere 375 Give It Some Thought 376 AIR POLLUTION IN REVIEW 377 VOCABULARY REVIEW 377
The Climate System 380 How Is Climate Change Detected? 380 Seafloor Sediment—A Storehouse of Climate Data 382 Oxygen-Isotope Analysis 382 Climate Change Recorded in Glacial Ice 383 Tree Rings—Archives of Environmental History 383 Other Types of Proxy Data 384 Natural Causes of Climate Change 385 Plate Tectonics and Climate Change 386 Volcanic Activity and Climate Change 386 Variations in Earth’s Orbit 388 BOX 14–1 Volcanism and Climate Change in the Geologic Past 390 Solar Variability and Climate 390 Eye on the Atmosphere 392 Human Impact on Global Climate 392 Carbon Dioxide, Trace Gases, and Climate Change 392 Rising CO2 Levels 393 The Atmosphere’s Response 393 The Role of Trace Gases 395 Eye on the Atmosphere 395 Climate-Feedback Mechanisms 397 Types of Feedback Mechanisms 397 Computer Models of Climate: Important yet Imperfect Tools 398 How Aerosols Influence Climate 398 Some Possible Consequences of Global Warming 399 Sea-Level Rise 399 The Changing Arctic 402 Eye on the Atmosphere 403 Increasing Ocean Acidity 404 The Potential for “Surprises” 404 Professional Profile: Michael Mann: Climate Change Scientist 405 Give It Some Thought 406 THE CHANGING CLIMATE IN REVIEW 406 VOCABULARY REVIEW 407
DAVID VAUGHAN/PHOTO RESEARCHERS, INC.
Contents The Polar Climates (E ) 436 The Tundra Climate (ET) 436 The Ice-Cap Climate (EF) 438 Highland Climates 439 Severe and Hazardous Weather: Drought—A Costly Atmospheric Hazard 440 Eye on the Atmosphere 442 Give It Some Thought 443 WORLD CLIMATES IN REVIEW 444 VOCABULARY REVIEW 446 PROBLEMS 447
MICHAEL GIANNECHINI/PHOTO RESEARCHERS, INC.
Phenomena 16 Optical of the Atmosphere
448
Focus on Concepts 449
15
World Climates
408
Focus on Concepts 409 Climate Classification 410 Climate Controls: A Summary 412 Latitude 412 Land and Water 413 Geographic Position and Prevailing Winds 413 BOX 15–1 Climate Diagrams 414 Mountains and Highlands 414 Ocean Currents 414 Pressure and Wind Systems 415 World Climates—An Overview 415 The Wet Tropics (Af, Am) 415 Temperature Characteristics 417 BOX 15–2 Clearing the Tropical Rain Forest— The Impact on Its Soils 418 Precipitation Characteristics 419 Tropical Wet and Dry (Aw) 419 Temperature Characteristics 419 Precipitation Characteristics 420 The Monsoon 421 The Cw Variant 422 The Dry Climates (B) 422 What Is Meant by “Dry”? 423 Subtropical Desert (BWh) and Steppe (BSh) 423 BOX 15–3 The Disappearing Aral Sea—A Large Lake Becomes a Barren Wasteland 425 West Coast Subtropical Deserts 426 Middle-Latitude Desert (BWk) and Steppe (BSk) 428 Humid Middle-Latitude Climates with Mild Winters (C ) 429 Humid Subtropical Climate (Cfa) 429 The Marine West Coast Climate (Cfb) 430 Eye on the Atmosphere 430 The Dry-Summer Subtropical (Mediterranean) Climate (Csa, Csb) 432 Humid Continental Climates with Severe Winters (D) 433 Humid Continental Climate (Dfa) 433 The Subarctic Climate (Dfc, Dfd) 435
Interactions of Light and Matter 450 Reflection 450 Refraction 450 Mirages 453 BOX 16–1 Are Highway Mirages Real? 454 Eye on the Atmosphere 455 Rainbows 455 Halos, Sun Dogs, and Solar Pillars 458 Glories 461 Other Optical Phenomena 461 Coronas 461 Iridescent Clouds 462 Eye on the Atmosphere 462 Give It Some Thought 463 OPTICAL PHENOMENA OF THE ATMOSPHERE IN REVIEW 464 VOCABULARY REVIEW 464
Appendix A Metric Units
465
Appendix B Explanation and Decoding of the Daily Weather Map 469 Appendix C Relative Humidity and Dew-Point Tables 476 Appendix D Laws Relating to Gases
478
Appendix E Newton’s Laws, Pressure–Gradient Force, and Coriolis Force 479 Appendix F Saffir–Simpson Hurricane Scale Appendix G Climate Data Glossary 488 Index 498
482
481
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Preface There are few aspects of the physical environment that influence our daily lives more than the phenomena we collectively call weather. The media regularly report a wide range of weather events as major news stories—an obvious reflection of people’s interest in and curiosity about the atmosphere. Not only does the atmosphere impact the lives of people, but people have a significant impact on the atmosphere as well. By altering the composition of Earth’s atmosphere, people have diminished the stratosphere’s ozone layer and created serious air-quality problems in urban and rural areas around the world. Moreover, human-generated emissions likely play an important role in global climate change, one of the most serious environmental issues facing humankind in the twenty-first century. In order to understand the weather phenomena that affect our daily lives and the serious environmental problems related to the atmosphere, it is important to develop an understanding of meteorological principles. A basic meteorology course takes advantage of our interest in and curiosity about the weather as well as our desire to understand the impact that people have on the atmospheric environment. The Atmosphere: An Introduction to Meteorology, 12th edition, is designed to meet the needs of students who enroll in such a course. It is our hope that the knowledge gained by taking a class and using this book will encourage many to actively participate in bettering the environment; some may even be sufficiently stimulated to continue their study of meteorology. Equally important, however, is our belief that a basic understanding of the atmosphere and its processes will greatly enhance appreciation of our planet and thereby enrich the reader’s life. In addition to being informative and up-to-date, The Atmosphere meets the need of beginning students for a readable and user-friendly text, a book that is a highly usable tool for learning basic meteorological principles and concepts.
UÊ
UÊ
UÊ
UÊ
New to the 12th Edition UÊ New Active Learning Path. Each chapter begins with Focus on Concepts, which identifies the knowledge and skills that students should master by the end of the chapter, helping students prioritize key concepts. Within the chapter, each major section concludes with a Concept Check that allows students to check their understanding and comprehension of important topics before moving on to the next section. Each chapter concludes with a new section called Give It Some Thought. These questions and problems challenge learners by involving them in activities that require higher-order thinking skills that include application, analysis, and synthesis of material in the chapter. UÊ Eye on the Atmosphere. Within every chapter are two or three images, often aerial or satellite views, that chalxvi
UÊ
lenge students to apply their understanding of basic facts and principles. A brief explanation of each image is followed by questions that serve to focus students on visual analysis and critical thinking tasks. Severe and Hazardous Weather. Atmospheric hazards adversely affect millions of people worldwide every day. Severe weather events have a significance and fascination that go beyond ordinary weather phenomena. Two entire chapters (Chapter 10, “Thunderstorms and Tornadoes,” and Chapter 11, “Hurricanes”) focus entirely on such topics. Moreover, the text contains 15 Severe and Hazardous Weather essays that are devoted to a broad variety of topics—heat waves, winter storms, floods, air pollution episodes, drought, wildfires, cold waves, and more. Professional Profiles. These essays present profiles of professionals who use meteorology in the real world, giving students a sense of professional applications and careers in the science. Included are profiles on research meteorologists, a military meteorologist, a climate scientist, a broadcast meteorologist, a storm chaser, and a senior hurricane specialist and warning coordination meteorologist. An Unparalleled Visual Program That Teaches. In addition to more than 150 new high-quality photos and satellite images, dozens of figures are new or redrawn by renowned geoscience illustrator Dennis Tasa. Numerous diagrams and maps are paired with photographs for greater effectiveness. Many new and revised art pieces also have additional labels that narrate the process being illustrated and guide students as they examine the figures. The result is a visual program that is clear and easy to understand. In addition, a new removable Cloud Guide appears at the back of the book, to help students with observation and forecasting out in the field. Significant Updating and Revision of Content. With the goal of keeping the text current and highly readable for beginning students, many discussions, case studies, and examples have been revised. This 12th edition represents perhaps the most extensive and thorough revision in the long history of this textbook. See the section “Revised and Updated Content” for more particulars. MyMeteorologyLab with Pearson eText. www.MyMeteorologyLab.com is a new resource both for student self-study and for instructors to manage their courses online and provide customizable assessments to students. MyMeteorologyLab’s assignable content includes Geoscience Animations, GEODe: Atmosphere tutorials, MapMasterTM interactive maps, a variety of chapter quizzes, and more. Students can also access the Pearson eText for The Atmosphere, 12th edition, Carbone’s Exercises for Weather & Climate interactive media, “In the News” RSS feeds, glossary flashcards, social networking features, and additional references and resources to extend learning beyond the text.
Preface
Distinguishing Features Readability The language of this book is straightforward and easy to understand. Clear, readable discussions with a minimum of technical language are the rule. The frequent headings and subheadings help students follow discussions and identify the important ideas presented in each chapter.
Visual Program Meteorology is highly visual, so art and photographs play a critical role in an introductory textbook. Our aim is to get maximum effectiveness from the visual component of the book. As in previous editions, Dennis Tasa, a gifted artist and respected science illustrator, has worked closely with the authors to plan and produce the diagrams, maps, graphs, and sketches that are so basic to student understanding. The result is art that is clear and easy to understand.
Focus on Basic Principles and Instructor Flexibility Although many topical issues are treated in the 12th edition of The Atmosphere, it should be emphasized that the main focus of this new edition remains the same as the focus of each of its predecessors—to promote student understanding of the basic principles of meteorology. Student use of the text is a primary concern, and the book’s adaptability to the needs and desires of the instructor is equally important. In keeping with this aim, the organization of the text remains intentionally traditional, allowing for maximum instructor flexibility in terms of the sequence and emphasis of topics.
Focus on Learning In addition to the new active learning features described earlier, The Atmosphere, 12th edition includes other important learning aids. Each chapter includes a number of Students Sometimes Ask features that address common student questions, a Chapter in Review that recaps all the major points, and a Vocabulary Review that provides a checklist of key terms with page references. In most chapters, Problems, many with quantitative orientation, are included. Most problems require only basic math skills and allow students to enhance their understanding by applying concepts and principles explained in the chapter. Each chapter ends by reminding students to go to the text’s outstanding premium website, www.MyMeteorologyLab.com, to access many useful learning tools.
Revised and Updated Content This 12th edition of The Atmosphere represents a thorough revision. In fact, it is likely the most thorough revision of text and figures in the long history of this text. Those familiar
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with the previous edition will see much that is new. The following are some examples:* UÊ Ê >«ÌiÀÊ £Ê Ì iÊ ÃiVÌÊ º >ÀÌ Ê >ÃÊ >Ê -ÞÃÌi»Ê ÃÊ Ã«fied and reduced in size, and the box on “The Origin and Evolution of Earth’s Atmosphere” is revised, updated, and rewritten. UÊ Ê >«ÌiÀÊ ÓÊ Ì iÊ L>ÃVÊ `ÃVÕÃÃÊ vÊ Ãi>ÃÃÊ ÃÊ ÀiÛÃi`Ê to provide a more understandable introduction to this important topic. “The Role of Gases in the Atmosphere” is revised and expanded, and “Earth’s Heat Budget” is reorganized and revised to improve clarity. UÊ Ê >«ÌiÀÊÎÊÃ}vV>ÌÊV >}iÃÊVÕ`iÊÌ ÀiiÊiÜÊÜÀ`Ê temperature maps and the all-new section “Heat Stress and Windchill.” UÊ Ê >«ÌiÀÊ {Ê Ì iÊ ÃiVÌÊ º6>«ÀÊ *ÀiÃÃÕÀiÊ >`Ê ->ÌÕÀ>tion” is substantially revised, and a new figure is added to support this important topic. “How Is Humidity Measured?” is simplified to improve clarity. UÊ Ê >«ÌiÀÊxÊÌ iÊ`ÃVÕÃÃÊvÊ >ÊÃÊÀiÛÃi`Ê>`ÊÕ«`>Ìi`]Ê the section “Precipitation Measurement” is simplified and reduced, and the section “Intentional Weather Modification” is reorganized and streamlined. UÊ >«ÌiÀÊÈÊLi}ÃÊÜÌ Ê>ÊÀiÛÃi`Ê>`ÊiÝ«>`i`ÊÌÀ`ÕVtory section. A new section titled “Why Does Air Pressure Vary?” replaces the section “Horizontal Variation in Air Pressure.” UÊ -}vV>ÌÊ«ÀÌÃÊvÊ >«ÌiÀÊÇÊ>ÀiÊV«iÌiÞÊÀiÜÀÌten and extensively reorganized. The sections “The Westerlies” and “Waves in the Westerlies” are combined and completely rewritten. The discussions of El Niño and La Niña are completely revised and include all-new figures. UÊ >«ÌiÀÊ nÊ VÕ`iÃÊ >Ê >iÜÊ V>ÃiÊ ÃÌÕ`ÞÊ vÊ >Ê V>ÃÃVÊ nor’easter that hit New England in January 2011. UÊ >«ÌiÀÊ Ê vÀiÀÞÊ º7i>Ì iÀÊ *>ÌÌiÀû®Ê >ÃÊ >Ê iÜÊ title, “Midlatitude Cyclones,” and has been completely revamped, with several new discussions and others that have been rewritten for greater clarity. UÊ / iÊ>ÌiÀ>ÊÊ >«ÌiÀÊ£ä]ʺ/ Õ`iÀÃÌÀÃÊ>`Ê/À>does,” includes new statistics and examples from spring 2011. It includes a new box on downbursts, new material on lightning and tornado destruction, and a Professional Profile that highlights a storm chaser. UÊ >«ÌiÀÊ ££]Ê Ì iÊ ÃiV`Ê vÊ ÌÜÊ V >«ÌiÀÃÊ Ì >ÌÊ vVÕÃÊ Ê severe weather, includes a revised introduction, updated coverage, a Professional Profile, and two revised case study boxes. UÊ Ê >«ÌiÀÊ £ÓÊ Ì iÊ ÃiVÌÃÊ º7i>Ì iÀÊ ÀiV>ÃÌ}»Ê >`Ê “Forecast Accuracy” are reorganized and significantly revised for greater clarity. The section “Satellites in Weather Forecasting” is updated and expanded. The chapter includes a Professional Profile of a research meteorologist. UÊ >«ÌiÀÊ £ÎÊ Li}ÃÊ ÜÌ Ê >Ê iÜÊ i>`}]Ê º/ iÊ / Ài>ÌÊ of Air Pollution.” The revised discussion “Secondary *For a complete and detailed list of changes, contact your local Pearson representative.
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Preface
Pollutants” is now divided into two parts. The new section “Air Quality Index” is accompanied by new art. A new box, “Viewing an Air Pollution Episode from Space,” presents a brief case study. UÊ >«ÌiÀÊ £{]Ê º/ iÊ >}}Ê >Ìi]»Ê Ü V Ê ÃÕÀÛiÞÃÊ both natural causes and the human impact on global climate, is thoroughly updated to include the most recent data available. The chapter includes material on trace gases, aerosols, computer models, sea-level rise, and ocean acidity. UÊ >«ÌiÀÊ £xÊ VÕ`iÃÊ >Ê iÜ]Ê ÀiÊ >VVÕÀ>Ìi]Ê >`Ê ÀiÊ readable map of world climates and a new chart showing the Köppen classification. UÊ Ê >«ÌiÀÊ £ÈÊ Ì iÊ ÃiVÌÊ ºÌiÀ>VÌÃÊ vÊ } ÌÊ >`Ê Matter” is completely rewritten and includes new supporting illustrations. The discussion of rainbows is reorganized and rewritten for greater clarity, while other sections are reduced and are more readable.
The Atmosphere Teaching and Learning Package The authors and publisher are pleased to present unparalleled media and supplements resources for students and instructors.
For You, the Student UÊ MyMeteorologyLab with Pearson eText www.MyMeteorologyLab.com is a new resource both for student self-study and for instructors to manage their courses online and assign customizable and automatically graded assessments to students. MyMeteorologyLab’s assignable content includes Geoscience Animations, GEODe: Atmosphere tutorials, MapMasterTM interactive maps, videos, a variety of chapter quizzes, and more. Students can also access the Pearson eText for The Atmosphere, 12th edition, Carbone’s Exercises for Weather & Climate interactive media, “In the News” RSS feeds, glossary flashcards, social networking features, and additional references and resources to extend learning beyond the text. UÊ Pearson eText for The Atmosphere, 12th edition Pearson eText for The Atmosphere, 12th edition gives you access to the text whenever and wherever you can access the Internet and includes powerful interactive and customization functions. UÊ Exercises for Weather and Climate, 8th edition by Greg Carbone [0321769651] This bestselling exercise manual’s 17 exercises encourage students to review important ideas and concepts through problem solving, simulations, and guided thinking. The graphics program and computer-based simulations and tutorials help students grasp key concepts. This manual is designed to compliment any introductory meteorology or weather and climate text.
UÊ Encounter Meteorology: Interactive Explorations of Earth Using Google Earth [0321815912] This workbook and premium website provides rich, interactive explorations of meteorology concepts through Google EarthTM explorations. All chapter explorations are available in print format as well as via online quizzes and downloadable PDFs, accommodating different classroom needs. Each worksheet is accompanied by corresponding Google Earth KMZ media files containing the placemarks, overlays, and annotations referred to in the worksheets, available for download from www.mygeoscienceplace.com. UÊ Encounter Geosystems: Interactive Explorations of Earth Using Google Earth [0321636996] This workbook and premium website provides rich, interactive explorations of physical geography concepts through Google Earth explorations. All chapter explorations are available in print format as well as via online quizzes and downloadable PDFs, accommodating different classroom needs. Each worksheet is accompanied by corresponding Google Earth KMZ media files containing the placemarks, overlays, and annotations referred to in the worksheets, available for download from www.mygeoscienceplace.com. UÊ Goode’s World Atlas, 22nd edition [0321652002] Goode’s World Atlas has been the world’s premiere educational atlas since 1923—and for good reason. It features over 250 pages of maps, from definitive physical and political maps to important thematic maps that illustrate the spatial aspects of many important topics. The 22nd edition includes 160 pages of new, digitally produced reference maps, as well as new thematic maps on global climate change, sea-level rise, CO2 emissions, polar ice fluctuations, deforestation, extreme weather events, infectious diseases, water resources, and energy production. UÊ Dire Predictions: Understanding Global Warming [0136044352] Periodic reports from the Intergovernmental Panel on Climate Change (IPCC) evaluate the risk of climate change brought on by humans. But the sheer volume of scientific data remains inscrutable to the general public, particularly to those who may still question the validity of climate change. In just over 200 pages, this practical text presents and expands upon the essential findings in a visually stunning and undeniably powerful way to the lay reader. Scientific findings that provide validity to the implications of climate change are presented in clear-cut graphic elements, striking images, and understandable analogies.
For You, the Instructor UÊ Geoscience Animation Library on DVD, 5th edition [0321716841] For engaging lectures, the Geoscience Animation Library includes more than 110 animations illuminating many difficult-to-visualize topics in physical geology, physical geography, oceanography, meterorology, and Earth science—created through a unique collaboration among Pearson’s leading geoscience authors.
Preface
UÊ MyMeteorologyLab www.MyMeteorologyLab.com helps instructors manage their courses online, with robust course management, gradebook, and diagnostic tools. Instructors can assign customizable and automatically graded assessments to students. Assignable content includes Geoscience Animations, GEODe: Atmosphere tutorials, MapMasterTM interactive maps, videos, a variety of chapter quizzes, and more. UÊ Instructor Resource DVD [0321780337] The Instructor Resource DVD provides high-quality electronic versions of photos and illustrations from the book, as well as customizable PowerPointTM lecture presentations, Classroom Response System questions in PowerPoint, and the Instructor Resource Manual and Test Bank in Microsoft Word and TestGen formats. The DVD also includes all the illustrations and photos from the text, in presentation-ready JPEG files, as well as digital transparencies. For easy reference and identification, all resources are organized by chapter. All of the elements on the DVD are also available online to professors at www.pearsonhighered.com/irc. UÊ Instructor Resource Manual by Neve Duncan Tabb (download only) [0321780329] The Instructor Resource Manual is intended as a resource for both new and experienced instructors. It includes a variety of lecture outlines, additional source materials, teaching tips, advice about how to integrate visual supplements (including the web-based resources), and various other ideas for the classroom. See www.pearsonhighered.com/irc. UÊ TestGen® Computerized Test Bank by Jennifer Johnson (download only) [0321780299] TestGen® is a computerized test generator that lets instructors view and edit Test Bank questions, transfer questions to tests, and print tests in a variety of customized formats. This Test Bank includes more than 2000 multiple-choice, fill-inthe-blank, and short-answer/essay questions. Questions are correlated to the revised U.S. National Geography Standards and Bloom’s Taxonomy to help instructors better map the assessments against both broad and specific teaching and learning objectives. The Test Bank is also available in Microsoft WordTM and is importable into Blackboard. See www.pearsonhighered.com/irc. UÊ Earth Report Geography Videos on DVD [0321662989] This three-DVD set is designed to help students visualize how human decisions and behavior have affected the environment and how individuals are taking steps toward recovery. With topics ranging from poor land management promoting the devastation of river systems in Central America to the struggles for electricity in China and Africa, these 13 videos from Television for the Environment’s global Earth Report series recognize
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the efforts of individuals around the world to unite and protect the planet.
Acknowledgments Writing a college textbook requires the talents and cooperation of many individuals. Working with Dennis Tasa, who is responsible for all of the text’s outstanding illustrations and much of the developmental work on GEODe: Atmosphere, is always special for us. We not only value his outstanding artistic talents and imagination but his friendship as well. Great thanks go to those colleagues who prepared indepth reviews or suggestions for new Give It Some Thought questions. Their critical comments and thoughtful input helped guide our work and clearly strengthened the 12th edition. We wish to thank: Jason Allard, Valdosta State University Deanna Bergondo, U.S. Coast Guard Academy William Conant, University of Arizona Ron Dowey, Harrisburg Area Community College– Harrisburg Douglas Gamble, University of North Carolina, Wilmington Mark Hildebrand, Southern Illinois University– Edwardsville Helenmary Hotz, University of Massachusetts–Boston Timothy and Jennifer Klingler, Delta College Mark Lemmon, Texas A&M University Jason Ortegren, University of West Florida Robert S. Rose, Tidewater Community College– Virginia Beach Roger D. Shew, University of North Carolina– Wilmington Steve Simpson, Highland Community College Eric Snodgrass, University of Illinois–UrbanaChampaign Andrew Van Tuyl, Galivan College We also want to acknowledge the team of professionals at Pearson Education. We sincerely appreciate the company’s continuing strong support for excellence and innovation. Our special thanks to the outstanding geography and meteorology team—Christian Botting, Crissy Dudonis, and Maureen McLaughlin. In addition to being great people to work with, all are committed to producing the best textbooks possible. The production team, led by Heidi Allgair at Element LLC and Ed Thomas at Pearson, has once again done an outstanding job. All are true professionals with whom we are very fortunate to be associated. Fred Lutgens Ed Tarbuck
Powerful pedagogy equips students with the skills to master the science.
NEW!
Focus on Concepts
Clear, testable learning outcomes at the beginning of each chapter focus students on key concepts and skills.
Concept Check 10.6
NEW!
1 How is thunder produced?
Concept Checks
2 Which is more common: sheet lightning or cloud-to-ground lightning?
3 What is heat lightning?
Concept Check 11.1 1 Define hurricane. What other names are used for this storm? 2 In what latitude zone do hurricanes develop? 3 Distinguish between the eye and the eye wall of a hurricane. How do conditions differ in these zones?
NEW!
Give It Some Thought 1. If you were asked to identify the coldest city in the
United States (or any other designated region), what statistics could you use? Can you list at least three different ways of selecting the coldest city? 2. The accompanying graph shows monthly high
and the differential heating of land and water influence the climate of this place. 5. The accompanying sketch map represents a hypothetical continent in the Northern Hemisphere. One isotherm has been placed on the map.
temperatures for Urbana, Illinois, and San Francisco, California. Although both cities are located at about the same latitude, the temperatures they experience are quite different. Which line on the graph represents Urbana and which represents San Francisco? How did you figure this out?
3. On which summer day would you expect the greatest
temperature range? Which would have the smallest range in temperature? Explain your choices. a. Cloudy skies during the day and clear skies at night b. Clear skies during the day and cloudy skies at night c. Clear skies during the day and clear skies at night d. Cloudy skies during the day and cloudy skies at night 4. The accompanying scene shows an island near the equator in the Indian Ocean. Describe how latitude, altitude,
a. Is the temperature higher at city A or city B? Explain. b. Is the season winter or summer? How are you able to determine this? c. Describe (or sketch) the position of this isotherm six months later. 6. The data below are mean monthly temperatures in °C for an inland location that lacks any significant ocean influence. Based on the annual temperature range, what is the approximate latitude of this place? Are these temperatures what you would normally expect for this latitude? If not, what control would explain these temperatures? J F M A M J J A S O N D 6.1 6.6 6.6 6.6 6.6 6.1 6.1 6.1 6.1 6.1 6.6 6.6 7. Refer to Figure 3–18. What causes the bend or kink in
the isotherms in the North Atlantic?
These questions appear at the end of each major section, giving students a chance to stop, check, and practice their understanding of key chapter concepts before moving on.
Give It Some Thought
GIST questions at the end of each chapter give students an opportunity to synthesize chapter concepts and practice higher-order thinking.
Problems “Problems” extend learning with deeper, quantitative treatments of chapter concepts.
Tools to refine students’ observation skills and emphasize the relevance of meteorology today. NEW!
NEW!
NEW!
Eye on the Atmosphere
Professional Profiles
These features ask students to inspect visualizations and data, practicing their critical thinking and visual analysis skills.
Severe and Hazardous Weather Essays
These essays profile a variety of professionals who use meteorology every These essays focus on the dramatic severe day, emphasizing opportunities in the field and hazardous weather phenomena that and the relevance of the science. increasingly impact our world. Warren Faidley: Storm Chaser
Iceland
Vo lc
a n ic
a sh
p lu
Norway me
Warren Faidley is a storm chaser. As an extreme weather photojournalist, he has survived more blizzards, tornadoes, hailstorms, and flying debris than he would like to remember. His images of town-dwarfing tornadoes and hurricane destruction have been used in movies and magazines, news programs, and textbooks. As a frequent witness to many violent weather events, he is often interviewed for news programs and storm documentaries. Faidley’s calendar revolves around storms: Spring is tornado season, summer is lightning season, and late summer to fall is hurricane season. All the while, he analyzes weather charts, second-guesses forecasts, and consults Doppler radar data the way most people consult city maps. He spends his days zigzagging across the farm roads and lonely highways of Oklahoma, Kansas, and other states to approach storms in progress.
It’s one part science and meteorology, and another part artistry. United Kingdom
(Image courtesy of NASA) In mid-April 2010, Iceland’s Eyjafjallajökull volcano produced an ash plume that rose nearly 50,000 feet into the atmosphere as it moved eastward across the North Atlantic. This event prompted authorities in the United Kingdom, Ireland, France, and Scandinavia to close airspace over their countries for fear that the volcanic ash, if sucked into an airplane’s turbines, could cause engine failure.
NEW!
Question 1 Based on the fact that Iceland is located in the zone of the polar easterlies (winds that blow from the northeast toward the southwest), explain why this ash plume moved primarily from west to east toward Northern Europe.
Getting near a storm is only half the job. “At the same time, I need to make images that convey what it looks like when a 2 × 4 goes through the side of a car.” One trick is finding spots of color like a red barn and green fields against gray storm clouds and a gray sky. “It’s one part science and meteorology, and another part artistry,” Faidley says. Faidley’s life has been entwined with extreme weather since childhood. He nearly drowned after being swept away in a flash flood at age 12, and he steered his bicycle into dust devils as a teen. By the mid-1980s, after earning a degree in journalism, he decided to become the country’s first full-time weather photojournalist. His first break was a bolt from the blue. Faidley snapped a photo of a white-hot arc of lightning striking a light pole, suffusing the Arizona night in an eeric purple glow. Another fork hit perilously near Faidley, almost killing him. But the episode ended on a happy note. Life magazine published the lightning photo in 1989, launching Faidley’s freelance career.
In 1992, Faidley followed up the lightning photo by obtaining some of the few existing shots of Hurricane Andrew in progress. Faidley hid under a shed in south Florida while the category 5 storm howled past.
There are moments of terror but also moments of absolute beauty. Storm chasing offers Faidley a heady mix of adrenaline and grace. “It’s very awe-inspiring. There are moments of terror but also moments of absolute beauty. Capturing a picture of an orange sky cut in half by the emerald green of a coming storm is fantastic.” Faidley is no mere thrill-seeker. He does a great deal of advance planning and takes the precautions required to come back alive. In the late 1990s, he designed and built the first tornado-resistant chase vehicle, installing impact-resistant glass, a NASCAR-type roll cage, and other safety features on his SUV. “The purpose isn’t to enable us to do something stupid, like penetrate a tornado.
Rather, it’s to offer us safety in case something unexpected happens such as a sign blowing off a motel and careening down the road.” He is always aware of escape roads whenever he’s in storm country, and he speaks regularly to the public about the importance of staying informed and knowing how to respond during a violent weather episode. When Faidley first began storm chasing in the 1980s, moment-by-moment weather infomation was hard to come by. Live weather radar on the Internet did not exist back then. Instead, he got to know National Weather Service forecasters and learned storm meteorology from them on the fly. Today, Faidley watches forecasts weeks ahead to ensure that he’s within driving range when the looming clouds appear. “I live a barnstorming, gypsy life driven by visual instinct,” Faidley says. “The canvas keeps changing but the canvas wants to kill you. It’s a juggling act.”
January 12, 2011— Classic Nor’easter
A
classic nor’easter moved up the east coast on January 12, 2011, dumping heavy snow on the New England states for the third time in three weeks (Figure 8-E). The storm began developing to the south a day earlier. As it moved northward along the coast, it merged with another system crossing from the Midwest. The satellite image in Figure 8-F shows that the storm had a distinctive comma shape—which forms from the counterclockwise circulation around a low-pressure center. Cold, humid mP air from the North Atlantic was drawn toward the storm center, producing dense clouds, especially on the north and west sides of the storm. In parts of New England, snow
fell as fast as 7.6centimeters (3 inches) per hour. More than 61 centimeters (24 inches) fell in many areas by the evening of January 12.
Airport near Hartford, Connecticut, set a one-day record with 57 centimeters (22 inches) of snow, and Wilmington, Vermont, received in excess of 91 centimeters (35 inches).
The storm left more than 100,000 people without electricity. Blizzard conditions—with visibility cut to less than 0.4 kilometer (0.25 mile) and gale-force winds for more than three hours—developed in parts of Connecticut and Massachusetts. The storm left more than 100,000 people without electricity and led to the shutdown of portions of Interstate 95 and the northeastern railroad service. Bradley International
Kathleen M. Wong
WARREN FAIDLEY is a well-known storm chaser and weather photographer. He is the author of The Ultimate Storm Survival Handbook and the autobiographical book Storm Chaser. (Photo courtesy of Warren Faidley)
FIGURE 8-E Digging out in Boston following the January 12, 2011 blizzard. (Photo by Michael Dwyer/ Alamy)
FIGURE 8-F Satellite image of a strong winter storm called a nor’easter along the coast of New England on January 12, 2011. In winter, a nor’easter exhibits a weather pattern in which strong northeast winds carry cold, humid mP air from the North Atlantic into New England and the middle Atlantic states. The combination of ample moisture and strong convergence can result in heavy snow. (NASA)
Cloud Guide
A fold out cloud guide at the back of the book provides students with a tool and reference for real world observation.
High Clouds: cloud bases above 6km (20,000 ft)
SHUTTERSTOCK
Cirrus These clouds are made exclusively of ice crystals. They are not as horizontally extensive as cirrostratus clouds.
JIM LEE/NOAA
Cirrostratus These are thin layered clouds composed of ice crystals. They are relatively indistinct and give the sky a whitish appearance.
Middle Clouds: cloud bases 2-6km (6,500–20,000 ft)
SHUTTERSTOCK
Cirrocumulus These high clouds can produce striking skies. Composed of ice crystals, they often contain linear bands, numerous patches of greater vertical development, or both.
DENNIS TASA
Contrails A contrail is a long, narrow cloud that is formed as exhaust from a jet aircraft condenses in cold air at high altitude. Upper level winds may gradually cause contrails to spread out.
SHUTTERSTOCK
Altocumulus These midlevel clouds are horizontally layered but exhibit varying thicknesses across their bases. Thicker areas can be arranged as parallel linear bands or as a series of individual puffs.
JIM LEE/NOAA
Altostratus These are midlevel, layered clouds that produce gray skies and obscure the Sun or Moon enough to make them appear as poorly defined bright spots. In this example, the setting sun brightens the clouds near the horizon but the gray appearance remains elsewhere.
SHUTTERSTOCK
Altocumulus (Lenticular) These clouds are marked by their lens-shaped appearance. They usually form downwind of mountain barriers as horizontal airflow is disrupted into a sequence of waves.
JIM LEE/NOAA
Altostratus (Multilayer) These are midlevel layered clouds that are dense enough to completely hide the Sun or Moon.
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The Atmosphere
MyMeteorologyLab Visualization and Practice An assortment of assignable, assessable media illustrate complex processes and bring concepts to life. www.MyMeteorologyLab.com
NEW!
Geoscience Animations
NEW!
These animations help students visualize difficult physical processes over space and time.
These interactive maps act as a mini-GIS tool, allowing students to layer various thematic maps to analyze spatial patterns and data at regional and global scales.
Exercises for Weather and Climate
GEODe: Atmosphere
These interactive simulations and exercises give students practice with problem solving and guided critical thinking.
A dynamic program that reinforces key concepts through animations, tutorials, interactive exercises, and review quizzes.
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Chapter 1 Forms of Condensation & Precipitation
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MyMeteorologyLab A Full Course Solution Assignable media, intuitive gradebook, and powerful diagnostics help you focus on teaching. www.MyMeteorologyLab.com
Course Management
Gradebook
MyMeteorologyLab is a full-featured online course management and homework system.
An easy-to-use and flexible gradebook helps identify struggling students.
Study Area (media, RSS feeds, flashcards,
eText
tools to help extend learning, master concepts, and prepare for exams.
MyMeteorologyLab includes the option of a Pearson eText version of The Atmosphere, with full search and annotation capability.
quizzes) Students have access to a rich set of study
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Introduction to the Atmosphere Earth’s atmosphere is unique. No other planet in our solar system has an atmosphere with the exact mixture of gases or the heat and moisture conditions necessary to sustain life as we know it. The gases that make up Earth’s atmosphere and the controls to which they are subject are vital to our existence. In this chapter we begin our examination of the ocean of air in which we all must live.
Hundreds of cars stranded on Chicago’s Lake Shore Drive on February 2, 2011, following a winter blizzard of historic proportions. (AP Photo/ Kiichiro Sato)
After completing this chapter, you should be able to: t Distinguish between weather and climate and name the basic elements of weather and climate. t List several important atmospheric hazards and identify those that are storm related. t Construct a hypothesis and distinguish between a scientific hypothesis and a scientific theory. t List and describe Earth’s four major spheres. t Define system and explain why Earth can be thought of as a system.
t List the major gases composing Earth’s atmosphere and identify those components that are most important meteorologically. t Explain why ozone depletion is a significant global issue. t Interpret a graph that shows changes in air pressure from Earth’s surface to the top of the atmosphere. t Sketch and label a graph showing the thermal structure of the atmosphere. t Distinguish between homosphere and heterosphere. 3
4
The Atmosphere: An Introduction to Meteorology
Focus on the Atmosphere ATMOSPHERE
Introduction to the Atmosphere ▸ Weather and Climate
Weather influences our everyday activities, our jobs, and our health and comfort. Many of us pay little attention to the weather unless we are inconvenienced by it or when it adds to our enjoyment of outdoor activities. Nevertheless, there are few other aspects of our physical environment that affect our lives more than the phenomena we collectively call the weather.
Weather in the United States The United States occupies an area that stretches from the tropics to the Arctic Circle. It has thousands of miles of coastline and extensive regions that are far from the influence of the ocean. Some landscapes are mountainous, and others are dominated by plains. It is a place where Pacific storms strike the West Coast, while the East is sometimes influenced by events in the Atlantic and the Gulf of Mexico. For those in the center of the country, it is common to experience weather events triggered when frigid southward-bound Canadian air masses clash with northward-moving tropical ones from the Gulf of Mexico. Stories about weather are a routine part of the daily news. Articles and items about the effects of heat, cold, floods, drought, fog, snow, ice, and strong winds are commonplace (Figure 1–1). Memorable weather events occur
Figure 1–1 Few aspects of our physical environment influence our daily lives more than the weather. Tornadoes are intense and destructive local storms of short duration that cause an average of about 55 deaths each year. (Photo by Wave RF/Photolibrary)
everywhere on our planet. The United States likely has the greatest variety of weather of any country in the world. Severe weather events, such as tornadoes, flash floods, and intense thunderstorms, as well as hurricanes and blizzards, are collectively more frequent and more damaging in the United States than in any other nation. Beyond its direct impact on the lives of individuals, the weather has a strong effect on the world economy, by influencing agriculture, energy use, water resources, transportation, and industry. Weather clearly influences our lives a great deal. Yet it is also important to realize that people influence the atmosphere and its behavior as well (Figure 1–2). There are, and will continue to be, significant political and scientific decisions to make involving these impacts. Answers to questions regarding air pollution and its control and the effects of various emissions on global climate are important examples. So there is a need for increased awareness and understanding of our atmosphere and its behavior.
Meteorology, Weather, and Climate The subtitle of this book includes the word meteorology. Meteorology is the scientific study of the atmosphere and the phenomena that we usually refer to as weather. Along with geology, oceanography, and astronomy, meteorology is considered one of the Earth sciences—the sciences that seek to understand our planet. It is important to point out that there are not strict boundaries among the Earth sciences; in many situations, these sciences overlap. Moreover, all of the Earth sciences involve an understanding and application of knowledge and principles from physics, chemistry, and biology. You will see many examples of this fact in your study of meteorology. Acted on by the combined effects of Earth’s motions and energy from the Sun, our planet’s formless and invisible envelope of air reacts by producing an infinite variety of weather, which in turn creates the basic pattern of global climates. Although not identical, weather and climate have much in common. Weather is constantly changing, sometimes from hour to hour and at other times from day to day. It is a term that refers to the state of the atmosphere at a given time and place. Whereas changes in the weather are continuous and sometimes seemingly erratic, it is nevertheless possible to arrive at a generalization of these variations. Such a description of aggregate weather conditions is termed climate. It is based on observations that have been accumulated over many decades. Climate is often defined simply as “average weather,” but this is an inadequate definition. In order to accurately portray the character of an area, variations and extremes must also be included, as well as the probabilities that such departures will take place. For example, it is necessary for farmers to know the average rainfall during the growing season, and it is also important to know the frequency of extremely wet and extremely dry years. Thus, climate is the sum of all statistical weather information that helps describe a place or region.
Chapter 1 Introduction to the Atmosphere
(a)
5
(b)
Figure 1–2 These examples remind us that people influence the atmosphere and its behavior. (a) Motor vehicles are a significant contributor to air pollution. This traffic jam was in Kuala Lumpur, Malaysia. (Photo by Ron Yue/Alamy) (b) Smoke bellows from a coal-fired electricity-generating plant in New Delhi, India, in June 2008. (AP Photo/Gurindes Osan)
Maps similar to the one in Figure 1–3 are familiar to everyone who checks the weather report in the morning newspaper or on a television station. In addition to showing predicted high temperatures for the day, this map shows other basic weather information about cloud cover, precipitation, and fronts. Suppose you were planning a vacation trip to an unfamiliar place. You would probably want to know what kind of weather to expect. Such information would help as you selected clothes to pack and could influence decisions regarding activities you might engage in during your stay. Unfortunately, weather forecasts that go beyond a few days are not very dependable. Thus, it would not be possible to get a reliable weather report about the conditions you are likely to encounter during your vacation. Instead, you might ask someone who is familiar with the area about what kind of weather to expect. “Are
thunderstorms common?” “Does it get cold at night?” “Are the afternoons sunny?” What you are seeking is information about the climate, the conditions that are typical for that
Students Sometimes Ask … Does meteorology have anything to do with meteors? Yes, there is a connection. Most people use the word meteor when referring to solid particles (meteoroids) that enter Earth’s atmosphere from space and “burn up” due to friction (“shooting stars”). The term meteorology was coined in 340BC, when the Greek philosopher Aristotle wrote a book titled Meteorlogica, which included explanations of atmospheric and astronomical phenomena. In Aristotle’s day anything that fell from or was seen in the sky was called a meteor. Today we distinguish between particles of ice or water in the atmosphere (called hydrometeors) and extraterrestrial objects called meteoroids, or meteors.
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The Atmosphere: An Introduction to Meteorology
cannot predict the weather. Although the place may usually (climatically) be warm, sunny, and dry during the 30s time of your planned vacation, you –0s 50s 20s may actually experience cool, over0s cast, and rainy weather. There is a –10s well-known saying that summarizes 20s this idea: “Climate is what you expect, 10s 50s 0s but weather is what you get.” 40s 10s 0s The nature of both weather and cli–0s 60s 20s mate is expressed in terms of the same 30s basic elements—those quantities or 50s properties that are measured regu70s 40s 10s larly. The most important are (1) the 40s 20s temperature of the air, (2) the humid80s ity of the air, (3)the type and amount 30s of cloudiness, (4) the type and amount of precipitation, (5) the pressure exert50s ed by the air, and (6) the speed and direction of the wind. These elements constitute the variables by which Rain T-storm Snow Ice Mostly sunny Partly cloudy Mostly cloudy weather patterns and climate types are depicted. Although you will study Figure 1–3 A typical newspaper weather map for a day in late December. The color these elements separately at first, keep bands show the high temperatures forecast for the day. in mind that they are very much interrelated. A change in one of the elements often produces changes in the others. place. Another useful source of such information is the great variety of climate tables, maps, and graphs that are available. For example, the map in Figure 1–4 shows the average percentage of possible sunshine in the United States for the month of November, and the graph in Figure 1–5 shows Concept Check 1.1 average daily high and low temperatures for each month, as well as extremes, for New York City. 1 Distinguish among meteorology, weather, and climate. Such information could, no doubt, help as you planned 2 List the basic elements of weather and climate. your trip. But it is important to realize that climate data
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Figure 1–4 Mean percentage of possible sunshine for November. Southern Arizona is clearly the sunniest area. By contrast, parts of the Pacific Northwest receive a much smaller percentage of the possible sunshine. Climate maps such as this one are based on many years of data.
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by all other weather events combined. Moreover, although severe storms and floods usually generate more attention, droughts can be just as devastating and carry an even bigger price tag. Between 1980 and 2010 the United States experienced 99 weather-related disasters in which overall damages and costs reached or exceeded $1 billion (Figure 1–7). The combined costs of these events exceeded $725 billion (normalized to 2007 dollars)! During the decade 1999–2008, an average of 629 direct weather fatalities occurred per year in the United States. During this span, the annual economic impacts of adverse weather on the national highway system alone exceeded $40 billion, and weather-related air traffic delays caused $4.2 billion in annual losses. At appropriate places throughout this book, you will have an opportunity to learn about atmospheric hazards. Two entire chapters (Chapter 10 and Chapter 11) focus
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Atmospheric Hazards: Assault by the Elements Natural hazards are a part of living on Earth. Every day they adversely affect literally millions of people worldwide and are responsible for staggering damages. Some, such as earthquakes and volcanic eruptions, are geological. Many others are related to the atmosphere. Occurrences of severe weather are far more fascinating than ordinary weather phenomena. A spectacular lightning display generated by a severe thunderstorm can elicit both awe and fear (Figure 1–6a). Of course, hurricanes and tornadoes attract a great deal of much-deserved attention. A single tornado outbreak or hurricane can cause billions of dollars in property damage, much human suffering, and many deaths. Of course, other atmospheric hazards adversely affect us. Some are storm related, such as blizzards, hail, and freezing rain. Others are not direct results of storms. Heat waves, cold waves, fog, wildfires, and drought are important examples (Figure 1–6b). In some years the loss of human life due to excessive heat or bitter cold exceeds that caused
This is a scene on a day in late April in southern Arizona’s Organ Pipe Cactus National Monument. (Photo by Michael Collier) Question 1 Write two brief statements about the locale in this image— one that relates to weather and one that relates to climate.
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The Atmosphere: An Introduction to Meteorology
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almost entirely on hazardous weather. In addition, a number of the book’s special-interest boxes are devoted to a broad variety of severe and hazardous weather, including heat waves, winter storms, floods, dust storms, drought, mudflows, and lightning. Every day our planet experiences an incredible assault by the atmosphere, so it is important to develop an awareness and understanding of these significant weather events.
Concept Check 1.2 1 List at least five storm-related atmospheric hazards. 2 What are three atmospheric hazards that are not directly storm related?
Figure 1–6 (a) Many people have incorrect perceptions of weather dangers and are unaware of the relative differences of weather threats to human life. For example, they are awed by the threat of hurricanes and tornadoes and plan accordingly on how to respond (for example, “Tornado Awareness Week” each spring) but fail to realize that lightning and winter storms can be greater threats. (Photo by Mark Newman/Superstock) (b) During the summer, dry weather coupled with lightning and strong winds contribute to wildfire danger. Millions of acres are burned each year, especially in the West. The loss of anchoring vegetation sets the stage for accelerated erosion when heavy rains subsequently occur. Near Boulder, Colorado, October 10, 2010. (AP Photo/ The Daily Camera, Paul Aiken)
The Nature of Scientific Inquiry As members of a modern society, we are constantly reminded of the benefits derived from science. But what exactly is the nature of scientific inquiry? Developing an understanding of how science is done and how scientists work is an important theme in this book. You will explore the difficulties of gathering data and some of the ingenious methods that have been developed to overcome these difficulties. You will also
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Once facts have been gathered and principles have been formulated to describe a natural 8 192 phenomenon, investigators try to explain how or why things happen in the manner 7 168 observed. They often do this by constructing a tentative (or untested) explanation, 6 144 which is called a scientific hypothesis. It is 5 best if an investigator can formulate more 120 than one hypothesis to explain a given set of 4 observations. If an individual scientist is un96 able to devise multiple hypotheses, others in 3 72 the scientific community will almost always develop alternative explanations. A spirited 2 48 debate frequently ensues. As a result, extensive research is conducted by proponents 1 24 of opposing hypotheses, and the results are made available to the wider scientific com0 0 munity in scientific journals. Before a hypothesis can become an accepted part of scientific knowledge, it Year must pass objective testing and analysis. If a hypothesis cannot be tested, it is not Figure 1–7 Between 1980 and 2010 the United States experienced 99 weatherscientifically useful, no matter how inrelated disasters in which overall damages and costs reached or exceeded $1 billion. teresting it might seem. The verification This bar graph shows the number of events that occurred each year and the damage process requires that predictions be made amounts in billions of dollars (normalized to 2007 dollars). The total losses for the 99 events exceeded $725 billion! For more about these extraordinary events see based on the hypothesis being considered www.ncdc.noaa.gov/oa/reports/billionz.html. (After NOAA) and the predictions be tested by being compared against objective observations of nature. Put another way, hypotheses must fit observasee examples of how hypotheses are formulated and tested, tions other than those used to formulate them in the first as well as learn about the development of some significant place. Hypotheses that fail rigorous testing are ultimately scientific theories. discarded. The history of science is littered with discarded All science is based on the assumption that the natuhypotheses. One of the best known is the Earth-centered ral world behaves in a consistent and predictable manner model of the universe—a proposal that was supported by that is comprehensible through careful, systematic study. the apparent daily motion of the Sun, Moon, and stars The overall goal of science is to discover the underlyaround Earth. ing patterns in nature and then to use this knowledge to make predictions about what should or should not be expected, given certain facts or circumstances. For Theory example, by understanding the processes and condiWhen a hypothesis has survived extensive scrutiny and tions that produce certain cloud types, meteorologists are when competing ones have been eliminated, a hypothesis often able to predict the approximate time and place of may be elevated to the status of a scientific theory. In evtheir formation. eryday language we may say, “That’s only a theory.” But a The development of new scientific knowledge involves scientific theory is a well-tested and widely accepted view some basic logical processes that are universally accepted. that the scientific community agrees best explains certain To determine what is occurring in the natural world, sciobservable facts. entists collect scientific facts through observation and Some theories that are extensively documented and measurement. The types of facts that are collected often extremely well supported are comprehensive in scope. An seek to answer a well-defined question about the natural example from the Earth sciences is the theory of plate tecworld, such as “Why does fog frequently develop in this tonics, which provides the framework for understanding place?” or “What causes rain to form in this cloud type?” the origin of mountains, earthquakes, and volcanic activity. Because some error is inevitable, the accuracy of a particuIt also explains the evolution of continents and ocean basins lar measurement or observation is always open to question. through time. As you will see in Chapter 14, this theory Nevertheless, these data are essential to science and serve also helps us understand some important aspects of climate as a springboard for the development of scientific theories change through long spans of geologic time. (Box 1–1).
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The Atmosphere: An Introduction to Meteorology
Box 1–1 Monitoring Earth from Space Scientific facts are gathered in many ways, including through laboratory experiments and field observations and measurements. Satellites provide another very important source of data. Satellite images give us perspectives that are difficult to gain from more traditional sources (Figure 1–A). Moreover, the high-tech instruments aboard many satellites enable scientists to gather information from remote regions where data are otherwise scarce. The image in Figure 1–B is from NASA’s Tropical Rainfall Measuring Mission (TRMM). TRMM is a research satellite designed to expand our understanding of Earth’s water (hydrologic) cycle and its role in our climate system. By covering the region between the latitudes 35° north and 35° south, it provides much-needed data on rainfall and the heat release associated with rainfall. Many types of measurements and images are possible. Instruments aboard the TRMM satellite have greatly expanded our ability to collect precipitation data. In addition to recording data for land areas, this satellite provides extremely precise measurements of rainfall over the oceans where conventional land-based instruments cannot see. This is especially important because much of Earth’s rain falls in ocean-covered tropical areas, and a great deal of the globe’s weather-producing energy comes from heat exchanges involved in the rainfall process. Until the TRMM, information on the intensity and amount of rainfall over the tropics was scanty. Such data are crucial to understanding and predicting global climate change.
FIGURE 1–A Satellite image of a massive winter storm on February 1, 2011. During a winter marked by several crippling storms, this one stands out. Heavy snow, ice, freezing rain, and frigid winds battered nearly two-thirds of the contiguous United States. In this image, the storm measures about 2000 kilometers (1240 miles) across. Satellites allow us to monitor the development and movement of major weather systems. (NASA)
FIGURE 1–B This map of rainfall for December 7–13, 2004, in Malaysia was constructed using TRMM data. Over 800 millimeters (32 inches) of rain fell along the east coast of the peninsula (darkest red area). The extraordinary rains caused extensive flooding and triggered many mudflows. (NASA/TRMM image)
Scientific Methods The processes just described, in which scientists gather facts through observations and formulate scientific hypotheses and theories, is called the scientific method. Contrary to popular belief, the scientific method is not a standard recipe that scientists apply in a routine manner to unravel the secrets of our natural world. Rather, it is an endeavor that involves creativity and insight. Rutherford and Ahlgren put it this
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way: “Inventing hypotheses or theories to imagine how the world works and then figuring out how they can be put to the test of reality is as creative as writing poetry, composing music, or designing skyscrapers.”*
*F. James Rutherford and Andrew Ahlgren, Science for All Americans (New York: Oxford University Press, 1990), p. 7.
Chapter 1 Introduction to the Atmosphere
There is not a fixed path that scientists always follow that leads unerringly to scientific knowledge. Nevertheless, many scientific investigations involve the following steps: (1) A question is raised about the natural world; (2) scientific data are collected that relate to the question (Figure 1–8); (3) questions are posed that relate to the data, and one or more working hypotheses are developed that may answer these questions; (4) observations and experiments are developed to test the hypotheses; (5) the hypotheses are accepted, modified, or rejected, based on extensive testing; (6) data and results are shared with the scientific community for critical and further testing. Other scientific discoveries may result from purely theoretical ideas that stand up to extensive examination. Some researchers use high-speed computers to simulate what is happening in the “real” world. These models are useful when dealing with natural processes that occur on very long time scales or take place in extreme or inaccessible locations. Still other scientific advancements have been made when a totally unexpected happening occurred during an experiment. These serendipitous discoveries are more than pure luck; as Louis Pasteur stated, “In the field of observation, chance favors only the prepared mind.” Scientific knowledge is acquired through several avenues, so it might be best to describe the nature of scientific
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Students Sometimes Ask… In class you compared a hypothesis to a theory. How is each one different from a scientific law? A scientific law is a basic principle that describes a particular behavior of nature that is generally narrow in scope and can be stated briefly—often as a simple mathematical equation. Because scientific laws have been shown time and time again to be consistent with observations and measurements, they are rarely discarded. Laws may, however, require modifications to fit new findings. For example, Newton’s laws of motion are still useful for everyday applications (NASA uses them to calculate satellite trajectories), but they do not work at velocities approaching the speed of light. For these circumstances, they have been supplanted by Einstein’s theory of relativity.
inquiry as the methods of science rather than the scientific method. In addition, it should always be remembered that even the most compelling scientific theories are still simplified explanations of the natural world.
Concept Check 1.3 1 How is a scientific hypothesis different from a scientific theory?
2 List the basic steps followed in many scientific investigations.
Figure 1–8 Gathering data and making careful observations are a basic part of scientific inquiry. (a) This Automated Surface Observing System (ASOS) installation is one of nearly 900 in use for data gathering as part of the U.S. primary surface observing network. (Photo by Bobbé Christopherson) (b) These scientists are working with a sediment core recovered from the ocean floor. Such cores often contain useful data about Earth’s climate history. (Photo by Science Source/Photo Researchers, Inc.)
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The Atmosphere: An Introduction to Meteorology
Students Sometimes Ask… Who provides all the data needed to prepare a weather forecast? Data from every part of the globe are needed to produce accurate weather forecasts. The World Meteorological Organization (WMO) was established by the United Nations to coordinate scientific activity related to weather and climate. It consists of 187 member states and territories, representing all parts of the globe. Its World Weather Watch provides up-to-the-minute standardized observations through member-operated observation systems. This global system involves more than 15 satellites, 10,000 landobservation and 7300 ship stations, hundreds of automated data buoys, and thousands of aircraft.
Earth’s Spheres The images in Figure 1–9 are considered to be classics because they let humanity see Earth differently than ever before. Figure 1–9a, known as “Earthrise,” was taken when the Apollo 8 astronauts orbited the Moon for the first time in December 1968. As the spacecraft rounded the Moon, Earth appeared to rise above the lunar surface. Figure 1–9b, referred to as “The Blue Marble,” is perhaps the most widely reproduced image of Earth; it was taken in December 1972 by the crew of Apollo 17 during the last manned lunar mission. These early views profoundly altered our conceptualizations of Earth and remain powerful images decades after they were first viewed. Seen from space, Earth is breathtaking in its beauty and startling in its solitude. The photos remind us that our home is, after all, a planet—small, selfcontained, and in some ways even fragile. Bill Anders, the
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Apollo 8 astronaut who took the “Earthrise” photo, expressed it this way: “We came all this way to explore the Moon, and the most important thing is that we discovered the Earth.” As we look closely at our planet from space, it becomes apparent that Earth is much more than rock and soil. In fact, the most conspicuous features in Figure 1–9a are not continents but swirling clouds suspended above the surface of the vast global ocean. These features emphasize the importance of water on our planet. The closer view of Earth from space shown in Figure 1–9b helps us appreciate why the physical environment is traditionally divided into three major parts: the solid Earth, the water portion of our planet, and Earth’s gaseous envelope. It should be emphasized that our environment is highly integrated and is not dominated by rock, water, or air alone. It is instead characterized by continuous interactions as air comes in contact with rock, rock with water, and water with air. Moreover, the biosphere, the totality of life forms on our planet, extends into each of the three physical realms and is an equally integral part of the planet. The interactions among Earth’s four spheres are incalculable. Figure 1–10 provides us with one easy-to-visualize example. The shoreline is an obvious meeting place for rock, water, and air. In this scene, ocean waves that were created by the drag of air moving across the water are breaking against the rocky shore. The force of the water can be powerful, and the erosional work that is accomplished can be great. On a human scale Earth is huge. Its surface area occupies 500,000,000 square kilometers (193 million square miles). We divide this vast planet into four independent parts. Because each part loosely occupies a shell around Earth, we call them spheres. The four spheres include the geosphere (solid Earth), the atmosphere (gaseous envelope), the hydrosphere (water portion), and the biosphere (life).
Figure 1–9 (a) View, called “Earthrise,” that greeted the Apollo 8 astronauts as their spacecraft emerged from behind the Moon. (NASA) (b) Africa and Arabia are prominent in this classic image called “The Blue Marble” taken from Apollo 17. The tan cloud-free zones over the land coincide with major desert regions. The band of clouds across central Africa is associated with a much wetter climate that in places sustains tropical rain forests. The dark blue of the oceans and the swirling cloud patterns remind us of the importance of the oceans and the atmosphere. Antarctica, a continent covered by glacial ice, is visible at the South Pole. (NASA)
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Figure 1–10 The shoreline is one obvious example of an interface—a common boundary where different parts of a system interact. In this scene, ocean waves (hydrosphere) that were created by the force of moving air (atmosphere) break against a rocky shore (geosphere). (Photo by Radius Images/ photolibrary.com)
It is important to remember that these spheres are not separated by well-defined boundaries; rather, each sphere is intertwined with all of the others. In addition, each of Earth’s four major spheres can be thought of as being composed of numerous interrelated parts.
The Geosphere Beneath the atmosphere and the ocean is the solid Earth, or geosphere. The geosphere extends from the surface to the center of the planet, a depth of about 6400 kilometers (nearly 4000 miles), making it by far the largest of Earth’s four spheres. Based on compositional differences, the geosphere is divided into three principal regions: the dense inner sphere, called the core; the less dense mantle; and the crust, which is the light and very thin outer skin of Earth. Soil, the thin veneer of material at Earth’s surface that supports the growth of plants, may be thought of as part of all four spheres. The solid portion is a mixture of weathered rock debris (geosphere) and organic matter from decayed plant and animal life (biosphere). The decomposed and disintegrated rock debris is the product of weathering processes that require air (atmosphere) and water (hydrosphere). Air and water also occupy the open spaces between the solid particles.
The Atmosphere Earth is surrounded by a life-giving gaseous envelope called the atmosphere (Figure 1–11). When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance. However, when compared to the thickness (radius) of the solid Earth (about 6400 kilometers [4000 miles]), the atmosphere is a very shallow layer. More than 99 percent of the atmosphere is within 30 kilometers (20 miles) of Earth’s surface. This thin blanket of air is nevertheless an integral part of the planet. It not only provides the air that we breathe but also acts to protect us from the dangerous radiation emitted by the Sun. The energy exchanges that continually occur between the atmosphere and Earth’s surface and between the atmosphere and space produce the effects we call weather. If, like the Moon, Earth had no atmosphere, our planet would not only be lifeless, but many of the processes and interactions that make the surface such a dynamic place could not operate.
The Hydrosphere Earth is sometimes called the blue planet. More than anything else, water makes Earth unique. The hydrosphere is a dynamic mass that is continually on the move, evaporating from the oceans to the atmosphere, precipitating to
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The Atmosphere: An Introduction to Meteorology Figure 1–11 This unique image of Earth’s atmosphere merging with the emptiness of space resembles an abstract painting. It was taken in June 2007 by a Space Shuttle crew member. The silvery streaks (callednoctilucent clouds) high in the blue area are at a height of about 80kilometers (50 miles). Air pressure at this height is less than one-thousandth of that at sea level. The reddish zone in the lower portion of the image is the densest part of the atmosphere. It is here, in a layer called the troposphere, that practically all weather phenomena occur. Ninety percent of Earth’s atmosphere occurs within just 16kilometers (10 miles) of the surface. (NASA)
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the land, and running back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth’s surface to an average depth of about 3800meters (12,500 feet). It accounts for about 97 percent of Earth’s water (Figure 1–12). However, the hydrosphere also includes the fresh water found in clouds, streams, lakes, and glaciers, as well as that found underground. Although these latter sources constitute just a tiny fraction of the total, they are much more important than their meager percentage indicates. Clouds, of course, play a vital role in many weather and climate processes. In addition to providing the fresh water that is so vital to life on land, streams, glaciers, and Oceans 97.2% groundwater are responsible for sculpting and creating many of our planet’s varied landforms.
(2.5 miles) and in boiling hot springs. Moreover, air currents can carry microorganisms many kilometers into the atmosphere. But even when we consider these extremes, life still must be thought of as being confined to a narrow band very near Earth’s surface. Plants and animals depend on the physical environment for the basics of life. However, organisms do more than just respond to their physical environment. Through
The Biosphere The biosphere includes all life on Earth (Figure 1–13). Ocean life is concentrated in the sunlit surface waters of the sea. Most life on land is also concentrated near the surface, with tree roots and burrowing animals reaching a few meters underground and flying insects and birds reaching a kilometer or so above the surface. A surprising variety of life forms are also adapted to extreme environments. For example, on the ocean floor, where pressures are extreme and no light penetrates, there are places where vents spew hot, mineral-rich fluids that support communities of exotic life-forms. On land, some bacteria thrive in rocks as deep as 4 kilometers
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Figure 1–12 Distribution of Earth’s water. Obviously, most of Earth’s water is in the oceans. Glacial ice represents about 85percent of all the water outside the oceans. When only liquid freshwater is considered, more than 90 percent is groundwater. (Glacier photo by Bernhard Edmaier/Photo Researchers, Inc.; stream photo by E.J. Tarbuck; and groundwater photo by Michael Collier)
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countless interactions, life forms help maintain and alter their physical environment. Without life, the makeup and nature of the geosphere, hydrosphere, and atmosphere would be very different.
Concept Check 1.4 1 Compare the height of the atmosphere to the thickness of the geosphere.
2 How much of Earth’s surface do oceans cover? 3 How much of the planet’s total water supply do the oceans represent?
4 List and briefly define the four spheres that constitute our environment.
Earth as a System
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Anyone who studies Earth soon learns that our planet is a dynamic body with many separate but highly interactive parts, or spheres. The atmosphere, hydrosphere, biosphere, and geosphere and all of their components can be studied separately. However, the parts are not isolated. Each is related in many ways to the others, producing a complex and continuously interacting whole that we call the Earth system.
Earth System Science
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Figure 1–13 (a) The ocean contains a significant portion of Earth’s biosphere. Modern coral reefs are unique and complex examples and are home to about 25 percent of all marine species.Because of this diversity, they are sometimes referred toas the ocean equivalent of tropical rain forests. (Photo by DarrylLeniuk/agefotostock) (b) Tropical rain forests are characterized by hundreds of different species per square kilometer. Climate has a strong influence on the nature of the biosphere. Life, in turn,influences the atmosphere. (Photo by agefotostock/SuperStock)
A simple example of the interactions among different parts of the Earth system occurs every winter as moisture evaporates from the Pacific Ocean and subsequently falls as rain in the hills of southern California, triggering destructive debris flows (Figure 1–14). The processes that move water from the hydrosphere to the atmosphere and then to the geosphere have a profound impact on the physical environment and on the plants and animals (including humans) that inhabit the affected regions. Scientists have recognized that in order to more fully understand our planet, they must learn how its individual components (land, water, air, and life-forms) are interconnected. This endeavor, called Earth system science, aims to study Earth as a system composed of numerous interacting parts, or subsystems. Using an interdisciplinary approach, those who practice Earth system science attempt to achieve the level of understanding necessary to comprehend and solve many of our global environmental problems. A system is a group of interacting, or interdependent, parts that form a complex whole. Most of us hear and use the term system frequently. We may service our car’s cooling system, make use of the city’s transportation system,
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The Atmosphere: An Introduction to Meteorology
Figure 1–14 This image provides an example of interactions among different parts of the Earth system. On January 10, 2005, extraordinary rains triggered this debris flow (popularly called a mudslide) in the coastal community of La Conchita, California. (AP Wideworld Photo)
and be a participant in the political system. A news report might inform us of an approaching weather system. Further, we know that Earth is just a small part of a larger system known as the solar system, which in turn is a subsystem of an even larger system called the Milky Way Galaxy.
The Earth System The Earth system has a nearly endless array of subsystems in which matter is recycled over and over again. One example that is described in Box 1–2 traces the movements of carbon among Earth’s four spheres. It shows us, for example, that
the carbon dioxide in the air and the carbon in living things and in certain rocks is all part of a subsystem described by the carbon cycle. The parts of the Earth system are linked so that a change in one part can produce changes in any or all of the other parts. For example, when a volcano erupts, lava from Earth’s interior may flow out at the surface and block a nearby valley. This new obstruction influences the region’s drainage system by creating a lake or causing streams to change course. The large quantities of volcanic ash and gases that can be emitted during an eruption might be blown high into the atmosphere and influence the amount of solar energy that can reach Earth’s surface. The result could be a drop in air temperatures over the entire hemisphere. Where the surface is covered by lava flows or a thick layer of volcanic ash, existing soils are buried. This causes the soil-forming processes to begin anew to transform the new surface material into soil (Figure 1–15). The soil that eventually forms will reflect the interactions among many parts of the Earth system—the volcanic parent material, the climate, and the impact of biological activity. Of course, there would also be significant changes in the biosphere. Some organisms and their habitats would be eliminated by the lava and ash, whereas new settings for life, such as the lake, would be created. The potential climate change could also impact sensitive life-forms. The Earth system is characterized by processes that vary on spatial scales from fractions of millimeters to thousands of kilometers. Time scales for Earth’s processes range from milliseconds to billions of years. As we learn about Earth, it becomes increasingly clear that despite significant separations in distance or time, many processes are connected, and a change in one component can influence the entire system. The Earth system is powered by energy from two sources. The Sun drives external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface. Weather and climate, ocean circulation, and erosional processes are driven by energy from the Sun. Earth’s interior is the second source of energy. Heat remaining from when our planet formed and heat that is continuously generated by radioactive decay power the internal processes that produce volcanoes, earthquakes, and mountains. Humans are part of the Earth system, a system in which the living and nonliving components are entwined and interconnected. Therefore, our actions produce changes in all the other parts. When we burn gasoline and coal, dispose of our wastes, and clear the land, we cause other parts of the system to respond, often in unforeseen ways. Throughout this book you will learn about some of Earth’s subsystems, including the hydrologic system and the climate system. Remember that these components and we humans are all part of the complex interacting whole we call the Earth system.
Concept Check 1.5 1 What is a system? List three examples. 2 What are the two sources of energy for the Earth system?
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Figure 1–15 When Mount St. Helens erupted in May 1980, the area shown here was buried by a volcanic mudflow. Now, plants are reestablished and new soil is forming. (Photo by Terry Donnelly/ Alamy)
Composition of the Atmosphere ATMOSPHERE
Introduction to the Atmosphere ▸ Composition of the Atmosphere
In the days of Aristotle, air was thought to be one of four fundamental substances that could not be further divided into constituent components. The other three substances were fire, earth (soil), and water. Even today the term air is sometimes used as if it were a specific gas, which of course it is not. The envelope of air that surrounds our planet is a mixture of many discrete gases, each with its own physical properties, in which varying quantities of tiny solid and liquid particles are suspended.
Major Components The composition of air is not constant; it varies from time to time and from place to place (see Box1–3). If the water vapor, dust, and other variable components were removed
from the atmosphere, we would find that its makeup is very stable up to an altitude of about 80 kilometers (50 miles). As you can see in Figure 1–16, two gases—nitrogen and oxygen—make up 99 percent of the volume of clean, dry air. Although these gases are the most plentiful components of the atmosphere and are of great significance to life on Earth, they are of little or no importance in affecting weather phenomena. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of a number of other gases.
Carbon Dioxide Carbon dioxide, although present in only minute amounts (0.0391 percent, or 391 parts per million), is nevertheless a meteorologically important constituent of air. Carbon dioxide is of great interest to meteorologists because it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Although the proportion of carbon dioxide in the atmosphere is relatively
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The Atmosphere: An Introduction to Meteorology
Box 1–2
The Carbon Cycle: One of Earth’s Subsystems
To illustrate the movement of material and energy in the Earth system, let us take a brief look at the carbon cycle (Figure 1–C). Pure carbon is relatively rare in nature. It is found predominantly in two minerals: diamond and graphite. Most carbon is bonded chemically to other elements to form compounds such as carbon dioxide, calcium carbonate, and the hydrocarbons found in coal and petroleum. Carbon is also the basic building block of life as it readily combines with hydrogen and oxygen to form the fundamental organic compounds that compose living things. In the atmosphere, carbon is found mainly as carbon dioxide (CO2). Atmospheric carbon dioxide is significant because it is a greenhouse gas, which means it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Because many of the processes that operate on Earth involve carbon dioxide, this gas is constantly moving into and out of the atmosphere. For example, through the process of photosynthesis, plants absorb carbon dioxide from the atmosphere to produce the essential organic compounds needed for growth. Animals that consume these plants (or consume other animals that eat plants) use these organic compounds as a source of energy and, through the process
Burning and decay of biomass
Carbon dioxide (0.0391% or 391 ppm)
All others
Weathering of granite Respiration by land organisms
Burning of fossil fuels
Burial of biomass
Photosynthesis and respiration of marine organisms
Deposition of carbonate sediments
CO2 dissolves in seawater
Lithosphere
Sediment and sedimentary rock
CO2 entering the atmosphere CO2 leaving the atmosphere
FIGURE 1–C Simplified diagram of the carbon cycle, with emphasis on the flow of carbon between the atmosphere and the hydrosphere, geosphere, and biosphere. The colored arrows show whether the flow of carbon is into or out of the atmosphere.
of respiration, return carbon dioxide to the atmosphere. (Plants also return some CO2 to the atmosphere via respiration.) Further,
Concentration in parts per million (ppm)
Argon (0.934%)
Weathering of carbonate Photosynthesis rock by vegetation
Volcanic activity
Neon (Ne) Helium (He) Methane (CH4) Krypton (Kr) Hydrogen (H2)
18.2 5.24 1.5 1.14 0.5
Oxygen (20.946%)
Nitrogen (78.084%)
Figure 1–16 Proportional volume of gases composing dry air. Nitrogen and oxygen obviously dominate.
when plants die and decay or are burned, this biomass is oxidized, and carbon dioxide is returned to the atmosphere.
uniform, its percentage has been rising steadily for more than a century. Figure 1–17 is a graph showing the growth in atmospheric CO2 since 1958. Much of this rise is attributed to the burning of ever-increasing quantities of fossil fuels, such as coal and oil. Some of this additional carbon dioxide is absorbed by the waters of the ocean or is used by plants, but more than 40 percent remains in the air. Estimates project that by sometime in the second half of the twentyfirst century, carbon dioxide levels will be twice as high as pre-industrial levels. Most atmospheric scientists agree that increased carbon dioxide concentrations have contributed to a warming of Earth’s atmosphere over the past several decades and will continue to do so in the decades to come. The magnitude of such temperature changes is uncertain and depends partly on the quantities of CO2 contributed by human activities in the years ahead. The role of carbon dioxide in the atmosphere and its possible effects on climate are examined in more detail in Chapters 2 and 14.
Chapter 1 Introduction to the Atmosphere
Not all dead plant material decays immediately back to carbon dioxide. A small percentage is deposited as sediment. Over long spans of geologic time, considerable biomass is buried with sediment. Under the right conditions, some of these carbon-rich deposits are converted to fossil fuels—coal, petroleum, or natural gas. Eventually some of the fuels are recovered (mined or pumped from a well) and burned to run factories and fuel our transportation system. One result of fossil-fuel combustion is the release of huge quantities of CO2 into the atmosphere. Certainly one of the most active parts of the carbon cycle is the movement of CO2 from the atmosphere to the biosphere and back again. Carbon also moves from the geosphere and hydrosphere to the atmosphere and back again. For example, volcanic activity early in Earth’s history is thought to be the source of much of the carbon dioxide found in the atmosphere. One way that carbon dioxide makes its way back to the hydrosphere and then to the solid Earth is by first combining with water to form carbonic acid (H2CO3), which then attacks the rocks that compose the geosphere. One product of this chemical weathering of solid rock is the soluble bicarbonate ion (2HCO3–), which is carried by groundwater and streams to the ocean. Here water-dwelling organisms extract this dissolved material to produce hard parts (shells) of calcium carbonate (CaCO3). When the organisms die, these skeletal
remains settle to the ocean floor as biochemical sediment and become sedimentary rock. In fact, the geosphere is by far Earth’s largest depository of carbon, where it is a constituent of a variety of rocks, the most abundant being limestone (Figure 1–D). Eventually the limestone may be exposed at Earth’s surface, where chemical weathering will cause the carbon stored in the rock to be released to the atmosphere as CO2.
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In summary, carbon moves among all four of Earth’s major spheres. It is essential to every living thing in the biosphere. In the atmosphere carbon dioxide is an important greenhouse gas. In the hydrosphere, carbondioxide is dissolved in lakes, rivers, and the ocean. In the geosphere, carbon is contained in carbonate-rich sediments and sedimentary rocks and is stored as organic matter dispersed through sedimentary rocks and as deposits of coal and petroleum.
FIGURE 1–D A great deal of carbon is locked up in Earth’s geosphere. England’s White Chalk Cliffs are an example. Chalk is a soft, porous type of limestone (CaCO3) consisting mainly of the hard parts of microscopic organisms called coccoliths (inset). (Photo by Prisma/SuperStock; inset by Steve Gschmeissner/Photo Researchers, Inc.)
Variable Components Air includes many gases and particles that vary significantly from time to time and place to place. Important examples include water vapor, aerosols, and ozone. Although usually present in small percentages, they can have significant effects on weather and climate.
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CO2 concentration (ppm)
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Water Vapor The amount of water vapor in the air varies considerably, from practically none at all up to about 4 percent by volume. Why is such a small fraction of the
360 350 340 330 320 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
Figure 1–17 Changes in the atmosphere’s carbon dioxide (CO2) as measured at Hawaii’s Mauna Loa Observatory. The oscillations reflect the seasonal variations in plant growth and decay in the Northern Hemisphere. During the first 10 years of this record (1958–1967), the average yearly CO2 increase was 0.81 ppm. During the last 10 years (2001–2010) the average yearly increase was 2.04 ppm. (Data from NOAA)
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The Atmosphere: An Introduction to Meteorology
Box 1–3
Origin and Evolution of Earth’s Atmosphere
The air we breathe is a stable mixture of 78 percent nitrogen, 21 percent oxygen, nearly 1 percent argon, and small amounts of gases such as carbon dioxide and water vapor. However, our planet’s original atmosphere 4.6 billion years ago was substantially different. Earth’s Primitive Atmosphere Early in Earth’s formation, its atmosphere likely consisted of gases most common in the early solar system: hydrogen, helium, methane, ammonia, carbon dioxide, and water vapor. The lightest of these gases, hydrogen and helium, escaped into space because Earth’s gravity was too weak to hold them. Most of the remaining gases were probably scattered into space by strong solar winds (vast streams of particles) from a young active Sun. (All stars, including the Sun, apparently experience a highly active stage early in their evolution, during which solar winds are very intense.) Earth’s first enduring atmosphere was generated by a process called outgassing, through which gases trapped in the planet’s interior are released. Outgassing from hundreds of active volcanoes still remains an important planetary function worldwide (Figure 1–E). However, early in Earth’s history, when massive heating and fluid-like motion occurred in the planet’s interior, the
gas output must have been immense. Based on our understanding of modern volcanic eruptions, Earth’s primitive atmosphere probably consisted of mostly water vapor,
carbon dioxide, and sulfur dioxide, with minor amounts of other gases and minimal nitrogen. Most importantly, free oxygen was not present.
FIGURE 1–E Earth’s first enduring atmosphere was formed by a process called outgassing, which continues today, from hundreds of active volcanoes worldwide. (Photo by Greg Vaughn/Alamy)
atmosphere so significant? The fact that water vapor is the source of all clouds and precipitation would be enough to explain its importance. However, water vapor has other roles. Like carbon dioxide, it has the ability to absorb heat given off by Earth, as well as some solar energy. It is therefore important when we examine the heating of the atmosphere. When water changes from one state to another, such as from a gas to a liquid or a liquid to a solid (see Figure 4–3,
Students Sometimes Ask… Could you explain a little more about why the graph in Figure 1–17 has so many ups and downs? Sure. Carbon dioxide is removed from the air by photosynthesis, the process by which green plants convert sunlight into chemical energy. In spring and summer, vigorous plant growth in the extensive land areas of the Northern Hemisphere removes carbon dioxide from the atmosphere, so the graph takes a dip. As winter approaches, many plants die or shed leaves. The decay of organic matter returns carbon dioxide to the air, causing the graph to spike upward.
p. 99), it absorbs or releases heat. This energy is termed latent heat, which means hidden heat. As you will see in later chapters, water vapor in the atmosphere transports this latent heat from one region to another, and it is the energy source that drives many storms. Aerosols The movements of the atmosphere are sufficient to keep a large quantity of solid and liquid particles suspended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay in the air very long. Still, many particles are microscopic and remain suspended for considerable periods of time. They may originate from many sources, both natural and human made, and include sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, ash and dust from volcanic eruptions, and more (Figure 1–18a). Collectively, these tiny solid and liquid particles are called aerosols. Aerosols are most numerous in the lower atmosphere near their primary source, Earth’s surface. Nevertheless, the upper atmosphere is not free of them, because some dust is
Chapter 1 Introduction to the Atmosphere
Oxygen in the Atmosphere As Earth cooled, water vapor condensed to form clouds, and torrential rains began to fill low-lying areas, which became the oceans. In those oceans, nearly 3.5 billion years ago, photosynthesizing bacteria began to release oxygen into the water. During photosynthesis, organisms use the Sun’s energy to produce organic material (energetic molecules of sugar containing hydrogen and carbon) from carbon dioxide (CO2) and water (H2O). The first bacteria probably used hydrogen sulfide (H2S) as the source of hydrogen rather than water. One of the earliest bacteria, cyanobacteria (once called blue-green algae), began to produce oxygen as a by-product of photosynthesis. Initially, the newly released oxygen was readily consumed by chemical reactions with other atoms and molecules (particularly iron) in the ocean (Figure 1–F). Once the available iron satisfied its need for oxygen and as the number of oxygen-generating organisms increased, oxygen began to build in the atmosphere. Chemical analyses of rocks suggest that a significant amount of oxygen appeared in the atmosphere as early as 2.2 billion years ago and increased steadily until it reached stable levels about 1.5 billion years ago. Obviously, the availability of free oxygen had a major impact on the development of life and vice versa. Earth’s atmosphere evolved together with its life-forms from an oxygen-free envelope to an oxygen-rich environment.
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FIGURE 1–F These ancient layered, iron-rich rocks, called banded iron formations, were deposited during a geologic span known as the Precambrian. Much of the oxygen generated as a by-product of photosynthesis was readily consumed by chemical reactions with iron to produce these rocks. (Photo by John Cancalosi/Photolibrary)
Another significant benefit of the “oxygen explosion” is that oxygen molecules (O2) readily absorb ultraviolet radiation and rearrange themselves to form ozone (O3). Today, ozone is concentrated above the surface in a layer called the stratosphere, where it absorbs much of the ultraviolet radiation that strikes the upper atmosphere.
carried to great heights by rising currents of air, and other particles are contributed by meteoroids that disintegrate as they pass through the atmosphere. From a meteorological standpoint, these tiny, often invisible particles can be significant. First, many act as surfaces on which water vapor may condense, an important function in the formation of clouds and fog. Second, aerosols can absorb or reflect incoming solar radiation. Thus, when an air-pollution episode is occurring or when ash fills the sky following a volcanic eruption, the amount of sunlight reaching Earth’s surface can be measurably reduced. Finally, aerosols contribute to an optical phenomenon we have all observed—the varied hues of red and orange at sunrise and sunset (Figure 1–18b). Ozone Another important component of the atmosphere is ozone. It is a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is not the same as the oxygen we breathe, which has two atoms per molecule (O2). There is very little ozone in the atmosphere. Overall, it represents just 3 out of every 10 million molecules. Moreover,
For the first time, Earth’s surface was protected from this type of solar radiation, which is particularly harmful to DNA. Marine organisms had always been shielded from ultraviolet radiation by the oceans, but the development of the atmosphere’s protective ozone layer made the continents more hospitable.
its distribution is not uniform. In the lowest portion of the atmosphere, ozone represents less than 1 part in 100 million. It is concentrated well above the surface in a layer called the stratosphere, between 10 and 50 kilometers (6 and 31 miles). In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet radiation emitted by the Sun. Ozone is then created when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This must happen in the presence of a third, neutral molecule that acts as a catalyst by allowing the reaction to take place without itself being consumed in the process. Ozone is concentrated in the 10- to 50-kilometer height range because a crucial balance exists there: The ultraviolet radiation from the Sun is sufficient to produce single atoms of oxygen, and there are enough gas molecules to bring about the required collisions. The presence of the ozone layer in our atmosphere is crucial to those of us who are land dwellers. The reason is that ozone absorbs the potentially harmful ultraviolet (UV) radiation from the Sun. If ozone did not filter a great deal of the ultraviolet radiation, and if the Sun’s UV rays reached
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The Atmosphere: An Introduction to Meteorology
Dust storm
Air pollution
(a)
(b)
Figure 1–18 (a) This satellite image from November 11, 2002, shows two examples of aerosols. First, a large dust storm is blowing across northeastern China toward the Korean Peninsula. Second, a dense haze toward the south (bottom center) is human-generated air pollution. (b) Dust in the air can cause sunsets to be especially colorful. (Satellite image courtesy of NASA; photo by elwynn/ Shutterstock)
the surface of Earth undiminished, land areas on our planet would be uninhabitable for most life as we know it. Thus, anything that reduces the amount of ozone in the atmosphere could affect the well-being of life on Earth. Just such a problem is described in the next section.
Concept Check 1.6 1 Is air a specific gas? Explain. 2 What are the two major components of clean, dry air? What proportion does each represent?
3 Why are water vapor and aerosols important constituents of Earth’s atmosphere?
4 What is ozone? Why is ozone important to life on Earth?
Ozone Depletion— A Global Issue The loss of ozone high in the atmosphere as a consequence of human activities is a serious global-scale environmental problem. For nearly a billion years Earth’s ozone layer has
protected life on the planet. However, over the past half century, people have unintentionally placed the ozone layer in jeopardy by polluting the atmosphere. The most significant of the offending chemicals are known as chlorofluorocarbons (CFCs). They are versatile compounds that are chemically stable, odorless, nontoxic, noncorrosive, and inexpensive to produce. Over several decades many uses were developed for CFCs, including as coolants for air-conditioning and refrigeration equipment, as cleaning solvents for electronic components, as propellants for aerosol sprays, and in the production of certain plastic foams.
Students Sometimes Ask… Isn’t ozone some sort of pollutant? Yes, you’re right. Although the naturally occurring ozone in the stratosphere is critical to life on Earth, it is regarded as a pollutant when produced at ground level because it can damage vegetation and be harmful to human health. Ozone is a major component in a noxious mixture of gases and particles called photochemical smog. It forms as a result of reactions triggered by sunlight that occur among pollutants emitted by motor vehicles and industries. Chapter13 provides more information about this.
Chapter 1 Introduction to the Atmosphere
No one worried about how CFCs might affect the atmosphere until three scientists, Paul Crutzen, F. Sherwood Rowland, and Mario Molina, studied the relationship. In 1974 they alerted the world when they reported that CFCs were probably reducing the average concentration of ozone in the stratosphere. In 1995 these scientists were awarded the Nobel Prize in chemistry for their pioneering work. They discovered that because CFCs are practically inert (that is, not chemically active) in the lower atmosphere, a portion of these gases gradually makes its way to the ozone layer, where sunlight separates the chemicals into their constituent atoms. The chlorine atoms released this way, through a complicated series of reactions, have the net effect of removing some of the ozone.
The Antarctic Ozone Hole Although ozone depletion by CFCs occurs worldwide, measurements have shown that ozone concentrations take an especially sharp drop over Antarctica during the Southern Hemisphere spring (September and October). Later, during November and December, the ozone concentration recovers to more normal levels (Figure 1–19). Between 1980, when it was discovered, and the early 2000s, this well-publicized ozone hole intensified and grew larger until it covered an area roughly the size of North America (Figure 1–20). The hole is caused in part by the relatively abundant ice particles in the south polar stratosphere. The ice boosts the effectiveness of CFCs in destroying ozone, thus causing a greater decline than would otherwise occur. The zone
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Area of North America Extent of 2006 ozone hole
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15 Extent of 2010 ozone hole
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of maximum depletion is confined to the Antarctic region by a swirling upper-level wind pattern. When this vortex weakens during the late spring, the ozone-depleted air is no longer restricted and mixes freely with air from other latitudes where ozone levels are higher. A few years after the Antarctic ozone hole was discovered, scientists detected a similar but smaller ozone thinning in the vicinity of the North Pole during spring and early summer. When this pool breaks up, parcels of ozonedepleted air move southward over North America, Europe, and Asia.
Effects of Ozone Depletion Because ozone filters out most of the damaging UV radiation in sunlight, a decrease in its concentration permits more of these harmful wavelengths to reach Earth’s surface. What are the effects of the increased ultraviolet radiation? Each 1 percent decrease in the concentration of stratospheric ozone increases the amount of UV radiation that reaches Earth’s surface by about 2 percent. Therefore, because ultraviolet radiation is known to induce skin cancer, ozone depletion seriously affects human health, especially among fair-skinned people and those who spend considerable time in the sun. The fact that up to a half million cases of these cancers occur in the United States annually means that ozone depletion could ultimately lead to many thousands more cases each year.* In addition to raising the risk of skin cancer, an increase in damaging UV radiation can negatively impact the human immune system, as well as promote cataracts, a clouding of the eye lens that reduces vision and may cause blindness if not treated. The effects of additional UV radiation on animal and plant life are also important. There is serious concern that crop yields and quality will be adversely affected. Some scientists also fear that increased UV radiation in the Antarctic will penetrate the waters surrounding the continent and impair or destroy the microscopic plants, called phytoplankton, that represent the base of the food chain. A decrease in phytoplankton, in turn, could reduce the population of copepods and krill that sustain fish, whales, penguins, and other marine life in the high latitudes of the Southern Hemisphere.
5
Montreal Protocol 0 Aug
Sep
Oct
Nov
Dec
Figure 1-19 Changes in the size of the Antarctic ozone hole during 2006 and 2010. The ozone hole in both years began to form in August and was well developed in September and October. As is typical, each year the ozone hole persisted through November and disappeared in December. At its maximum, the area of the ozone hole was about 22 million square kilometers in 2010, an area nearly as large as all of North America.
What has been done to protect the atmosphere’s ozone layer? Realizing that the risks of not curbing CFC emissions were difficult to ignore, an international agreement known as the Montreal Protocol on Substances That Deplete the Ozone Layer was concluded under the auspices of the United Nations in late 1987. The protocol established legally binding controls * For more on this, see Severe and Hazardous Weather: “The Ultraviolet Index,” p. 49.
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The Atmosphere: An Introduction to Meteorology
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Figure 1–20 The two satellite images show ozone distribution in the Southern Hemisphere on the days in September 1979 and 2010 when the ozone hole was largest. The dark blue shades over Antarctica correspond to the region with the sparsest ozone. The ozone hole is not technically a “hole” where no ozone is present but is actually a region of exceptionally depteted ozone in the stratosphere over the Antarctic that occurs in the spring. The small graph traces changes in the maximum size of the ozone hole, 1980–2010. (NOAA)
on the production and consumption of gases known to cause ozone depletion. As the scientific understanding of ozone depletion improved after 1987 and substitutes and alternatives became available for the offending chemicals, the Montreal Protocol was strengthened several times. More than 190 nations eventually ratified the treaty. The Montreal Protocol represents a positive international response to a global environment problem. As a result of the action, the total abundance of ozone-depleting gases in the atmosphere has started to decrease in recent years. According to the U.S. Environmental Protection Agency (U.S. EPA), the ozone layer has not grown thinner since 1998 over most of the world.* If the nations of the world continue to follow the provisions of the protocol, the decreases are expected to continue throughout the twenty-first century. Some offending chemicals are still increasing but will begin to decrease in coming decades. Between 2060 and 2075, the abundance of ozone-depleting gases is projected to fall to values that existed before the Antarctic ozone hole began to form in the 1980s.
Concept Check 1.7 1 What are CFCs, and what is their connection to the ozone problem?
2 During what time of year is the Antarctic ozone hole well developed?
3 Describe three effects of ozone depletion. 4 What is the Montreal Protocol?
* U.S. EPA, Achievements in Stratospheric Ozone Protection, Progress Report. EPA-430-R-07-001, April 2007, p. 5.
Vertical Structure of the Atmosphere ATMOSPHERE
Introduction to the Atmosphere ▸ Extent of the Atmosphere/Thermal Structure of the Atmosphere
To say that the atmosphere begins at Earth’s surface and extends upward is obvious. However, where does the atmosphere end and where does outer space begin? There is no sharp boundary; the atmosphere rapidly thins as you travel away from Earth, until there are too few gas molecules to detect.
Pressure Changes To understand the vertical extent of the atmosphere, let us examine the changes in atmospheric pressure with height. Atmospheric pressure is simply the weight of the air above. At sea level the average pressure is slightly more than 1000 millibars. This corresponds to a weight of slightly more than 1 kilogram per square centimeter (14.7 pounds per square inch). Obviously, the pressure at higher altitudes is less (Figure 1–21). One-half of the atmosphere lies below an altitude of 5.6kilometers (3.5 miles). At about 16 kilometers (10 miles), 90 percent of the atmosphere has been traversed, and above 100 kilometers (62 miles) only 0.00003 percent of all the gases composing the atmosphere remain. At an altitude of 100 kilometers the atmosphere is so thin that the density of air is less than could be found in the most perfect artificial vacuum at the surface. Nevertheless, the atmosphere continues to even greater heights. The truly
Chapter 1 Introduction to the Atmosphere
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Kathy Orr, Broadcast Meteorologist not the ‘rip and read’ of years gone by. We take data from the supercomputers in Washington or models by the Navy and make our own forecasts. There are some services that provide forecasts locally and nationally, but they’re not located where we are. I can look out the window and tell whether those forecasts are going to be accurate or not.” As a weathercaster, Orr has worked to promote education in science and math. For three years, she led a community program called Kidcasters. By offering children a chance to present the weather on TV, Orr hoped to interest elementary school children in science and math. For the past nine summers, she has conducted a similar program called Orr at the Shore. Each program highlights environmental issues along the New Jersey coast. KATHY ORR is an award-winning broadcast meteorologist in Philadelphia. (Photo courtesy of Kathy Orr)
Kathy Orr is a trusted and familiar face on the airwaves of Philadelphia. As chief meteorologist for CBS3, Orr has kept the City of Brotherly Love abreast of the weather for 18 years and earned 10 regional Emmy awards in the process.
Orr calls being a television weathercaster a dream come true. Orr calls being a television weathercaster a dream come true. Growing up in Syracuse, New York, Orr operated her own miniature weather station and marveled at the snow squalls that howled across Lake Ontario. “It could be a sunny afternoon, then the wind would blow over the lake. All of a sudden there was a blinding blizzard,” shesays. When not watching the skies, Orr stayed glued to her family’s TV set. At the time, she
couldn’t see how to combine her two major interests. “There weren’t any women doing the weather on television back then. There were also not a lot of meteorologists on TV; it was less about the science and more for comic relief,” she says. She majored in broadcasting at Syracuse University and went on to earn a second degree in meteorology at the State University of New York at Oswego. There she learned the basis for the snow squalls that transfixed her as a girl. “These kinds of phenomena are associated with being on the downwind side of a Great Lake. When wind comes along, the lake acts like a snowmaking machine.” While still in school, Orr landed a job as the weathercaster on a Syracuse station’s brand-new morning show. She’s remained a television meteorologist ever since. Today, Orr says, being a trained meteorologist “is definitely a competitive advantage. It’s
rarefied nature of the outer atmosphere is described very well by Richard Craig: The earth’s outermost atmosphere, the part above a few hundred kilometers, is a region of extremely low density. Near sea level, the number of atoms and molecules in a cubic centimeter of air is about 2 3 1019; near 600 km, it is only about 2 3 107, which is the sea-level value divided by a million million. At sea level, an atom or molecule can be expected, on the average, to move about 7 3 1026 cm before colliding with another particle; at the 600-km level, this distance, called the “mean free path,” is about 10km. Near
My job is to explain complicated ideas to people in an uncomplicated way. Orr continues to promote science literacy by volunteering for the American Meteorological Society’s DataStreme Atmosphere Project. As a DataStreme mentor, she has visited dozens of schools to train teachers in the science of meteorology. The teachers then promote the use of weather lessons in their districts to pique student interest in science, mathematics, and technology. Orr considers her forecasts educational as well. “My job is to explain complicated ideas to people in an uncomplicated way.” Being a weathercaster, Orr says, is demanding but also exhilarating. “In TV, the hours are crazy. If you work mornings, you’re up at 2 AM; if nights, you’re up until midnight. So you really have to love it. But if you do, you’ll find a way. And I feel blessed to have done this for so long.”
sea level, an atom or molecule, on the average, undergoes about 7 3 109 such collisions each second; near 600km, this number is reduced to about 1 each minute.*
The graphic portrayal of pressure data (Figure 1–21) shows that the rate of pressure decrease is not constant. Rather, pressure decreases at a decreasing rate with an increase in altitude until, beyond an altitude of about 35 kilometers (22 miles), the decrease is negligible. *Richard Craig, The Edge of Space: Exploring the Upper Atmosphere (New York: Doubleday & Company, Inc., 1968), p. 130.
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The Atmosphere: An Introduction to Meteorology
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32 Capt. Kittinger, USAF 1961 31.3 km (102,800 ft) Air pressure = 9.6 mb
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Figure 1–21 Atmospheric pressure changes with altitude. The rate of pressure decrease with an increase in altitude is not constant. Rather, pressure decreases rapidly near Earth’s surface and more gradually at greater heights.
Put another way, data illustrate that air is highly compressible—that is, it expands with decreasing pressure and becomes compressed with increasing pressure. Consequently, traces of our atmosphere extend for thousands of kilometers beyond Earth’s surface. Thus, to say where the atmosphere ends and outer space begins is arbitrary and, to a large extent, depends on what phenomenon one is studying. It is apparent that there is no sharp boundary. In summary, data on vertical pressure changes show that the vast bulk of the gases making up the atmosphere is very near Earth’s surface and that the gases gradually merge with the emptiness of space. When compared with the size of the solid Earth, the envelope of air surrounding our planet is indeed very shallow.
Temperature Changes By the early twentieth century much had been learned about the lower atmosphere. The upper atmosphere was partly known from indirect methods. Data from balloons and kites had revealed that the air temperature dropped with increasing height above Earth’s surface. This phenomenon is felt by anyone who has climbed a high mountain and is obvious in pictures of snow-capped mountaintops rising above snowfree lowlands (Figure 1–22).
This jet is cruising at an altitude of 10 kilometers (6.2 miles). (Photo by interlight/Shutterstock) Question 1 Refer to the graph in Figure 1–21. What is the approximate air pressure where the jet is flying? Question 2 About what percentage of the atmosphere is below the jet (assuming that the pressure at the surface is 1000 millibars)?
Although measurements had not been taken above a height of about 10 kilometers (6 miles), scientists believed that the temperature continued to decline with height to a value of absolute zero (–273°C) at the outer edge of the atmosphere. In 1902, however, the French scientist Leon Philippe Teisserenc de Bort refuted the notion that temperature
Figure 1–22 Temperatures drop with an increase in altitude in the troposphere. Therefore, it is possible to have snow on a mountaintop and warmer, snow-free lowlands below. (Photo by David Wall/Alamy)
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Chapter 1 Introduction to the Atmosphere
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decreases continuously with an increase in altitude. In studying the results of more than 200 balloon launchings, Teisserenc de Bort found that the temperature stopped decreasing and leveled off at an altitude between 8 and 12 kilometers (5 and 7.5 miles). This surprising discovery was at first doubted, but subsequent data-gathering confirmed his findings. Later, through the use of balloons and rocket-sounding techniques, the temperature structure of the atmosphere up to great heights became clear. Today the atmosphere is divided vertically into four layers on the basis of temperature (Figure 1–23).
Troposphere The bottom layer in which we live, where temperature decreases with 50 30 Stratopause an increase in altitude, is the troposphere. 40 The term was coined in 1908 by Teisserrenc de Bort and literally means the region where 20 STRATOSPHERE 30 Maximum ozone air “turns over,” a reference to the apprecia20 ble vertical mixing of air in this lowermost 10 zone. Tropopause 10 The temperature decrease in the tropoMt. Everest TROPOSPHERE sphere is called the environmental lapse –100 – 90 – 80 –70 – 60 – 50 – 40 – 30 – 20 –10 10 20 30 30 50˚C 0 rate. Its average value is 6.5°C per kilometer (3.5°F per 1000 feet), a figure known as 80 100 120˚F 0 60 –140 –120 –100 – 80 – 60 – 40 – 20 20 40 32 the normal lapse rate. It should be emphaTemperature sized, however, that the environmental lapse rate is not a constant but rather can be highly variable and must be regularly mea- Figure 1–23 Thermal structure of the atmosphere. sured. To determine the actual environmental lapse rate as well as gather information about vertical changes in air pressure, wind, The troposphere is the chief focus of meteorologists and humidity, radiosondes are used. A radiosonde is an because it is in this layer that essentially all important instrument package that is attached to a balloon and transweather phenomena occur. Almost all clouds and certainly mits data by radio as it ascends through the atmosphere all precipitation, as well as all our violent storms, are born (Figure 1–24). The environmental lapse rate can vary durin this lowermost layer of the atmosphere. This is why the ing the course of a day with fluctuations of the weather, troposphere is often called the “weather sphere.” as well as seasonally and from place to place. Sometimes shallow layers where temperatures actually increase with Stratosphere Beyond the troposphere lies the height are observed in the troposphere. When such a reverstratosphere; the boundary between the troposphere and sal occurs, a temperature inversion is said to exist.* the stratosphere is known as the tropopause. Below the The temperature decrease continues to an average tropopause, atmospheric properties are readily transferred height of about 12 kilometers (7.5 miles). Yet the thickby large-scale turbulence and mixing, but above it, in the ness of the troposphere is not the same everywhere. It stratosphere, they are not. In the stratosphere, the temperareaches heights in excess of 16 kilometers (10 miles) in the ture at first remains nearly constant to a height of about 20 tropics, but in polar regions it is more subdued, extendkilometers (12 miles) before it begins a sharp increase that ing to 9 kilometers (5.5 miles) or less (Figure 1–25). Warm continues until the stratopause is encountered at a height of surface temperatures and highly developed thermal mixabout 50 kilometers (30 miles) above Earth’s surface. Higher ing are responsible for the greater vertical extent of the trotemperatures occur in the stratosphere because it is in this posphere near the equator. As a result, the environmental layer that the atmosphere’s ozone is concentrated. Recall lapse rate extends to great heights; and despite relatively that ozone absorbs ultraviolet radiation from the Sun. Conhigh surface temperatures below, the lowest tropospheric sequently, the stratosphere is heated by the Sun. Although temperatures are found aloft in the tropics and not at the the maximum ozone concentration exists between 15 and 30 poles. kilometers (9 and 19 miles), the smaller amounts of ozone above this height range absorb enough UV energy to cause the higher observed temperatures. *Temperature inversions are described in greater detail in Chapter 13.
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Pole
Tropopause
27
24 Equator
21 Tropical tropopause Altitude (km)
18
15
Middle latitude tropopause
12 Polar tropopause 9
6
3
–70 –60 –50 –40 –30 –20 –10 Temperature (˚C)
10
20
Figure 1–25 Differences in the height of the tropopause. The variation in the height of the tropopause, as shown on the small inset diagram, is greatly exaggerated. Figure 1–24 A lightweight instrument package, the radiosonde, is suspended below a 2-meter-wide weather balloon. As the radiosonde is carried aloft, sensors measure pressure, temperature, and relative humidity. A radio transmitter sends the measurements to a ground receiver. By tracking the radiosonde in flight, information on wind speed and direction aloft is also obtained. Observations where winds aloft are obtained are called “rawinsonde” observations. Worldwide, there are about 900 upper-air observation stations. Through international agreements, data are exchanged among countries. (Photo by Mark Burnett/ Photo Researchers, Inc.)
Mesosphere In the third layer, the mesosphere, temperatures again decrease with height until at the mesopause, some 80 kilometers (50 miles) above the surface, the average temperature approaches 290°C (2130°F). The coldest temperatures anywhere in the atmosphere occur at the mesopause. The pressure at the base of the mesosphere is only about one-thousandth that at sea level. At the mesopause, the atmospheric pressure drops to just one-millionth that at sea level. Because accessibility is difficult, the mesosphere is one of the least explored regions of the atmosphere. The reason is that it cannot be reached by the highest-flying airplanes and research balloons, nor is it accessible to the
lowest-orbiting satellites. Recent technical developments are just beginning to fill this knowledge gap. Thermosphere The fourth layer extends outward from the mesopause and has no well-defined upper limit. It is the thermosphere, a layer that contains only a tiny fraction of the atmosphere’s mass. In the extremely rarified air of this outermost layer, temperatures again increase, due to the absorption of very shortwave, high-energy solar radiation by atoms of oxygen and nitrogen. Temperatures rise to extremely high values of more than 1000°C (1800°F) in the thermosphere. But such temperatures are not comparable to those experienced near Earth’s surface. Temperature is defined in terms of the average speed at which molecules move. Because the gases of the thermosphere are moving at very high speeds, the temperature is very high. But the gases are so sparse that collectively they possess only an insignificant quantity of heat. For this reason, the temperature of a satellite orbiting Earth in the thermosphere is determined chiefly by the amount of solar
Chapter 1 Introduction to the Atmosphere
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radiation it absorbs and not by the high temperature of the almost nonexistent surrounding air. If an astronaut inside were to expose his or her hand, the air in this layer would not feel hot.
Concept Check 1.8 1 Does air pressure increase or decrease with an increase in altitude? Is the rate of change constant or variable? Explain.
2 Is the outer edge of the atmosphere clearly defined? Explain. 3 The atmosphere is divided vertically into four layers on the basis of temperature. List these layers in order from lowest to highest. In which layer does practically all of our weather occur?
4 Why does temperature increase in the stratosphere? 5 Why are temperatures in the thermosphere not strictly comparable to those experienced near Earth’s surface?
Vertical Variations in Composition In addition to the layers defined by vertical variations in temperature, other layers, or zones, are also recognized in the atmosphere. Based on composition, the atmosphere is often divided into two layers: the homosphere and the heterosphere. From Earth’s surface to an altitude of about 80 kilometers (50 miles), the makeup of the air is uniform in terms of the proportions of its component gases. That is, the composition is the same as that shown earlier, in Figure 1–16. This lower uniform layer is termed the homosphere, the zone of homogeneous composition. In contrast, the very thin atmosphere above 80 kilometers is not uniform. Because it has a heterogeneous composition, the term heterosphere is used. Here the gases are arranged into four roughly spherical shells, each with a distinctive composition. The lowermost layer is dominated by molecular nitrogen (N2), next, a layer of atomic oxygen (O) is encountered, followed by a layer dominated by helium (He) atoms, and finally a region consisting largely of hydrogen (H) atoms. The stratified nature of the gases making up the heterosphere varies according to their weights. Molecular nitrogen is the heaviest, and so it is lowest. The lightest gas, hydrogen, is outermost.
Ionosphere Located in the altitude range between 80 to 400 kilometers (50 to 250 miles), and thus coinciding with the lower portions of the thermosphere and heterosphere, is an electrically charged layer known as the ionosphere. Here molecules of nitrogen and atoms of oxygen are readily ionized as they
When this weather balloon was launched, the surface temperature was 17°C. It is now at an altitude of 1 kilometer. (Photo by David R. Frazier/ Photo Researchers, Inc.) Question 1 What term is applied to the instrument package being carried aloft by the balloon? Question 2 In what layer of the atmosphere is the balloon? Question 3 If average conditions prevail, what air temperature is the instrument package recording? How did you figure this out? Question 4 How will the size of the balloon change, if at all, as it rises through the atmosphere? Explain.
absorb high-energy shortwave solar energy. In this process, each affected molecule or atom loses one or more electrons and becomes a positively charged ion, and the electrons are set free to travel as electric currents. Although ionization occurs at heights as great as 1000kilometers (620 miles) and extends as low as perhaps 50 kilometers (30 miles), positively charged ions and negative electrons are most dense in the range of 80 to 400 kilometers (50 to 250 miles). The concentration of ions is not great below this zone because much of the short-wavelength radiation needed for ionization has already been depleted.
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The Atmosphere: An Introduction to Meteorology
In addition, the atmospheric density at this level results in a large percentage of free electrons being swiftly captured by positively charged ions. Beyond the 400-kilometer (250-mile) upward limit of the ionosphere, the concentration of ions is low because of the extremely low density of the air. Because so few molecules and atoms are present, relatively few ions and free electrons can be produced. The electrical structure of the ionosphere is not uniform. It consists of three layers of varying ion density. From bottom to top, these layers are called the D, E, and F layers, respectively. Because the production of ions requires direct solar radiation, the concentration of charged particles changes from day to night, particularly in the D and E zones. That is, these layers weaken and disappear at night and reappear during the day. The uppermost layer, or F layer, on the other hand, is present both day and night. The density of the atmosphere in this layer is very low, and positive ions and electrons do not meet and recombine as rapidly as they do at lesser heights, where density is higher. Consequently, the concentration of ions and electrons in the F layer does not change rapidly, and the layer, although weak, remains through the night.
its Southern Hemisphere counterpart, the aurora australis (southern lights), appear in a wide variety of forms. Sometimes the displays consist of vertical streamers in which there can be considerable movement. At other times the auroras appear as a series of luminous expanding arcs or as a quiet glow that has an almost foglike quality. The occurrence of auroral displays is closely correlated in time with solar-flare activity and, in geographic location, with Earth’s magnetic poles. Solar flares are massive magnetic storms on the Sun that emit enormous amounts of energy and great quantities of fast-moving atomic particles. As the clouds of protons and electrons from the solar storm approach Earth, they are captured by its magnetic field, which in turn guides them toward the magnetic poles. Then, as the ions impinge on the ionosphere, they energize the atoms of oxygen and molecules of nitrogen and cause them to emit light—the glow of the auroras. Because the occurrence of solar flares is closely correlated with sunspot activity, auroral displays increase conspicuously at times when sunspots are most numerous.
The Auroras
Concept Check 1.9
As best we can tell, the ionosphere has little impact on our daily weather. But this layer of the atmosphere is the site of one of nature’s most interesting spectacles, the auroras (Figure 1–26). The aurora borealis (northern lights) and
1 Distinguish between the homosphere and the heterosphere. 2 What is the ionosphere? Where in the atmosphere is it located? 3 What is the primary cause of the auroras?
Figure 1–26 Aurora borealis (northern lights) as seen from Alaska. The same phenomenon occurs toward the South Pole, where it is called the aurora australis (southern lights). (Photo by agefotostock/ SuperStock)
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Give It Some Thought 1. Determine which statements refer to weather and which refer to climate. (Note: One statement includes aspects of both weather and climate.) a. The baseball game was rained out today. b. January is Omaha’s coldest month. c. North Africa is a desert. d. The high this afternoon was 25°C. e. Last evening a tornado ripped through central Oklahoma. f. I am moving to southern Arizona because it is warm and sunny. g. Thursday’s low of –20°C is the coldest temperature ever recorded for that city. h. It is partly cloudy. 2. After entering a dark room, you turn on a wall switch, but the light does not come on. Suggest at least three hypotheses that might explain this observation. 3. Making accurate measurements and observations is a basic part of scientific inquiry. The accompanying radar image, showing the distribution and intensity of precipitation associated with a storm, provides one example. Identify three additional images in this chapter that illustrate ways in which scientific data are gathered. Suggest advantages that might be associated with each example.
greenhousegases have increased global average temperatures. b. One or two studies suggest that hurricance intensity is increasing. 5. Refer to Figure 1–21 to answer the following questions. a. If you were to climb to the top of Mount Everest, how many breaths of air would you have to take at that altitude to equal one breath at sea level? b. If you are flying in a commercial jet at an altitude of 12 kilometers, about what percentage of the atmosphere’s mass is below you? 6. If you were ascending from the surface of Earth to the top of the atmosphere, which one of the following would be most useful for determining the layer of the atmosphere you were in? Explain. a. Doppler radar b. Hygrometer (humidity) c. Weather satellite d. Barometer (air pressure) e. Thermometer (temperature) 7. The accompanying photo provides an example of interactions among different parts of the Earth system. It is a view of a mudflow that was triggered by extraordinary rains. Which of Earth’s four “spheres” were involved in this natural disaster that buried a small town on the Philippine island ofLeyte?Describe how each contributed to the mudflow.
(Photo by AP Photo/Pat Roque)
(Image by National Weather Service)
4. During a conversation with your meteorology professor, she makes the two statements listed below. Which can be considered a hypothesis? Which is more likely a theory? a. After several decades, the science community hasdetermined that human-generated
8. Where would you expect the thickness of the troposphere (that is, the distance between Earth’s surface and the tropopause) to be greater: over Hawaii or Alaska? Why? Do you think it is likely that the thickness of the troposphere over Alaska is different in January than in July? If so, why?
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The Atmosphere: An Introduction to Meteorology
INTRODUCTION TO THE ATMOSPHERE IN REVIEW ●
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Meteorology is the scientific study of the atmosphere. Weather refers to the state of the atmosphere at a given time and place. It is constantly changing, sometimes from hour to hour and other times from day to day. Climate is an aggregate of weather conditions, the sum of all statistical weather information that helps describe a place or region. The nature of both weather and climate is expressed in terms of the same basic elements, those quantities or properties measured regularly. The most important elements are (1) air temperature, (2) humidity, (3) type and amount of cloudiness, (4) type and amount of precipitation, (5) air pressure, and (6) the speed and direction of the wind. All science is based on the assumption that the natural world behaves in a consistent and predictable manner. The process by which scientists gather facts through observation and careful measurement and formulate scientific hypotheses and theories is often referred to as the scientific method. Earth’s four spheres include the atmosphere (gaseous envelope), the geosphere (solid Earth), the hydrosphere (water portion), and the biosphere (life). Each sphere is composed of many interrelated parts and is intertwined with all the other spheres. Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the Earth system. Earth system science uses an interdisciplinary approach to integrate the knowledge of several academic fields in the study of our planet and its global environmental problems. A system is a group of interacting parts that form a complex whole. The two sources of energy that power the Earth system are (1) the Sun, which drives the external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface, and (2) heat from Earth’s interior that powers the internal processes that produce volcanoes, earthquakes, and mountains. Air is a mixture of many discrete gases, and its composition varies from time to time and place to place. After water vapor, dust, and other variable components are removed, two gases, nitrogen and oxygen, make up 99 percent of the volume of the remaining clean, dry air. Carbon dioxide, although present in only minute amounts (0.0391 percent, or 391 ppm), is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. The variable components of air include water vapor, dust particles, and ozone. Like carbon dioxide, water vapor can absorb heat given off by Earth as well as some solar energy. When water vapor changes from one state to another, it absorbs or releases heat. In the atmosphere, water vapor transports this latent (“hidden”) heat from one place to another, and it is the energy source that helps drive many storms. Aerosols (tiny solid and liquid particles) are meteorologically important because these often-invisible particles act as surfaces on which water can condense and are also absorbers and reflectors of incoming solar radiation. Ozone, a form of oxygen that combines three oxygen atoms into each molecule (O3), is a gas concentrated in the 10- to 50-kilometer height range in the atmosphere that absorbs the potentially harmful ultraviolet (UV) radiation from the Sun.
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Over the past half century, people have placed Earth’s ozone layer in jeopardy by polluting the atmosphere with chlorofluorocarbons (CFCs), which remove some of the gas. Ozone concentrations take an especially sharp drop over Antarctica during the Southern Hemisphere spring (September and October). Ozone depletion seriously affects human health, especially among fair-skinned people and those who spend considerable time in the Sun. The Montreal Protocol, concluded under the auspices of the United Nations, represents a positive international response to the ozone problem. No sharp boundary to the upper atmosphere exists. The atmosphere simply thins as you travel away from Earth, until there are too few gas molecules to detect. Traces of atmosphere extend for thousands of kilometers beyond Earth’s surface. Using temperature as the basis, the atmosphere is divided into four layers. The temperature decrease in the troposphere, the bottom layer in which we live, is called the environmental lapse rate. Its average value is 6.5°C per kilometer, a figure known as the normal lapse rate. The environmental lapse rate is not a constant and must be regularly measured using radiosondes. The thickness of the troposphere is generally greater in the tropics than in polar regions. Essentially all important weather phenomena occur in the troposphere. Beyond the troposphere lies the stratosphere; the boundary between the troposphere and stratosphere is known as the tropopause. In the stratosphere, the temperature at first remains constant to a height of about 20kilometers (12 miles) before it begins a sharp increase due to the absorption of ultraviolet radiation from the Sun by ozone. The temperatures continue to increase until the stratopause is encountered at a height of about 50 kilometers (30 miles). In the mesosphere, the third layer, temperatures again decrease with height until the mesopause, some 80 kilometers (50 miles) above the surface. The fourth layer, the thermosphere, with no well-defined upper limit, consists of extremely rarefied air. Temperatures here increase with an increase in altitude. The atmosphere is often divided into two layers, based on composition. The homosphere (zone of homogeneous composition), from Earth’s surface to an altitude of about 80kilometers (50 miles), consists of air that is uniform in terms of the proportions of its component gases. Above 80 kilometers, the heterosphere (zone of heterogenous composition) consists of gases arranged into four roughly spherical shells, each with a distinctive composition. The stratified nature of the gases in the heterosphere varies according to their weights. Occurring in the altitude range between 80 and 400 kilometers (50 and 250 miles) is an electrically charged layer known as the ionosphere. Here molecules of nitrogen and atoms of oxygen are readily ionized as they absorb high-energy, shortwave solar energy. Three layers of varying ion density make up the ionosphere. Auroras (the aurora borealis, northern lights, and its Southern Hemisphere counterpart the aurora australis, southern lights) occur within the ionosphere. Auroras form as clouds of protons and electrons ejected from the Sun during solar-flare activity enter the atmosphere near Earth’s magnetic poles and energize the atoms of oxygen and molecules of nitrogen, causing them to emit light—the glow of the auroras.
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VOCABULARY REVIEW aerosols (p. 21) air (p. 17) atmosphere (p. 13) aurora australis (p. 29) aurora borealis (p. 29) biosphere (p. 15) climate (p. 5) elements of weather and climate (p. 7)
environmental lapse rate (p. 26) geosphere (p. 13) hydrosphere (p. 14) hypothesis (p. 9) ionosphere (p. 29) mesopause (p. 27) mesosphere (p. 27) meteorology (p. 4) ozone (p. 21)
radiosonde (p. 26) stratopause (p. 27) stratosphere (p. 27) system (p. 16) theory (p. 10) thermosphere (p. 27) tropopause (p. 27) troposphere (p. 26) weather (p. 5)
PROBLEMS 1. Refer to the newspaper-type weather map in Figure 1–3 to answer the following: a. Estimate the predicted high temperatures in central New York State and the northwest corner of Arizona. b. Where is the coldest area on the weather map? Where is the warmest? c. On this weather map, H stands for the center of a region of high pressure. Does it appear as though high pressure is associated with precipitation or fair weather? d. Which is warmer—central Texas or central Maine? Would you normally expect this to be the case? 2. Refer to the graph in Figure 1–5 to answer the following questions about temperatures in New York City: a. What is the approximate average daily high temperature in January? In July? b. Approximately what are the highest and lowest temperatures ever recorded? 3. Refer to the graph in Figure 1–7. Which year had the greatest number of billion-dollar weather disasters? How many events occurred that year? In which year was the damage amount greatest? 4. Refer to the graph in Figure 1–21 to answer the following: a. Approximately how much does the air pressure drop (in millibars) between the surface and 4 kilometers? (Use a surface pressure of 1000 millibars.) b. How much does the pressure drop between 4 and 8 kilometers? c. Based on your answers to parts a and b, answer the following: With an increase in altitude, air pressure decreases at a(n) (constant, increasing, decreasing) rate. Underline the correct answer.
5. If the temperature at sea level were 23°C, what would the air temperature be at a height of 2 kilometers, under average conditions? 6. Use the graph of the atmosphere’s thermal structure (Figure1–23) to answer the following: a. What are the approximate height and temperature of the stratopause? b. At what altitude is the temperature lowest? What is the temperature at that height? 7. Answer the following questions by examining the graph in Figure 1–25: a. In which one of the three regions (tropics, middle latitudes, poles) is the surface temperature lowest? b. In which region is the tropopause encountered at the lowest altitude? The highest? What are the altitudes and temperatures of the tropopause in those regions? 8. a. On a spring day a middle-latitude city (about 40°N latitude) has a surface (sea-level) temperature of 10°C. If vertical soundings reveal a nearly constant environmental lapse rate of 6.5°C per kilometer and a temperature at the tropopause of –55°C, what is the height of the tropopause? b. On the same spring day a station near the equator has a surface temperature of 25°C, 15°C higher than the middle-latitude city mentioned in part a. Vertical soundings reveal an environmental lapse rate of 6.5°C per kilometer and indicate that the tropopause is encountered at 16 kilometers. What is the air temperature at the tropopause?
Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Introduction to the Atmosphere.
Heating Earth’s Surface and Atmosphere From our everyday experiences, we know that the Sun’s rays feel warmer and paved roads become much hotter on clear, sunny days compared to overcast days. Pictures of snowcapped mountains remind us that temperatures decrease with altitude. And we know that the fury of winter is always replaced by the newness of spring. What you may not know is that these are manifestations of the same phenomenon that causes the blue color of the sky and the red color of a brilliant sunset. All these are a result of the interaction of solar radiation with Earth’s atmosphere and its land–sea surface.
This power plant in Andalucia, Spain, produces clean thermoelectric power from the Sun. (Photo by Kevin Foy/Alamy)
After completing this chapter, you should be able to:
t Explain what causes the Sun angle and length of daylight to change during the year and describe how these changes produce the seasons.
t Calculate the noon Sun angle for any latitude on the equinoxes and the solstices.
t Define temperature and explain how it is different from the total kinetic energy contained in a substance.
t Contrast the concepts of latent heat and sensible heat. t List and describe the three mechanisms of heat transfer.
t Sketch and label a diagram that shows the fate of incoming solar radiation.
t Explain what causes blue skies and red sunsets. t Explain what is meant by the statement “The atmosphere is heated from the ground up.”
t Describe the role of water vapor and carbon dioxide in producing the greenhouse effect.
t Sketch and label a diagram illustrating Earth’s heat budget.
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The Atmosphere: An Introduction to Meteorology
Earth–Sun Relationships ATMOSPHERE
Heating Earth’s Surface and Atmosphere ▸ Understanding Seasons
The amount of solar energy received at any location varies with latitude, time of day, and season of the year. Contrasting images of polar bears and perpetual ice and palm trees along a tropical beach serve to illustrate the extremes (Figure 2–1). The unequal heating of Earth’s land–sea surface creates
winds and drives the ocean’s currents, which in turn transport heat from the tropics toward the poles in an unending attempt to balance energy inequalities. The consequences of these processes are the phenomena we call weather. If the Sun were “turned off,” global winds and ocean currents would quickly cease. Yet as long as the Sun shines, winds will blow and weather will persist. So to understand how the dynamic weather machine works, we must know why different latitudes receive different quantities of solar energy and why the amount of solar energy received changes during the course of a year to produce the seasons.
Earth’s Motions Earth has two principal motions—rotation and revolution. Rotation is the spinning of Earth on its axis that produces the daily cycle of day and night. The other motion, revolution, refers to Earth’s movement in a slightly elliptical orbit around the Sun. The distance between Earth and Sun averages about 150 million kilometers (93 million miles). Because Earth’s orbit is not perfectly circular, however, the distance varies during the course of a year. Each year, on about January 3, our planet is about 147.3 million kilometers (91.5 million miles) from the Sun, closer than at any other time—a position called perihelion. About six months later, on July 4, Earth is about 152.1 million kilometers (94.5 million miles) from the Sun, farther away than at any other time—a position called aphelion. Although Earth is closest to the Sun and receives up to 7 percent more energy in January than in July, this difference plays only a minor role in producing seasonal temperature variations, as evidenced by the fact that Earth is closest to the Sun during the Northern Hemisphere winter.
(a)
What Causes the Seasons?
(b)
Figure 2–1 An understanding of Earth-Sun relationships is basic to an understanding of weather and climate. (a) In tropical latitudes, temperature contrasts during the year are modest. (Photo by Maria Skaldina/Shutterstock) (b) In polar regions, seasonal temperature contrasts can be dramatic. (Photo by Michael Collier)
If variations in the distance between the Sun and Earth do not cause seasonal temperature changes, what does? The gradual but significant change in day length certainly accounts for some of the difference we notice between summer and winter. Furthermore, a gradual change in the angle (altitude) of the Sun above the horizon is also a major contributing factor (Figure 2–2). For example, someone living in Chicago, Illinois, experiences the noon Sun highest in the sky in late June. But as summer gives way to autumn, the noon Sun appears lower in the sky, and sunset occurs earlier each evening. The seasonal variation in the angle of the Sun above the horizon affects the amount of energy received at Earth’s surface in two ways. First, when the Sun is directly overhead (at a 90° angle), the solar rays are most concentrated and thus most intense. At lower Sun angles, the rays become more spread out and less intense (Figure 2–3). You have probably experienced this when using a flashlight. If the beam strikes a surface perpendicularly, a small intense spot is produced. By contrast, if the flashlight beam strikes at any other angle, the area illuminated is larger—but noticeably dimmer. Second, but of less significance, the angle of the Sun determines the path solar rays take as they pass through the
Chapter 2 Heating Earth’s Surface and Atmosphere June 21-22
Longest day Day and night equal
March 21-22 September 22-23
E
E
Sun angle 73 1/2°
Sun angle 50° S
N
December 21-22
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W (a) Summer solstice at 40°N latitude Shortest day
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Noon Sun
Sun angle 33 1/2°
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Figure 2–2 Daily paths of the Sun for a place located at 40° north latitude for the (a) summer solstice; (b) spring or fall equinox, and (c) winter solstice and for a place located at 80° north latitude at the (d) summer solstice.
atmosphere (Figure 2–4). When the Sun is directly overhead, the rays strike the atmosphere at a 90° angle and travel the shortest possible route to the surface. This distance is referred to as 1 atmosphere. However, rays entering the atmosphere at a 30° angle must travel twice this distance before reaching the surface, while rays at a 5° angle travel through
a distance roughly equal to the thickness of 11 atmospheres (Table 2–1). The longer the path, the greater the chance that sunlight will be dispersed by the atmosphere, which reduces the intensity at the surface. These conditions account for the fact that we cannot look directly at the midday Sun but we enjoy gazing at a sunset.
1 it un
1 unit
nit
1u
90˚
1 unit
45˚ 1.4 units
30˚ 2 units
Figure 2–3 Changes in the Sun’s angle cause variations in the amount of solar energy reaching Earth’s surface. The higher the angle, the more intense the solar radiation.
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The Atmosphere: An Introduction to Meteorology 231/2 ˚ N Atmosphere 90˚ 661/2˚
In summary, the most important reasons for variations in the amount of solar energy reaching a particular location are the seasonal changes in the angle at which the Sun’s rays strike the surface and changes in the length of daylight.
Earth’s Orientation
Sun's rays
30˚
0˚ 231/2 ˚
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Figure 2–4 Rays striking Earth at a low angle (near the poles) must traverse more of the atmosphere than rays striking at a high angle (around the equator) and thus are subject to greater depletion by reflection and absorption.
It is important to remember that Earth’s shape is spherical. Hence, on any given day, the only places that will receive vertical (90°) rays from the Sun are located along one particular line of latitude. As we move either north or south of this location, the Sun’s rays strike at decreasing angles. Thus, the nearer a place is situated to the latitude receiving the vertical rays of the Sun, the higher will be its noon Sun, and the more concentrated will be the radiation it receives.
What causes fluctuations in Sun angle and length of daylight during the course of a year? Variations occur because Earth’s orientation to the Sun continually changes. Earth’s axis (the imaginary line through the poles around which Earth rotates) is not perpendicular to the plane of its orbit around the Sun—called the plane of the ecliptic. Instead, it is tilted 23 1/2° from the perpendicular, called the inclination of the axis. If the axis were not inclined, Earth would lack seasons. Because the axis remains pointed in the same direction (toward the North Star), the orientation of Earth’s axis to the Sun’s rays is constantly changing (Figure 2–5). For example, on one day in June each year, Earth’s position in orbit is such that the Northern Hemisphere is “leaning” 23 1/2° toward the Sun (left in Figure 2–5). Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere “leans” 23 1/2° away from the Sun (Figure 2–5, right). On days between these extremes, Earth’s axis is leaning at amounts less than 23 1/2° to the rays of the Sun. This change in orientation causes the spot where the Sun’s rays are vertical to make an annual migration from 23 1/2° north of the equator to 23 1/2° south of the equator. In turn, this migration causes the angle of the noon Sun to vary by up
Equinox March 21-22 Sun vertical at equator
Arctic Circle Tropic of Cancer Equator Tropic of Capricorn
231/2˚
Sun
Solstice June 21-22 Sun vertical at Latitude 231/2˚ N
Solstice December 21-22 Sun vertical at Latitude 231/2˚ S
Orbit
Figure 2–5 Earth–Sun relationships.
Equinox September 22-23 Sun vertical at equator
Chapter 2 Heating Earth’s Surface and Atmosphere TABLE 2–1 Distance Radiation Must Travel Through the Atmosphere Angle of Sun Above Horizon
Students Sometimes Ask . . . Is the Sun ever directly overhead anywhere in the United States?
Equivalent Number of Atmospheres Sunlight Must Pass Through
90° (directly overhead)
1.00
80°
1.02
70°
1.06
60°
1.15
50°
1.31
40°
1.56
30°
2.00
20°
2.92
10°
5.70
5°
10.80
0° (at horizon)
45.00
Yes, but only in the state of Hawaii. Honolulu, located on the island of Oahu at about 21° north latitude, experiences a 90° Sun angle twice each year—once at noon on about May 27 and again at noon on about July 20. All of the other states are located north of the Tropic of Cancer and therefore never experience the vertical rays of the Sun.
Solstices and Equinoxes Based on the annual migration of the direct rays of the Sun, four days each year are especially significant. On June 21 or 22, the vertical rays of the Sun strike 23 1/2° north latitude (23 1/2° north of the equator), a line of latitude known as the Tropic of Cancer (Figure 2–5). For people living in the Northern Hemisphere, June 21 or 22 is known as the summer solstice, the first “official” day of summer (see Box 2–1). Six months later, on December 21 or 22, Earth is in an opposite position, with the Sun’s vertical rays striking at 23 1/2° south latitude. This line is known as the Tropic of Capricorn. For those in the Northern Hemisphere, December 21 or 22 is the winter solstice, the first day of winter. However,
to 47° 1 23 1/2 1 23 1/2 2 for many locations during a year. A midlatitude city such as New York, for instance, has a maximum noon Sun angle of 73 1/2° when the Sun’s vertical rays have reached their farthest northward location in June and a minimum noon Sun angle of 26 1/2° six months later (Figure 2–6).
N Data: Sun's rays Location: 40° N 261/2 ˚ Date: December 22
Loc
atio
1
Location of 90° Sun: 23 /2° S
Tro pi
1
63 /2°
co
n: 4
0° N
fC
anc
er (
Calculations:
23 1 /2 ˚ N
)
Step 1: Distance in degrees between 231/2° S and 40° N = 631/2° Step 2:
Sun's rays
Equ
ato
90˚
39
Tro pi
co
90 –631/2° Noon Sun angle at 40° N 261/2 ° = on December 22
fC
apr
ico
r (0
°)
rn (
23 1 /2˚ S
)
S
Figure 2–6 Calculating the noon Sun angle. Recall that on any given day, only one latitude receives vertical (90°) rays of the Sun. A place located 1° away (either north or south) receives an 89° angle; a place 2° away, an 88° angle; and so forth. To calculate the noon Sun angle, simply find the number of degrees of latitude separating the location you want to know about from the latitude that is receiving the vertical rays of the Sun. Then subtract that value from 90°. The example in this figure illustrates how to calculate the noon Sun angle for a city located at 40° north latitude on December 22 (winter solstice).
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The Atmosphere: An Introduction to Meteorology
Box 2-1
When Are the Seasons?
Have you ever been caught in a snowstorm around Thanksgiving, even though winter does not begin until December 21? Or perhaps you have endured several consecutive days of 100° temperatures although summer has not “officially” started? The idea of dividing the year into four seasons originated from the Earth–Sun relationships discussed in this chapter (Table 2–A). This astronomical definition of the seasons defines winter (Northern Hemisphere) as the period from the winter solstice (December 21–22) to the spring equinox (March 21–22) and so forth. This is also the definition used most widely by the news media, yet it is not unusual for portions of the United States and Canada to have significant snowfalls weeks before the “official” start of winter (Table 2–A). Because the weather phenomena we normally associate with each season do not coincide well with the astronomical seasons, meteorologists prefer to divide the year into four three-month periods based primarily on temperature. Thus, winter is defined as December, January, and February, the three coldest months of the year in the Northern Hemisphere. Summer is defined as the three warmest months, June, July, and August. Spring and autumn are the transition periods between these two seasons (Figure 2–A). Inasmuch as these four three-month periods better reflect the temperatures and weather that we associate with the respective seasons, this definition of the seasons is more useful for meteorological discussions.
TABLE 2–A Occurrence of the Seasons in the Northern Hemisphere Season
Astronomical Season
Climatological Season
Spring
March 21 or 22 to June 21 or 22
March, April, May
Summer
June 21 or 22 to September 22 or 23
June, July, August
Autumn
September 22 or 23 to December 21 or 22
September, October, November
Winter
December 21 or 22 to March 21 or 22
December, January, February
FIGURE 2–A Trees turning bright colors before losing their leaves is a common fall scene in the middle latitudes. (Photo by Corbis/SuperStock).
on this same day people in the Southern Hemisphere are experiencing their summer solstice. The equinoxes occur midway between the solstices. September 22 or 23 is the date of the autumnal (fall) equinox in the Northern Hemisphere, and March 21 or 22 is the date of the spring equinox (also called the vernal equinox). On these dates the vertical rays of the Sun strike the equator (0° latitude) because Earth’s position is such that its axis is tilted neither toward nor away from the Sun. The length of daylight versus darkness is also determined by the position of Earth relative to the Sun’s rays. The length of daylight on June 21, the summer solstice in the Northern Hemisphere, is greater than the length of night.
This fact can be established by examining Figure 2–7, which illustrates the circle of illumination—that is, the boundary separating the dark half of Earth from the lighted half. The length of daylight is established by comparing the fraction of a line of latitude that is on the “day” side of the circle of illumination with the fraction on the “night” side. Notice that on June 21 all locations in the Northern Hemisphere experience longer periods of daylight than darkness (Figure 2–7). By contrast, during the December solstice the length of darkness exceeds the length of daylight at all locations in the Northern Hemisphere. For example, consider New York City (about 40° north latitude), which has about 15 hours of daylight on June 21 and only 9 hours on December 21.
Chapter 2 Heating Earth’s Surface and Atmosphere
23 1 /2 ˚
40˚
66 1 /2˚
N
N 9h
24 hrs. 15
0˚ 23 1 /2 ˚
12
12
hrs
.
Sun's rays
hrs
.
40˚
13 1 /2 h
23 1 /2 ˚
rs.
15
hrs
.
10 1 /2 h
hrs
.
24
rs.
9h
66 1 /2˚
rs.
rs.
40˚
rs.
10 1 /2 h
13 1 /2 h
66 1 /2˚
41
rs.
0˚ hrs
.
S
S (a) June Solstice (Northern Hemisphere summer)
66 1 /2 ˚
40˚
23 1 /2 ˚
(b) December Solstice (Northern Hemisphere winter)
N 661/2˚
Arctic Circle
40˚
12 hrs. Sun's rays
12 hrs .
231/2˚ Tropic of Cancer
0˚
12 hrs .
Equator 12 hrs. Tropic of Capricorn 12 hrs.
231/2˚ 40˚ (c) Spring/Fall Equinox
S
Also note from Table 2–2 that the farther north a location is from the equator on June 21, the longer the period of daylight. When you reach the Arctic Circle (66 1/2° north latitude), the length of daylight is 24 hours. Places located at or north of the Arctic Circle experience the “midnight Sun,” which does not set for a period that ranges from one day to about six months (Figure 2–8). As a review of the characteristics of the summer solstice for the Northern Hemisphere, examine Figure 2–7 and Table 2–2 and consider the following: 1. The date of occurrence is June 21 or 22. 2. The vertical rays of the Sun are striking the Tropic of Cancer (23 1/2° north latitude). 3. Locations in the Northern Hemisphere are experiencing their longest length of daylight and highest Sun angle. (The opposite is true for the Southern Hemisphere.) 4. The farther north a location is from the equator, the longer the period of daylight, until the Arctic Circle is reached, where the length of daylight becomes 24 hours. (The opposite is true for the Southern Hemisphere.) The winter solstice facts are the reverse. It should now be apparent why a midlatitude location is warmest in the summer—when the days are longest and the angle of the Sun is highest. During an equinox (meaning “equal night”), the length of daylight is 12 hours everywhere on Earth because the cir-
Figure 2-7 Characteristics of the solstices and equinoxes.
cle of illumination passes directly through the poles, thus dividing the lines of latitude in half. These seasonal changes, in turn, cause the month-tomonth variations in temperature observed at most locations outside the tropics. Figure 2–9 shows mean monthly temperatures for selected cities at different latitudes. Notice that the cities located at more poleward latitudes experience larger temperature differences from summer to winter than do cities located nearer the equator. Also notice that temperature minimums for Southern Hemisphere locations occur in July,
TABLE 2–2
Length of Daylight
Latitude (degrees)
Summer Solstice
Winter Solstice
Equinoxes
12 hr
12 hr
12 hr
10
12 hr 35 min
11 hr 25 min
12 hr
20
13 hr 12 min
10 hr 48 min
12 hr
30
13 hr 56 min
10 hr 04 min
12 hr
40
14 hr 52 min
9 hr 08 min
12 hr
50
16 hr 18 min
7 hr 42 min
12 hr
60
18 hr 27 min
5 hr 33 min
12 hr
70
2 mo
0 hr 00 min
12 hr
80
4 mo
0 hr 00 min
12 hr
90
6 mo
0 hr 00 min
12 hr
Figure 2–8 Multiple exposures of the midnight Sun representative of midsummer in the high latitudes. This example shows the midnight Sun in Norway. (Photo by Martin Woike/agephotostock)
40
St. Louis, Missouri 39˚ N
36
1 Do the annual variations in Earth–Sun distance adequately account for seasonal temperature changes? Explain.
2 Use a simple sketch to show why the intensity of solar radiation striking Earth’s surface changes when the Sun angle changes.
3 Briefly explain the primary cause of the seasons. 4 What is the significance of the Tropic of Cancer and the
100 90
28
80
24 70
20 16
60 Capetown, South Africa 34˚ S
8
50 40
4 0
30
–4 20
Winnipeg, Manitoba 50° N
–12
Temperature (˚F)
12
–8
Concept Check 2.1
Iquitos, Peru 4˚ S
32
Temperature (˚C)
whereas they occur in January for most places in the Northern Hemisphere. In summary, seasonal fluctuations in the amount of solar energy reaching various places on Earth’s surface are caused by the migrating vertical rays of the Sun and the resulting variations in Sun angle and length of daylight. All locations situated at the same latitude have identical Sun angles and lengths of daylight. If the Earth–Sun relationships previously described were the only controls of temperature, we would expect these places to have identical temperatures as well. Obviously, such is not the case. Although the angle of the Sun above the horizon and the length of daylight are the primary controls of temperature, other factors must be considered—a topic that will be addressed in Chapter 3.
10
–16
–20 Point Barrow, Alaska 71˚ N
– 24
–10
–28 –32
–20 J
F
M
A
M
J
J
A
S
O
N
D
Month
Tropic of Capricorn?
5 After examining Table 2–2, write a general statement that relates the season, latitude, and the length of daylight.
42
Figure 2–9 Mean monthly temperatures for five cities located at different latitudes. Note that Cape Town, South Africa, experiences winter in June, July, and August.
Chapter 2 Heating Earth’s Surface and Atmosphere
43
This image, which shows the first sunrise of 2008 at the South Pole, was taken at the U.S. Amundsen– Scott Station. At the moment the Sun cleared the horizon, the weathered American flag was seen whipping in the wind above a sign marking the location of the geographic South Pole. (NASA) Question 1 What was the approximate date that this photograph was taken? Question 2 How long after this photo was taken did the Sun set at the South Pole? Question 3 Over the course of one year, what is the highest position the Sun can reach (measured in degrees) at the South Pole? On what date does this occur?
Students Sometimes Ask . . . Where Is the Land of the Midnight Sun? Any place located north of the Arctic Circle (66 1/2° north latitude) or south of the Antarctic Circle (66 1/2° south latitude) experiences 24 hours of continuous daylight (or darkness) at least one day each year. The closer a place is to a pole, the longer the period of continuous daylight (or darkness). Someone stationed at either pole will experience six months of continuous daylight followed by six months of darkness. Thus, the Land of the Midnight Sun refers to any location where the latitude is greater than 66 1/2°, such as the northern portions of Alaska, Canada, Russia, and Scandinavia, as well as most of Antarctica.
Energy, Temperature, and Heat The universe is made up of a combination of matter and energy. The concept of matter is easy to grasp because it is the “stuff” we can see, smell, and touch. Energy, on the other hand, is abstract and therefore more difficult to describe and understand. Energy comes to Earth from the Sun in the form of electromagnetic radiation, which we see as light and feel as heat. There are countless places and situations where energy is present—in the food we eat, the water located at the top of a waterfall, and the waves that break along the shore.
Forms of Energy Energy can be defined simply as the capacity to do work. Work is done whenever matter moves. Common examples include the chemical energy from gasoline that powers
automobiles, the heat energy from stoves that excites water molecules (boils water), and the gravitational energy that has the capacity to move snow down a mountain slope in the form of an avalanche. These examples illustrate that energy takes many forms and can also change from one form to another. For example, the chemical energy in gasoline is first converted to heat in the engine of an automobile, which is then converted to mechanical energy that moves the automobile along. You are undoubtedly familiar with some of the common forms of energy, such as heat, chemical, nuclear, radiant (light), and gravitational energy. Energy is also placed into one of two major categories: kinetic energy and potential energy. Kinetic Energy Energy associated with an object by virtue of its motion is described as kinetic energy. A simple example of kinetic energy is the motion of a hammer when driving a nail. Because of its motion, the hammer is able to move another object (do work). The faster the hammer is swung, the greater its kinetic energy (energy of motion). Similarly, a larger (more massive) hammer possesses more kinetic energy than a smaller one, provided that both are swung at the same velocity. Likewise, the winds associated with a hurricane possess much more kinetic energy than do light, localized breezes because they are both larger in scale (cover a larger area) and travel at higher velocities. Kinetic energy is also significant at the atomic level. All matter is composed of atoms and molecules that are continually vibrating, and by virtue of this motion have kinetic energy. For example, when a pan of water is placed over a fire, the water becomes warmer because the fire causes the water molecules to vibrate faster. Thus, when a solid, liquid, or gas is heated, its atoms or molecules move faster and possess more kinetic energy.
44
The Atmosphere: An Introduction to Meteorology
Potential Energy As the term implies, potential energy has the capability to do work. For example, large hailstones suspended by an updraft in a towering cloud have potential energy. Should the updraft subside, these hailstones may fall to Earth and do destructive work on roofs and vehicles. Many substances, including wood, gasoline, and the food you eat, contain potential energy, which is capable of doing work given the right circumstances.
Temperature Humans think of temperature as how warm or cold an object is with respect to some standard measure. However, our senses are often poor judges of warm and cold (see Students Sometimes Ask, p. 46). Temperature is a measure of the average kinetic energy of the atoms or molecules in a substance. When a substance gains energy, its particles move faster and its temperature rises. By contrast, when energy is lost, the atoms and molecules vibrate more slowly and its temperature drops. In the United States the Fahrenheit scale is used most often for everyday expressions of temperature. However, scientists and the majority of other countries use the Celsius and Kelvin temperature scales. A discussion of these scales is provided in Chapter 3. It is important to note that temperature is not a measure of the total kinetic energy of an object. For example, a cup of boiling water has a much higher temperature than a bathtub of lukewarm water. However, the quantity of water in the cup is small, so it contains far less kinetic energy than the water in the tub. Much more ice would melt in the tub of lukewarm water than in the cup of boiling water. The temperature of the water in the cup is higher because the atoms and molecules are vibrating faster, but the total amount of kinetic energy (also called thermal energy) is much smaller because there are fewer particles.
Students Sometimes Ask . . . What would the seasons be like if Earth were not tilted on its axis? The most obvious change would be that all locations on the globe would experience 12 hours of daylight every day of the year. Moreover, for any latitude, the Sun would always follow the path it does during an equinox. There would be no seasonal temperature changes, and daily temperatures would be roughly equivalent to the “average” for that location.
Heat We define heat as energy transferred into or out of an object because of temperature differences between that object and its surroundings. If you hold a hot mug of coffee, your hand will begin to feel warm or even hot. By contrast, when you hold an ice cube, heat is transferred from your hand to the ice cube. Heat flows from a region of higher temperature to one
of lower temperature. Once the temperatures become equal, heat flow stops. Meteorologists further subdivide heat into two categories, latent heat and sensible heat. Latent heat is the energy involved when water changes from one state of matter to another—when liquid water evaporates and becomes water vapor, for example. During evaporation heat is required to break the hydrogen bonds between water molecules that occurs when water vapor escapes a water body. Because the most energetic water molecules escape, the average kinetic energy (temperature) of the water body drops. Therefore, evaporation is a cooling process, something you may have experienced upon stepping out, dripping wet, from a swimming pool or bathtub. The energy absorbed by the escaping water vapor molecules is termed latent heat (meaning “hidden”) because it does not result in a temperature increase. The latent heat stored in water vapor is eventually released in the atmosphere during condensation—when water vapor returns to the liquid state during cloud formation. Therefore, latent heat is responsible for transporting considerable amounts of energy from Earth’s land–sea surface to the atmosphere. By contrast, sensible heat is the heat we can feel and measure with a thermometer. It is called sensible heat because it can be “sensed.” Warm air that originates over the Gulf of Mexico and flows into the Great Plains in the winter is an example of the transfer of sensible heat.
Concept Check 2.2 1 Distinguish between heat and temperature. 2 Describe how latent heat is transferred from Earth’s land–sea surface to the atmosphere.
3 Compare latent heat and sensible heat.
Mechanisms of Heat Transfer ATMOSPHERE
Heating Earth’s Surface and Atmosphere ▸ Solar Radiation
The flow of energy can occur in three ways: conduction, convection, and radiation (Figure 2–10). Although they are presented separately, all three mechanisms of heat transfer can operate simultaneously. In addition, these processes may transfer heat between the Sun and Earth and between Earth’s surface, the atmosphere, and outer space.
Conduction Anyone who attempts to pick up a metal spoon left in a boiling pot of soup realizes that heat is transmitted along the entire length of the spoon. The transfer of heat in this manner is called conduction. The hot soup causes the molecules at the lower end of the spoon to vibrate more rapidly.
Chapter 2 Heating Earth’s Surface and Atmosphere
45
tains numerous air spaces that impair the flow of heat. This is why wild animals may burrow into a snowbank to escape the “cold.” The snow, like a down-filled comforter, does not supply heat; it simply retards the loss of the animal’s own body heat.
Conduction Convection
Convection Radiation
Figure 2–10 The three mechanisms of heat transfer: conduction, convection, and radiation.
These molecules and free electrons collide more vigorously with their neighbors and so on up the handle of the spoon. Thus, conduction is the transfer of heat through electron and molecular collisions from one molecule to another. The ability of substances to conduct heat varies considerably. Metals are good conductors, as those of us who have touched a hot spoon have quickly learned. Air, in contrast, is a very poor conductor of heat. Consequently, conduction is important only between Earth’s surface and the air immediately in contact with the surface. As a means of heat transfer for the atmosphere as a whole, conduction is the least significant and can be disregarded when considering most meteorological phenomena. Objects that are poor conductors, such as air, are called insulators. Most objects that are good insulators, such as cork, plastic foam, or goose down, contain many small air spaces. The poor conductivity of the trapped air gives these materials their insulating value. Snow is also a poor conductor (good insulator), and like other insulators, it con-
Much of the heat transport in Earth’s atmosphere and oceans occurs by convection. Convection is heat transfer that involves the actual movement or circulation of a substance. It takes place in fluids (liquids such as water and gases such as air) where the material is able to flow. The pan of water being heated over a campfire in Figure 2–10 illustrates the nature of a simple convective circulation. The fire warms the bottom of the pan, which conducts heat to the water inside. Because water is a relatively poor conductor, only the water in close proximity to the bottom of the pan is heated by conduction. Heating causes water to expand and become less dense. Thus, the hot, buoyant water near the bottom of the pan rises, while the cooler, denser water above sinks. As long as the water is heated from the bottom and cools near the top, it will continue to “turn over,” producing a convective circulation. In a similar manner, some of the air in the lowest layer of the atmosphere that was heated by radiation and conduction is transported by convection to higher layers of the atmosphere. For example, on a hot, sunny day the air above a plowed field will be heated more than the air above the surrounding woodlands. As warm, less dense air above the plowed field buoys upward, it is replaced by the cooler air above the woodlands (Figure 2–11). In this way a convective flow is established. The warm parcels of rising air are called thermals and are what hang-glider pilots use to keep their crafts soaring. Convection of this type not only transfers heat but also transports moisture aloft. The result is an increase in cloudiness that frequently can be observed on warm summer afternoons.
Convection
Wa
(a)
(b)
r
rm
m
air
Convection
Wa
Rising thermal
Condensation level
a ir
C o o l a ir
Cool air
Solar heating
Figure 2–11 (a) Heating of Earth’s surface produces thermals of rising air that transport heat and moisture aloft. (b) The rising air cools, and if it reaches the condensation level, clouds form. Rising warmer air and descending cooler air are an example of convective circulation.
46
The Atmosphere: An Introduction to Meteorology
On a much larger scale is the global convective circulation of the atmosphere, which is driven by the unequal heating of Earth’s surface. These complex movements are responsible for the redistribution of heat between hot equatorial regions and frigid polar latitudes and will be discussed in detail in Chapter 7.
Students Sometimes Ask . . . In the morning when I get out of bed, why does the tile flooring in the bathroom feel much colder than the carpeted area, even though both materials are the same temperature? The difference you feel is due mainly to the fact that floor tile is a much better conductor of heat than carpet. Hence, heat is more rapidly conducted from your bare feet to the tile floor than from your bare feet to the carpeted floor. Even at room temperature (20°C [68°F]), objects that are good conductors can feel chilly to the touch. (Remember, body temperature is about 37°C [98.6°F].)
Atmospheric circulation consists of vertical as well as horizontal components, so both vertical and horizontal heat transfer occurs. Meteorologists often use the term convection to describe the part of the atmospheric circulation that involves upward and downward heat transfer. By contrast, the term advection is used to denote the primarily horizontal component of convective flow. (The common term for advection is “wind,” a phenomenon we will examine closely in later chapters.) Residents of the midlatitudes often experience the effects of heat transfer by advection. For example, when frigid Canadian air invades the Midwest in January, it brings bitterly cold winter weather.
Radiation The third mechanism of heat transfer is radiation. Unlike conduction and convection, radiation is the only mechanism of heat transfer that can travel through the vacuum of space and thus is responsible for solar energy reaching Earth. Solar Radiation The Sun is the ultimate source of energy that drives the weather machine. We know that the Sun emits light and heat energy as well as the rays that darken skin pigmentation. Although these forms of energy constitute a major portion of the total energy that radiates from the Sun, they are only a part of a large array of energy called radiation, or electromagnetic radiation. This array or spectrum of electromagnetic energy is shown in Figure 2–12. All types of radiation, whether X-rays, radio waves, or heat waves, travel through the vacuum of space at 300,000 kilometers (186,000 miles) per second, a value known as the speed of light. To help visualize radiant energy, imagine ripples made in a calm pond when a pebble is tossed in. Like the waves produced in the pond, electromagnetic waves come in various sizes, or wavelengths—the distance from one crest to the next (Figure 2–12). Radio waves have the longest wavelengths, up to tens of kilometers in length. Gamma waves are the shortest, at less than one-billionth of a centimeter long. Visible light is roughly in the middle of this range. Radiation is often identified by the effect that it produces when it interacts with an object. The retinas of our eyes, for instance, are sensitive to a range of wavelengths called visible light. We often refer to visible light as white light because it appears “white” in color. It is easy to show, however, that white light is really an array of colors, each color corresponding to a specific range of wavelengths. By using a prism, white light can be divided into the colors
Microwave
0.4
FM VHF
UHF
Micrometers 0.5 0.6
0.7
AM
VLF
Radio Infrared 100 GHz Near Far Visible
1 GHz
1 100 Micrometers
Ultraviolet Near Far X rays “Hard”
“Soft”
Gamma rays
Wavelength (meters)
10–14
10–12
10–10
10–8
10–6
10–4
10–2
Figure 2–12 The electromagnetic spectrum, illustrating the wavelengths and names of various types of radiation.
1
102
104
Chapter 2 Heating Earth’s Surface and Atmosphere
47
It is important to note that the Sun emits all forms of radiation, as shown in Figure 2–12, but in varying quantities. Over 95 percent of all solar radiation is emitted in wavelengths between 0.1 and 2.5 micrometers, with much of this energy concentrated in the visible and near-infrared parts of the electromagnetic spectrum (Figure 2–14). The narrow band of visible light, between 0.4 and 0.7 micrometer, represents over 43 percent of the total energy emitted. The bulk of the remainder lies in the infrared zone (49 percent) and ultraviolet (UV) section (7 percent). Less than 1 percent of solar radiation is emitted as X-rays, gamma rays, and radio waves.
Laws of Radiation Figure 2–13 Visible light consists of an array of colors we commonly call the “colors of the rainbow.” Rainbows are relatively common phenomena produced by the bending and reflection of light by drops of water. There is more about rainbows in Chapter 16. (Photo by David Robertson/Alamy)
To obtain a better appreciation of how the Sun’s radiant energy interacts with Earth’s atmosphere and land–sea surface, it is helpful to have a general understanding of the basic radiation laws. Although the mathematics of these laws is beyond the scope of this text, the concepts are fundamental to understanding radiation:
Radiation intensity
1. All objects continually emit radiant energy over a of the rainbow, from violet with the shortest wavelength, range of wavelengths.* Thus, not only do hot objects 0.4 micrometer (μm) (1 micrometer is one-millionth of a such as the Sun continually emit energy, but Earth does meter), to red with the longest wavelength, 0.7 micrometer as well, even the polar ice caps. (Figure 2–13). Located adjacent to the color red, and having a longer wavelength, is infrared radiation, which cannot be seen by *The temperature of the object must be above a theoretical value called the human eye but is detected as heat. Only the infrared enabsolute zero (–273°C) in order to emit radiant energy. The letter K is used ergy that is nearest the visible part of the spectrum is intense for values on the Kelvin temperature scale. For more explanation, see the enough to be felt as heat and is referred to as near infrared. section on “Temperature Scales” in Chapter 3. On the opposite side of the visible range, located next to violet, the energy emitted is called ultraviolet radiation and consists of wavelengths that may cause sunburned skin. Although we divide radiant energy into Radiation categories based on our ability to perceive from Sun them, all wavelengths of radiation behave (10,000°F) similarly. When an object absorbs any form of electromagnetic energy, the waves exRadiation cite subatomic particles (electrons). This emitted 43% results in an increase in molecular motion from Earth 49% (59°F) and a corresponding increase in tempera7% ture. Thus, electromagnetic waves from the Sun travel through space and, upon being 0.1 0.5 1.0 1.5 2.0 10 20 30 40 50 Wavelength in micrometers absorbed, increase the molecular motion Ultra- Visible Infrared of other molecules—including those that violet make up the atmosphere, Earth’s land–sea Longwave Shortwave surfaces, and human bodies. One important difference among the Figure 2–14 Comparison of the intensity of solar radiation and radiation emitted various wavelengths of radiant energy is by Earth. Because of the Sun’s high surface temperature, most of its energy is radiated that shorter wavelengths are more energetic. at wavelengths shorter than 4 micrometers, with the greatest intensity in the visible This accounts for the fact that relatively range of the electromagnetic spectrum. Earth, in contrast, radiates most of its energy short (high-energy) ultraviolet waves can in wavelengths longer than 4 micrometers, primarily in the infrared band. Thus, we damage human tissue more readily than call the Sun’s radiation shortwave and Earth’s radiation longwave. (From PHYSICAL can similar exposures to longer-wavelength GEOGRAPHY: A LANDSCAPE APPRECIATION, 9th Edition, by Tom L. McKnight and radiation. The damage can result in skin Darrell Hess, © 2008. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.) cancer and cataracts.
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The Atmosphere: An Introduction to Meteorology
Box 2-2
Radiation Laws Gregory J. Carbone *
All bodies radiate energy. Both the rate and the wavelength of radiation emitted depend on the temperature of the radiating body.
By contrast, Earth has an average temperature of only 288 K. If we round the value to 300 K, we have:
Stefan–Boltzmann Law This law mathematically expresses the rate of radiation emitted per unit area:
E 5 1 5.67 3 10 28 W /m2 K4 2 1 300 K 2 4
E 5 sT
4
E, the rate of radiation emitted by a body, is proportional to the fourth power of the body’s temperature (T ). The Stefan– Boltzmann constant 1 s 2 is equal to 5.67 3 10 28 W /m2 K4. Compare the difference between the radiation emission from the Sun and Earth. The Sun, with an average temperature of 6000 K, emits 73,483,200 watts per square meter 1 Wm 22 2 : E 5 1 5.67 3 10 28 W /m2 K4 2 1 6000 K 2 4 5 73,483,200 W /m2
5 459 W /m2
Wien’s constant (C) is equal to 2898 mmK. If we use the Sun and Earth as examples, we find: l max 1 Sun 2 5
2898 mmK 5 0.483 mm 6000 K
and:
The Sun has a temperature that is approximately 20 times higher than that of Earth and thus it emits approximately 160,000 times more radiation per unit area. This makes sense because 204 5 160,000. Wien’s Displacement Law Wien’s displacement law describes mathematically the relationship between the temperature (T ) of a radiating body and its wavelength of maximum emission 1 lmax 2 :
lmax 1 Earth 2 5
2898 mmK 5 9.66 mm 300 K
Note that the Sun radiates its maximum energy within the visible portion of the electromagnetic spectrum. The cooler Earth radiates its maximum energy in the infrared portion of the electromagnetic spectrum.
*
lmax 5 C /T
2. Hotter objects radiate more total energy per unit area than do colder objects. The Sun, which has a surface temperature of 6000 K (10,000°F), emits about 160,000 times more energy per unit area than does Earth, which has an average surface temperature of 288 K (59°F). (This concept is called the Stefan–Boltzmann law and is expressed mathematically in Box 2–2.) 3. Hotter objects radiate more energy in the form of short-wavelength radiation than do cooler objects. We can visualize this law by imagining a piece of metal that, when heated sufficiently (as occurs in a blacksmith’s shop), produces a white glow. As it cools, the metal emits more of its energy in longer wavelengths and glows a reddish color. Eventually, no light is given off, but if you place your hand near the metal, the still longer infrared radiation will be detectable as heat. The Sun radiates maximum energy at 0.5 micrometer, which is in the visible range (Figure 2–14). The maximum radiation emitted from Earth occurs at a wavelength of 10 micrometers, well within the infrared (heat) range. Because the maximum Earth radiation is roughly 20 times longer than the maximum solar radiation, it is often referred to as longwave radiation, whereas solar radiation is called shortwave radiation. (This concept, known as Wien’s displacement law, is expressed mathematically in Box 2–2.) 4. Objects that are good absorbers of radiation are also good emitters. Earth’s surface and the Sun are nearly perfect radiators because they absorb and radiate with nearly 100 percent efficiency. By contrast, the gases that compose our atmosphere are selective absorbers and
Professor Carbone is a faculty member in the Department of Geography at the University of South Carolina.
emitters of radiation. For some wavelengths the atmosphere is nearly transparent (little radiation absorbed). For others, however, it is nearly opaque (absorbs most of the radiation that strikes it). Experience tells us that the atmosphere is quite transparent to visible light emitted by the Sun because it readily reaches Earth’s surface. To summarize, although the Sun is the ultimate source of radiant energy, all objects continually radiate energy over a range of wavelengths. Hot objects, such as the Sun, emit mostly shortwave (high-energy) radiation. By contrast, most objects at everyday temperatures (Earth’s surface and atmosphere) emit longwave (low-energy) radiation. Objects that are good absorbers of radiation, such as Earth’s surface, are also good emitters. By contrast, most gases are good absorbers (emitters) only in certain wavelengths but poor absorbers in other wavelengths.
Concept Check 2.3 1 Describe the three basic mechanisms of energy transfer. Which mechanism is least important meteorologically?
2 What is the difference between convection and advection? 3 Compare visible, infrared, and ultraviolet radiation. For each, indicate whether it is considered shortwave or longwave radiation.
4 In what part of the electromagnetic spectrum does the Sun radiate maximum energy? How does this compare to Earth?
5 Describe the relationship between the temperature of a radiating body and the wavelengths it emits.
Chapter 2 Heating Earth’s Surface and Atmosphere
49
The Ultraviolet Index*
M
ost people welcome sunny weather. On warm days, when the sky is cloudless and bright, many spend a great deal of time outdoors “soaking up” the sunshine (Figure 2–B). For many, the goal is to develop a dark tan, one that sunbathers often describe as “healthy looking.” Ironically, there is strong evidence that too much sunshine (specifically, too much ultraviolet radiation) can lead to serious health problems, mainly skin cancer and cataracts of the eyes. Since June 1994 the National Weather Service (NWS) has issued the next-day ultraviolet index (UVI) for the United States to warn the public of potential health risks of exposure to sunlight (Figure 2–C). The UV index is determined by taking into account the predicted cloud cover and reflectivity of the surface, as well as the Sun angle and atmospheric depth for each forecast location. Because atmospheric ozone strongly absorbs ultraviolet radiation, the extent of the ozone layer is also considered. The UVI values lie on a scale from 0to 15, with larger values representing greatest risk. The U.S. Environmental Protection Agency has established five exposure categories
based on UVI values— Low, Moderate, High, 0 1 2 Very High, and Extreme (Table 2–B). Precautionary measures have been developed for each category. The public is advised to minimize outdoor activities when the UVI is Very High or Extreme. Sunscreen with a sun-protection factor (SPF) of 15 or higher is recommended for all exposed June 8, 2008 skin. This is especially important after swimming or while sunbathing, even on cloudy days with the UVI in the Low category.
UV Index 3
The public is advised to minimize outdoor activities when the UVI is Very High or Extreme. Table 2–B shows the range of minutes to burn for the most susceptible skin type (pale or milky white) for each exposure category. Note that the exposure that results in sunburn varies from over 60 minutes for the Low category to less than 10 minutes for the Extreme category. It takes approximately five
4
5
6
7
8
9 10 11 12 13 14 15
FIGURE 2–C UV index forecast for June 8, 2008.To view the current UVI forecast, go to www.epa.gov/sunwise/uvindex.html.
times longer to cause sunburn of the least susceptible skin type, brown to dark. The most susceptible skin type develops red sunburn, painful swelling, and skin peeling when exposed to excessive sunlight. By contrast, the least susceptible skin type rarely burns and shows very rapid tanning response. *Based on material prepared by Professor Gong-Yuh Lin, a faculty member in the Department of Geography atCalifornia State University, Northridge.
TABLE 2–B The UV Index: Minutes to Burn for the Most Susceptible Skin Type
UVI Value
FIGURE 2–B Exposing sensitive skin to too much solar ultraviolet radiation has potential health risks. (Photo by Eddie Gerald/Rough Guides)
Exposure Category
Description
Minutes to Burn . 60
0–2
Low
Low danger from the Sun’s UV rays for the average person.
3–5
Moderate
Moderate risk from unprotected Sun exposure. Take precautions during the midday, when Sun is strongest.
40–60
6–7
High
Protection against sunburn is needed. Cover up, wear a hat and sunglasses, and use sunscreen.
25–40
8–10
Very High
Try to avoid the Sun between 11 am and 4 pm. Otherwise, cover up and use sunscreen.
10–25
Extreme
Take all precautions. Unprotected skin will burn in minutes. Do not pursue outdoor activities if possible. If outdoors, apply sunscreen liberally every 2 hours.
11–15
, 10
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The Atmosphere: An Introduction to Meteorology
What Happens to Incoming Solar Radiation? ATMOSPHERE
Heating Earth’s Surface and Atmosphere ▸ What Happens to Incoming Solar Radiation?
(Figure 2–16b). Whether solar radiation is reflected or scattered depends largely on the size of the intervening particles and the wavelength of the light. Reflection and Earth’s Albedo About 30 percent of the solar energy that reaches our planet is reflected back to space (Figure 2–15). Included in this figure is the amount sent skyward by backscattering. This energy is lost to Earth and does not play a role in heating the atmosphere or Earth’s surface. The fraction of radiation that is reflected by an object is called its albedo. The albedo for Earth as a whole (planetary albedo) is 30 percent. The amount of light reflected from Earth’s land–sea surface represents only about 5 percent of the total planetary albedo (Figure 2–15). Not surprisingly, clouds are largely responsible for most of Earth’s “brightness” as seen from space. The high reflectivity of clouds is experienced when looking down on a cloud during an airline flight. In comparison to Earth, the Moon, which has neither clouds nor an atmosphere, has an average albedo of only 7 percent. Although a full Moon appears bright, the much brighter and larger Earth would, by comparison, provide far more light for an astronaut’s “Earth-lit” Moon walk at night. Figure 2–17 gives the albedos for various surfaces. Fresh snow and thick clouds have high albedos (good reflectors). By contrast, dark soils and parking lots have low albedos and thus absorb much of the radiation they receive. In the case of a lake or the ocean, note that the angle at which the Sun’s rays strike the water surface greatly affects its albedo.
When radiation strikes an object, three different things may occur simultaneously. First, some of the energy may be absorbed. Recall that when radiant energy is absorbed, the molecules begin to vibrate faster, which causes an increase in temperature. The amount of energy absorbed by an object depends on the intensity of the radiation and the object’s absorptivity. In the visible range, the degree of absorptivity is largely responsible for the brightness of an object. Surfaces that are good absorbers of all wavelengths of visible light appear black in color, whereas light-colored surfaces have a much lower absorptivity. That is why wearing light-colored clothing on a sunny summer day may help keep you cooler. Second, substances such as water and air, which are transparent to certain wavelengths of radiation, may simply transmit energy—allowing it to pass through without being absorbed. Third, some radiation may “bounce off” the object without being absorbed or transmitted. In summary, radiation may be absorbed, transmitted, or redirected (reflected or scattered). Figure 2–15 shows the fate of incoming solar radiation averaged for the entire globe. On average, about 50 percent of incoming solar energy is absorbed at Earth’s surface. Another 30 percent is reflected and scattered back to space by the atmosphere, clouds, and reflective surfaces such as snow and water, and about 20 percent is absorbed by clouds and atmospheric gases. What determines whether solar radiation will Solar radiation be transmitted to the surface, scattered, reflected 100% back to space, or absorbed by the gases and particles in the atmosphere? As you will see, it depends greatly upon the wavelength of the radiation, as well as the size and nature of the intervening material.
Reflection and Scattering
30% lost to space by reflection and scattering 5% backscattered to space by the atmosphere
20% reflected from clouds
Reflection is the process whereby light bounces back from an object at the same angle and intensity (Figure 2–16a). By contrast, scattering produces a larger number of weaker rays, traveling in different directions. Scattering disperses light both forward and backward (backscattering)
20% of radiation absorbed by atmosphere and clouds
Figure 2–15 Average distribution of incoming solar radiation, by percentage. More solar energy is absorbed by Earth’s surface than by the atmosphere.
50% of direct and diffused radiation absorbed by land and sea
5% reflected from land–sea surface
Chapter 2 Heating Earth’s Surface and Atmosphere
Scattering
51
Thick clouds 70 – 80% Thin clouds 25 – 30%
Snow 50 – 90%
Reflection
Forest 5 – 10%
Grass 5 – 25% (a)
(b)
Figure 2–16 Reflection and scattering. (a) Reflected light bounces back from a surface at the same angle at which it strikes that surface and with the same intensity. (b) When a beam of light is scattered, it results in a larger number of weaker rays, traveling in different directions. Usually more energy is scattered in the forward direction than is backscattered.
Scattering and Diffused Light Although incoming solar radiation travels in a straight line, small dust particles and gas molecules in the atmosphere scatter some of this energy in different directions. The result, called diffused light, explains how light reaches the area under the limbs of a tree and how a room is lit in the absence of direct sunlight. Further, on clear days scattering accounts for the brightness and the blue color of the daytime sky. In contrast, bodies like the Moon and Mercury, which are without atmospheres, have dark skies and “pitch-black” shadows, even during daylight hours. Overall, about one-half of solar radiation that is absorbed at Earth’s surface arrives as diffused (scattered) light. Blue Skies and Red Sunsets Recall that sunlight appears white, but it is composed of all colors. Gas molecules more effectively scatter blue and violet light that have shorter wavelengths than they scatter red and orange. This accounts for the blue color of the sky and the orange and red colors seen at sunrise and sunset (Figure 2–18). On clear days, you can look in any direction away from the direct Sun and see a blue sky, the wavelength of light more readily scattered by the atmosphere. Conversely, the Sun appears to have an orange-to-reddish hue when viewed near the horizon (Figure 2–19). This is the result of the great distance solar radiation must travel through the atmosphere before it reaches your eyes (see Table 2–1). During its travel, most of the blue and violet
Light roof 35 – 50%
Wet plowed field 15 – 25% Asphalt 5 – 10%
Water 5 – 80% (varies with sun angle)
Dark roof 10 – 15%
Sandy beach 20 – 40%
Figure 2–17 Albedo (reflectivity) of various surfaces. In general, light-colored surfaces tend to be more reflective than dark-colored surfaces and thus have higher albedos.
wavelengths are scattered out. Consequently, the light that reaches your eyes consists mostly of reds and oranges. The reddish appearance of clouds during sunrise and sunset also results because the clouds are illuminated by light from which the blue color has been removed by scattering.
Figure 2–18 At sunset, clouds often appear red because they are illuminated by sunlight in which most of the blue light has been lost due to scattering. (Photo by Michael Collier)
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The Atmosphere: An Introduction to Meteorology
Absorption of Solar Radiation
Midday sun
Midday– observer sees whitish sun, blue sky
Sunset– observer sees reddish sunset
Figure 2–19 Short wavelengths (blue and violet) of visible light are scattered more effectively than are longer wavelengths (redand orange). Therefore, when the Sun is overhead, an observer can look in any direction and see predominantly blue light that was selectively scattered by the gases in the atmosphere. By contrast, at sunset, the path that light must take through the atmosphere is much longer. Consequently, most of the blue light is scattered before it reaches an observer. Thus, the Sun appears reddish in color.
The most spectacular sunsets occur when large quantities of fine dust or smoke particles penetrate the stratosphere. For three years after the great eruption of the Indonesian volcano Krakatau in 1883, brilliant sunsets occurred worldwide. In addition, the European summer that followed this colossal explosion was cooler than normal, which has been attributed to the loss of incoming solar radiation due to an increase in backscattering. Large particles associated with haze, fog, and smog scatter light more equally in all wavelengths. Because no color is predominant over any other, the sky appears white or gray on days when large particles are abundant. Scattering of sunlight by haze, water droplets, or dust particles makes it possible for us to observe bands (or rays) of sunlight called crepuscular rays. These bright fan-shaped bands are most commonly seen when the Sun shines through a break in the clouds, as shown in Figure 2–20. Crepuscular rays can also be observed around twilight, when towering clouds cause alternating lighter and darker bands (light rays and shadows) to streak across the sky. In summary, the color of the sky gives an indication of the number of large or small particles present. Numerous small particles produce red sunsets, whereas large particles produce white (gray) skies. Thus, the bluer the sky, the less polluted, or dryer, the air.
Although Earth’s surface is a relatively good absorber (effectively absorbing most wavelengths of solar radiation), the atmosphere is not. As a result, only 20 percent of the solar radiation that reaches Earth is absorbed by gases in the atmosphere (see Figure 2–15). Much of the remaining incoming solar radiation transmits through the atmosphere and is absorbed by the Earth’s land–sea surface. The atmosphere is a less effective absorber because gases are selective absorbers (and emitters) of radiation. Sun at sunset Freshly fallen snow is another example of a selective absorber. Snow is a poor absorber of visible light (reflecting up to 85 percent) and, as a result, the temperature directly above a snow-covered surface is colder than it would otherwise be because much of the incoming radiation is reflected away. By contrast, snow is a very good absorber (absorbing up to 95 percent) of the infrared (heat) radiation that is emitted from Earth’s surface. As the ground radiates heat upward, the lowest layer of snow absorbs this energy and, in turn, radiates most of the energy downward. Thus, the depth at which a winter’s frost can penetrate into the ground is much less when the ground is snow covered than in an equally cold region without snow—giving credence to the statement “The ground is blanketed with snow.” Farmers who plant winter wheat desire a deep snow cover because it insulates their crops from bitter winter temperatures.
Figure 2–20 Crepuscular rays are produced when haze scatters light. Crepuscular rays are most commonly seen when the Sun shines through a break in the clouds. (Photo by Tetra Images/ Jupiter Images)
Chapter 2 Heating Earth’s Surface and Atmosphere
Concept Check 2.4 1 Prepare and label a simple sketch that shows what happens to incoming solar radiation.
2 Why does the daytime sky usually appear blue? 3 Why may the sky have a red or orange hue near sunrise or sunset?
4 What is the primary factor that causes the albedo of some materials to vary depending on their location or time of day?
53
Earth’s surface is emitted at wavelengths between 2.5 and 30 micrometers, placing it in the long end of the infrared band of the electromagnetic spectrum. Heating of the atmosphere requires an understanding of how gases interact with the short wavelength incoming solar radiation and the long wavelength outgoing radiation emitted by Earth (Figure 2–21, top).
Heating the Atmosphere When a gas molecule absorbs radiation, this energy is transformed into internal molecular motion, which is detectable as a rise in temperature (sensible heat). The lower part of Figure 2–21 gives the absorptivity of the principal atmospheric gases. Note that nitrogen, the most
Radiation intensity
Solar radiation
Radiation emitted by Earth
100% Nitrogen N2
Eye on the Atmosphere
Absorptivity
0% 100%
0% 100%
Water vapor H2O 0% 100%
0%
This image was taken by astronauts aboard the International Space Station as it traveled over the west coast of South America. On average, these astronauts experience 16 sunrises and sunsets during a 24-hour orbital period. The separation between day and night is marked by a line called the terminator (NASA). Question 1 Locate the terminator in this image. Would you describe it as a sharp line? Explain.
Carbon dioxide CO2
0.1
Oxygen and ozone O2 and O3 0.2
0.3 0.4
0.6 0.8 1
Visible radiation
Ultraviolet
Visible
Shortwave radiation
1.5 2
3
4 5 6
8 10
20 30
Atmospheric window
Infrared Longwave radiation
Question 2 Are the astronauts looking at a sunrise or a sunset?
The Role of Gases in the Atmosphere ATMOSPHERE
Heating Earth’s Surface and Atmosphere ▸ The Greenhouse Effect
Figure 2–21 shows that the majority of solar radiation is emitted in wavelengths shorter than 2.5 micrometers— shortwave radiation. By contrast, most radiation from
Figure 2–21 The effectiveness of selected gases of the atmosphere in absorbing incoming shortwave radiation (left side of graph) and outgoing longwave terrestrial radiation (right side). The blue areas represent the percentage of radiation absorbed by the various gases. The atmosphere as a whole is quite transparent to solar radiation between 0.3 and 0.7 micrometer, which includes the band of visible light. Most solar radiation falls in this range, explaining why a large amount of solar radiation penetrates the atmosphere and reaches Earth’s surface. Also, note that longwave infrared radiation in the zone between 8 and 12 micrometers can escape the atmosphere most readily. This zone is called the atmospheric window.
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The Atmosphere: An Introduction to Meteorology
abundant constituent in the atmosphere (78 percent), is a relatively poor absorber of incoming solar radiation. The only significant absorbers of incoming solar radiation are water vapor, oxygen, and ozone, which account for most of the solar energy absorbed directly by the atmosphere. Oxygen and ozone are efficient absorbers of high-energy, shortwave radiation. Oxygen removes most of the shorterwavelength UV radiation high in the atmosphere, and ozone absorbs UV rays in the stratosphere between 10 and 50 kilometers (6 and 30 miles). The absorption of UV energy in the stratosphere accounts for the high temperatures experienced there. More importantly, without the removal of most UV radiation, human life would not be possible because UV energy disrupts our genetic code. Looking at the bottom of Figure 2–21, you can see that for the atmosphere as a whole, none of the gases are effective absorbers of radiation with wavelengths between 0.3 and 0.7 micrometer. This region of the spectrum corresponds to the visible light band, which constitutes about 43 percent of the energy radiated by the Sun. Because the atmosphere is a poor absorber of visible radiation, most of this energy is transmitted to Earth’s surface. Thus, we say that the atmosphere is nearly transparent to incoming solar radiation and that direct solar energy is not an effective “heater” of Earth’s atmosphere.
Students Sometimes Ask . . . What causes leaves on deciduous trees to change color each fall? The leaves of all deciduous trees contain the pigment chlorophyll, which gives them a green color. The leaves of some trees also contain the pigment carotene, which is yellow, and still others produce a class of pigments that appear red in color. During summer, leaves are factories that generate sugar from carbon dioxide and water by the action of light on chlorophyll. As the dominant pigment, chlorophyll causes the leaves of most trees to appear green. The shortening days and cool nights of autumn trigger changes in deciduous trees. With a drop in chlorophyll production, the green color of the leaves fades, allowing other pigments to be seen. If the leaves contain carotene, as do birch and hickory, they will change to bright yellow. Other trees, such as red maple and sumac, display the brightest reds and purples in the autumn landscape.
We can also see in Figure 2–21 that the atmosphere is generally a relatively efficient absorber of longwave (infrared) radiation emitted by Earth (see the bottom right of Figure 2–21). Water vapor and carbon dioxide are the principal absorbing gases, with water vapor absorbing about 60 percent of the radiation emitted by Earth’s surface. Therefore, water vapor, more than any other gas, accounts for the warm temperatures of the lower troposphere, where it is most highly concentrated. Although the atmosphere is an effective absorber of radiation emitted by Earth’s surface, it is nevertheless quite
transparent to the band of radiation between 8 and 12 micrometers. Notice in Figure 2–21 (lower right) that the gases in the atmosphere (N2, CO2, H2O) absorb minimal energy in these wavelengths. Because the atmosphere is transparent to radiation between 8 and 12 micrometers, much as window glass is transparent to visible light, this band is called the atmospheric window. Although other “atmospheric windows” exist, the one located between 8 and 12 micrometers is the most significant because it is located where Earth’s radiation is most intense. By contrast, clouds that are composed of tiny liquid droplets (not water vapor) are excellent absorbers of the energy in the atmospheric window. Clouds absorb outgoing radiation and radiate much of this energy back to Earth’s surface. Thus, clouds serve a purpose similar to window blinds because they effectively block the atmospheric window and lower the rate at which Earth’s surface cools. This explains why nighttime temperatures remain higher on cloudy nights than on clear nights. Because the atmosphere is largely transparent to solar (shortwave) radiation but more absorptive of the longwave radiation emitted by Earth, the atmosphere is heated from the ground up. This explains the general drop in temperature with increased altitude in the troposphere. The farther from the “radiator” (Earth’s surface), the colder it gets. On average, the temperature drops 6.5°C for each kilometer (3.5°F per 1000 feet) increase in altitude, a figure known as the normal lapse rate. The fact that the atmosphere does not acquire the bulk of its energy directly from the Sun but is heated by Earth’s surface is of utmost importance to the dynamics of the weather machine.
The Greenhouse Effect Research of “airless” planetary bodies such as the Moon have led scientists to determine that if Earth had no atmosphere, it would have an average surface temperature below freezing. However, Earth’s atmosphere “traps” some of the outgoing radiation, which makes our planet habitable. The extremely important role the atmosphere plays in heating Earth’s surface has been named the greenhouse effect. As discussed earlier, cloudless air is largely transparent to incoming shortwave solar radiation and, hence, transmits it to Earth’s surface. By contrast, a significant fraction of the longwave radiation emitted by Earth’s land–sea surface is absorbed by water vapor, carbon dioxide, and other trace gases in the atmosphere. This energy heats the air and increases the rate at which it radiates energy, both out to space and back toward Earth’s surface. Without this complicated game of “pass the hot potato,” Earth’s average temperature would be –18°C (0°F) rather than the current temperature of 15°C (59°F) (Figure 2–22). These absorptive gases in our atmosphere make Earth habitable for humans and other life forms. This natural phenomenon was named the greenhouse effect because greenhouses are heated in a similar manner
Chapter 2 Heating Earth’s Surface and Atmosphere
All outgoing longwave energy is reradiated directly back to space
Incoming shortwave solar radiation (a) Airless bodies like the Moon
Figure 2–22 The greenhouse effect. (a) All incoming solar radiation reaches the surface of airless bodies such as the Moon. However, all of the energy that is absorbed by the surface is radiated directly back to space. This causes the lunar surface to have a much lower average surface temperature than Earth. Because the Moon experiences days and nights that are about 2 weeks long, the lunar days are hot and the nights are frigid. (b) On bodies with modest amounts of greenhouse gases, such as Earth, much of the short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the surface. This energy is then emitted from the surface as longer-wavelength radiation, which is absorbed by greenhouse gases in the atmosphere. Some of the energy absorbed will be radiated back to the surface and is responsible for keeping Earth’s surface 33°C (59°F) warmer than it would be otherwise. (c) Bodies with abundant greenhouse gases, such as Venus, experience extraordinary greenhouse warming, which is estimated to raise its surface temperature by 523°C (941°F).
(Figure 2–22). The glass in a greenhouse allows shortwave solar radiation to enter and be absorbed by the objects inside. These objects, in turn, radiate energy but at longer wavelengths, to which glass is nearly opaque. The heat, therefore, is “trapped” in the greenhouse. Although this analogy is widely used, it has been shown that air inside greenhouses attains higher temperatures than outside air in part due to the restricted exchange of warmer air inside and cooler air outside. Nevertheless, the term “greenhouse effect” is still used to describe atmospheric heating. Media reports frequently and erroneously identify the greenhouse effect as the “villain” in the global warming problem. However, the greenhouse effect and global warming are different concepts. Without the greenhouse effect, Earth would be uninhabitable. Scientists have mounting evidence that human activities (particularly the release of carbon dioxide into the atmosphere) are responsible for a rise in global temperatures (see Chapter 14). Thus, hu-
Some outgoing longwave radiation absorbed by greenhouse gases
55
Greenhouse gases reradiate some energy Earthward
Incoming shortwave solar radiation (b) Bodies with modest amounts of greenhouse gases like Earth
Most outgoing longwave radiation absorbed by greenhouse gases
Greenhouse gases reradiate considerable energy toward the Venusian surface
Incoming shortwave solar radiation (c) Bodies with abundant greenhouse gases like Venus
mans are compounding the effects of an otherwise natural process (the greenhouse effect). It is incorrect to equate the greenhouse phenomenon, which makes life possible, with global warming—which involves undesirable changes to our atmosphere caused by human activities.
Concept Check 2.5 1 Explain why the atmosphere is heated chiefly by radiation from Earth’s surface rather than by direct solar radiation.
2 Which gases are the primary heat absorbers in the lower atmosphere? Which gas is most influential in weather?
3 What is the atmospheric window? How is it “closed”? 4 How does Earth’s atmosphere act as a greenhouse? 5 What is the “villain” in the global warming problem?
The Atmosphere: An Introduction to Meteorology
Annual Energy Balance
Students Sometimes Ask . . .
Figure 2–23 illustrates Earth’s annual energy budget. For simplicity we will use 100 units to represent the solar radiation intercepted at the outer edge of the atmosphere. You have already seen that, of the total radiation that reaches Earth, roughly 30 units (30 percent) are reflected and scattered back to space. The remaining 70 units are absorbed, 20 units within the atmosphere and 50 units by Earth’s land–sea surface. How does Earth transfer this energy back to space? If all of the energy absorbed by our planet were radiated directly and immediately back to space, Earth’s heat budget would be simple—100 units of radiation received and 100 units returned to space. In fact, this does happen over time (minus small quantities of energy that become locked up in biomass that may eventually become fossil fuel). What complicates the heat budget is the behavior of certain greenhouse gases, particularly water vapor and carbon dioxide. As you learned, these greenhouse gases absorb a large share of outward-directed infrared radiation and radiate much of that energy back to Earth. This “recycled” energy significantly increases the radiation received by Earth’s surface. In addition to the 50 units received directly from the Sun, Earth’s surface receives longwave radiation emitted downward by the atmosphere (94 units).
Is Venus so much hotter than Earth because it is closer to the Sun? Proximity to the Sun is actually not the primary factor. On Earth, water vapor and carbon dioxide are the primary greenhouse gases and are responsible for elevating Earth’s average surface temperature by 33°C (59°F). However, greenhouse gases make up less than 1 percent of Earth’s atmosphere. By contrast, the Venusian atmosphere is much denser and consists of 97 percent carbon dioxide. Thus, the Venusian atmosphere experiences extraordinary greenhouse warming, which is estimated to raise its surface temperature by 523°C (941°F)—hot enough to melt lead.
Earth’s Heat Budget Globally, Earth’s average temperature remains relatively constant, despite seasonal cold spells and heat waves. This stability indicates that a balance exists between the amount of incoming solar radiation and the amount of radiation emitted back to space; otherwise, Earth would be getting progressively colder or progressively warmer. The annual balance of incoming and outgoing radiation is called Earth’s heat budget.
Incoming Solar Radiation +100 units
Outgoing radiation lost to space –100 units –30 units of solar radiation reflected back and scattered to space
–12 units of longwave radiation to which the atmosphere is transparent
+23 units released to the atmosphere by condensation (latent heat)
+7 units absorbed by the atmosphere
+8 units of longwave radiation absorbed by the atmosphere
94 units
+20 units absorbed by atmosphere and clouds
–58 units emitted to space by the atmosphere
+50 units of solar radiation absorbed by Earth's surface
–7 units lost from Earth’s surface by conduction and convection
+
–23 units lost by evaporation
+
Figure 2–23 Heat budget of Earth and the atmosphere. These estimates of the average global energy budget come from satellite observations and radiation studies. As more data are accumulated, these numbers will be modified. (Data from Kiehl, Trenberth, Liou, and others)
114 units
56
–20 units lost by longwave = radiation
Longwave energy exchange between Earth's surface and the atmosphere
–50 units of energy lost by Earth's surface
Chapter 2 Heating Earth’s Surface and Atmosphere
This infrared (IR) image produced by the GOES-14 satellite displays cold objects as bright white and hot objects as black. The hottest (blackest) features shown are land surfaces, and the coldest (whitest) features are the tops of towering storm clouds. Recall that we cannot see infrared (thermal) radiation,
57
but we have developed artificial detectors capable of extending our vision into the long-wavelength portion of the electromagnetic spectrum.
IR image. One is a well-developed tropical stormnamed Hurricane Bill. Can you locate Hurricane Bill?
Question 1 Several areas of cloud development and potential storms are shown on this
Question 2 What is an advantage of IR images over visible images?
A balance is maintained because all the energy absorbed by Earth’s surface is returned to the atmosphere and eventually radiated back to space. Earth’s surface loses energy through a variety of processes: the emission of longwave radiation; conduction and convection; and energy loss to Earth’s surface through the process of evaporation—latent heat (Figure 2–23). Most of the longwave radiation emitted skyward is re-absorbed by the atmosphere. Conduction results in the transfer of energy between Earth’s surface to the air directly above, while convection carries the warm air located near the surface upward as thermals (7 units). Earth’s surface also loses a substantial amount of energy (23 units) through evaporation. This occurs because energy is required for liquid water molecules to leave the
surface of a body of water and change to its gaseous form, water vapor. The energy lost by a water body is carried into the atmosphere by molecules of water vapor. Recall that the heat used to evaporate water does not produce a temperature change and is referred to as latent heat (hidden heat). If the water vapor condenses to form cloud droplets, the energy will be detectable as sensible heat (heat we can feel and measure with a thermometer). Thus through the process of evaporation, water molecules carry latent heat into the atmosphere, where it is eventually released. In summary, a careful examination of Figure 2–23 confirms that the quantity of incoming solar radiation is, over time, balanced by the quantity of longwave radiation that is radiated back to space.
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The Atmosphere: An Introduction to Meteorology
Box 2-3
Solar Power
Nearly 95 percent of the world’s energy needs are derived from fossil fuels, primarily oil, coal, and natural gas. Current estimates indicate that, at the present rate of consumption, we have enough fossil fuels to last about 150 years. However, with the rapid increase in demand by developing countries, the rate of consumption continues to climb. Thus, reserves will be in short supply sooner rather than later. How can a growing demand for energy be met without radically altering the planet we inhabit? Although no clear answer has yet emerged, we must consider greater use of alternate energy sources, such as solar and wind power. The term solar energy generally refers to the direct use of the Sun’s rays to supply energy for the needs of people. The simplest, and perhaps most widely used, passive solar collectors are south-facing windows. As shortwave sunlight passes through the glass, its energy is absorbed by objects in the room that, in turn, radiate longwave heat that warms the air in the room. More elaborate systems used for home heating involve active solar collectors. These roof-mounted devices are normally large blackened boxes that are covered with glass. The heat they collect can be transferred to where it is needed by circulating air or fluids through pipes. Solar collectors are also used successfully to heat water for domestic and commercial needs. In Israel, for example, about 80 percent of all homes are equipped with solar collectors that provide hot water. Research is currently under way to improve the technologies for concentrating sunlight. One method uses parabolic troughs as solar energy collectors. Each collector resembles a large tube that has been cut in half. Their highly polished surfaces reflect sunlight onto a collection pipe. A fluid (usually oil) runs through the pipe and is heated by the concentrated sunlight. The fluid can reach temperatures of over 400°F and is typically used to make steam that drives a turbine to produce electricity. Another type of collector uses photovoltaic (solar) cells that convert the Sun’s energy
FIGURE 2–D Nearly cloudless deserts, such as California’s Mojave Desert, are prime sites for photovoltaic cells that convert solar radiation directly into electricity. (Photo by Jim West/Alamy)
directly into electricity. Photovoltaic cells are usually connected together to create solar panels in which sunlight knocks electrons into higher-energy states, to produce electricity (Figure 2–D). For many years, solar cells were used mainly to power calculators and novelty devices. Today, however, large photovoltaic power stations are connected to electric grids to supplement other powergenerating facilities. The leading countries in photovoltaic capacity are Germany, Japan, Spain, and the United States. The high cost of solar cells makes generating solar electricity more expensive than electricity created by other sources. As the cost of fossil fuels continues to increase, advances in photovoltaic technology should narrow the price differential. A new technology being developed, called the Stirling dish, converts thermal energy to electricity by using a mirror array to focus the Sun’s rays on the receiver end of a Stirling engine (Figure 2–E). The internal side of a receiver then heats hydrogen gas, causing it to expand. The pressure created by the expanding gas drives a piston, which turns a small electric generator.
FIGURE 2–E This Stirling dish is located near Phoenix, Arizona. (Photo by Brian Green/Alamy)
Chapter 2 Heating Earth’s Surface and Atmosphere
Latitudinal Heat Balance Because incoming solar radiation is roughly equal to the amount of outgoing radiation, on average, worldwide temperatures remain constant. However, the balance of Deficit
N. Pole Outgoing radiation
38° latitude
Incoming radiation
incoming and outgoing radiation that is applicable for the entire planet is not maintained at each latitude. Averaged over the entire year, a zone around Earth that lies within 38° latitude of the equator receives more solar radiation than is lost to space (Figure 2–24). The opposite is true for higher latitudes, where more heat is lost through radiation emitted by Earth than is received from the Sun. We might conclude that the tropics are getting hotter and the poles are getting colder. But that is not the case. Instead, the global wind systems and, to a lesser extent, the oceans act as giant thermal engines, transferring surplus heat from the tropics poleward. In effect, the energy imbalance drives the winds and the ocean currents. It should be of interest to those who live in the middle latitudes—in the Northern Hemisphere, from the latitude of New Orleans at 30° north to the latitude of Winnipeg, Manitoba, at 50° north—that most heat transfer takes place across this region. Consequently, much of the stormy weather experienced in the middle latitudes can be attributed to this unending transfer of heat from the tropics toward the poles. These processes are discussed in more detail in later chapters.
Surplus
Equator
Figure 2–24 Latitudinal heat balance averaged over the entire year. In the zone extending 38° on both sides of the equator, the amount of incoming solar radiation exceeds the loss from outgoing Earth radiation. The reverse is true for the middle and high polar latitudes, where losses from outgoing Earth radiation exceed gains from incoming solar radiation.
59
Concept Check 2.6 1 The tropics receive more solar radiation than is lost. Why then don’t the tropics keep getting hotter?
2 What two phenomena are driven by the imbalance of heating that exists between the tropics and poles?
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The Atmosphere: An Introduction to Meteorology
Give It Some Thought 1. How would our seasons be affected if Earth’s axis
6. Which of the three mechanisms of heat transfer is
were not inclined 2312° to the plane of its orbit but were instead perpendicular? 2. Describe the seasons if Earth’s axis were inclined 40°. Where would the Tropics of Cancer and Capricorn be located? How about the Arctic and Antarctic Circles? 3. The accompanying four diagrams (labeled a–d) are intended to illustrate the Earth–Sun relationships that produce the seasons. a. Which one of these diagrams most accurately shows this relationship? b. Identify what is inaccurately shown in each of the other three diagrams.
most significant in each of the following situations: a. Driving a car with the seat heater turned on b. Sitting in an outdoor hot tub c. Lying inside a tanning bed d. Driving a car with the air conditioning turned on 7. During a “shore lunch” on a fishing trip to a remote location, the fishing guide will sometimes place a pail of lake water next to the cooking fire, as shown in the accompanying illustration. When the water in the pail begins to boil, the guide will lift the pail from the fire and “impress” the guests by placing their other hand on the bottom. Use what you have learned about the three mechanisms of heat transfer to explain why the guide’s hands aren’t burned by touching the bottom of the pail.
8. The Sun shines continually at the North Pole for six
months, from the spring equinox until the fall equinox, yet temperatures never get very warm. Explain why this is the case. 9. The accompanying image shows an area of our galaxy where stars having surface temperatures much hotter than the Sun have recently formed. Imagine that an Earth-like planet formed around one of these stars, at a distance where it receives the same intensity of light as Earth. Use the laws of radiation to explain why this planet may not provide a habitable environment for humans.
4. On what date is Earth closest to the Sun? On that date,
what season is it in the Northern Hemisphere? Explain this apparent contradiction. 5. When a person in the United States watches the Sun move through the sky during a typical day, he or she sees it travel from left to right. However, on the same day, a person in Australia will see the Sun travel across the sky from right to left. Explain or make a sketch that illustrates why this difference occurs.
(NASA)
Chapter 2 Heating Earth’s Surface and Atmosphere
10. Rank the following according to the wavelength
61
the ash and debris this volcano spewed high into the atmosphere?
of radiant energy each emits, from the shortest wavelength to the longest: a. A light bulb with a filament glowing at 4000°C b. A rock at room temperature c. A car engine at 140°C 11. Figure 2–15 shows that about 30 percent of the Sun’s energy is reflected or scattered back to space. If Earth’s albedo were to increase to 50 percent, how would you expect Earth’s average surface temperature to change? 12. Explain why Earth’s equatorial regions are not becoming warmer, despite the fact that they receive more incoming solar radiation than they radiate back to space. 13. The accompanying photo shows the explosive 1991 eruption of Mount Pinatubo in the Philippines. How would you expect global temperatures to respond to
(U.S. Geological Survey)
HEATING EARTH’S SURFACE AND ATMOSPHERE IN REVIEW ●
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The seasons are caused by changes in the angle at which the Sun’s rays strike the surface and the changes in the length of daylight at each latitude. These seasonal changes are the result of the tilt of Earth’s axis as it revolves around the Sun. Energy is the ability to do work. The two major categories of energy are (1) kinetic energy, which can be thought of as energy of motion, and (2) potential energy, energy that has the capability to do work. Temperature is a measure of the average kinetic energy of the atoms or molecules in a substance. Heat is the transfer of energy into or out of an object because of temperature differences between that object and its surroundings. Latent heat (hidden heat) is the energy involved when water changes from one state of matter to another. During evaporation, for example, energy is stored by the escaping water vapor molecules, and this energy is eventually released when water vapor condenses to form water droplets in clouds. Conduction is the transfer of heat through matter by electron and molecular collisions between molecules. Because air is a poor conductor, conduction is significant only between Earth’s surface and the air immediately in contact with the surface. Convection is heat transfer that involves the actual movement or circulation of a substance. Convection is an important mechanism of heat transfer in the atmosphere, where warm air rises and cooler air descends. Radiation or electromagnetic radiation consists of a large array of energy that includes X-rays, visible light, heat waves, and radio waves that travel as waves of various
●
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sizes. Shorter wavelengths of radiation have greater energy. These are four basic laws of radiation: (1) All objects emit radiant energy; (2) hotter objects radiate more total energy per unit area than colder objects; (3) the hotter the radiating body, the shorter is the wavelength of maximum radiation; and (4) objects that are good absorbers of radiation are also good emitters. Approximately 50 percent of the solar radiation that strikes the atmosphere reaches Earth’s surface. About 30 percent is reflected back to space. The fraction of radiation reflected by a surface is called its albedo. The remaining 20 percent of the energy is absorbed by clouds and the atmosphere’s gases. Radiant energy absorbed at Earth’s surface is eventually radiated skyward. Because Earth has a much lower surface temperature than the Sun, its radiation is in the form of longwave infrared radiation. Because the atmospheric gases, primarily water vapor and carbon dioxide, are more efficient absorbers of terrestrial (longwave) radiation, the atmosphere is heated from the ground up. The selective absorption of Earth radiation by atmospheric gases, mainly water vapor and carbon dioxide, that results in Earth’s average temperature being warmer than it would be otherwise is referred to as the greenhouse effect. The greenhouse effect is a natural phenomenon that makes Earth habitable. Human activities that release greenhouse gases (primarily carbon dioxide) are the “villains” of global warming—not the “greenhouse effect” as it is often, but inaccurately, portrayed.
62 ● ●
The Atmosphere: An Introduction to Meteorology Because of the annual balance that exists between incoming and outgoing radiation, called the heat budget, Earth’s average temperature remains relatively constant. Averaged over the entire year, a zone around Earth between 38° north and 38° south receives more solar radiation than is lost to space. The opposite is true for
higher latitudes, where more heat is lost through outgoing longwave radiation than is received. It is this energy imbalance between the low and high latitudes that drives the weather system and in turn transfers surplus heat from the tropics poleward.
VOCABULARY REVIEW absorptivity (p. 50) advection (p. 46) albedo (p. 50) aphelion (p. 36) atmospheric window (p. 54) autumnal (fall) equinox (p. 40) backscattering (p. 50) circle of illumination (p. 40) conduction (p. 45) convection (p. 45) diffused light (p. 51) energy (p. 43) greenhouse effect (p. 54) heat (p. 44)
heat budget (p. 56) inclination of the axis (p. 38) infrared radiation (p. 47) kinetic energy (p. 43) latent heat (p. 44) longwave radiation (p. 48) perihelion (p. 36) plane of the ecliptic (p. 38) potential energy (p. 44) radiation, or electromagnetic radiation (p. 46) reflection (p. 50) revolution (p. 36) rotation (p. 36) scattering (p. 50)
sensible heat (p. 44) shortwave radiation (p. 48) spring (vernal) equinox (p. 40) summer solstice (p. 39) temperature (p. 44) thermal (p. 45) Tropic of Cancer (p. 39) Tropic of Capricorn (p. 39) ultraviolet radiation (p. 47) visible light (p. 46) wavelength (p. 46) winter solstice (p. 39)
PROBLEMS 1. Refer to Figure 2–6 and calculate the noon Sun angle on June 21 and December 21 at 50° north latitude, 0° latitude (the equator), and 20° south latitude. Which of these latitudes has the greatest variation in noon Sun angle between summer and winter? 2. For the latitudes listed in Problem 1, determine the length of daylight and darkness on June 21 and December 21 (refer to Table 2–2). Which of these latitudes has the largest seasonal variation in length of daylight? Which latitude has the smallest variation?
Sun’s rays
it
3. Calculate the noon Sun angle at your location for the equinoxes and solstices. 4. If Earth had no atmosphere, its longwave radiation emission would be lost quickly to space, making the planetapproximately 33 K cooler. Calculate the rate ofradiation emitted (E), and the wavelength of maximumradiation emission 1 lmax 2 for Earth at 255 K. (Hint: See Box 2–2.) 5. The intensity of solar radiation can be calculated using trigonometry, as shown in Figure 2–25. For simplicity, consider a solar beam of 1 unit width. The surface area over which the beam would be spread changes with Sun angle, such that: 1 unit Surface area 5 sin 1 Sun angle 2
1
un
Sun angle Surface area
Figure 2–25 Calculating solar intensity. Therefore, if the Sun angle at solar noon is 56°: Surface area 5
1 unit 1 unit 5 5 1.206 units sin 56° 0.829
Using this method and your answers to Problem 3, calculate the intensity of solar radiation (surface area) for your location at noon during the summer and winter solstices.
Chapter 2 Heating Earth’s Surface and Atmosphere
30 20
20° 18°
30 5 10
30
15 20
25
12° 10° 8°
20 25 15
231/2°
5
10 r
4°
231/2° 0°
10
5
6°
Au gu st
14°
em be
25
Se
20
0°
30
pt
15
2°
20
25
2°
ch ar M
30
4°
15
5
8°
15
10°
10 5
tob
10
er
6°
Oc
Latitude of Vertical Noon Sun
10 20
10 5
16°
ly Ju
June 20 30
10
22°
r il Ap
7. Use Figure 2–6 and the analemma in Figure 2–26 to calculate the noon Sun angle at your location (latitude) on the following dates: a. September 7 b. July 5 c. January 1
N 24°
May
6. Figure 2–26 is an analemma—a graph used to determine the latitude where the overhead noon Sun is located for any date. To determine the latitude of the overhead noon Sun from the analemma, find the desired date on the graph and read the coinciding latitude along the left axis. Determine the latitude of the overhead noon Sun for the following dates. Remember to indicate north (N) or south (S). a. March 21 b. June 5 c. December 10
25
The analemma
12°
Februa
ry
20 25
14° 30 16°
be
25 30
24° S
5
r
10
Figure 2–26
Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Heating Earth’s Surface and Atmosphere.
15
20
20 15
Decembe
25
30
5
10
25
ary
u
Jan
r
15 20
m
22°
10
30
ve
20°
10
15
5 No
18°
5
20
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Temperature Temperature is one of the basic elements of weather and climate. When someone asks what the weather is like outside, air temperature is often the first element we mention. From everyday experience, we know that temperatures vary on different time scales: seasonally, daily, and even hourly. Moreover, we all realize that substantial temperature differences exist from one place to another. In Chapter 2 you learned how air is heated and examined the role of Earth–Sun relationships in causing temperature variations from season to season and from latitude to latitude. In this chapter you will focus on several other aspects of this very important atmospheric property, including factors other than Earth– Sun relationships, that act as temperature controls. You will also look at how temperature is measured and expressed and see that temperature data can be of very practical value to us all. Applications include calculations that are useful in evaluating energy consumption, crop maturity, and human comfort.
Death Valley, California. Summertime temperatures here are among the highest anywhere in the Western Hemisphere. For more about Death Valley’s extraordinary temperatures, see Box 3–1. (Photo by Michael Collier)
After completing this chapter you should be able to:
t Calculate five commonly used types of temperature data and interpret a map that depicts temperature data using isotherms.
t List the principal controls of temperature and use examples to describe their effects.
t Explain why water and land heat and cool differently.
t Interpret the patterns depicted on world maps of January and July temperatures and on a world map of annual temperature ranges.
t Discuss the basic daily and annual cycles of air temperature.
t Explain how different types of thermometers work and why the placement of thermometers is an important factor in obtaining accurate readings.
t Distinguish among Fahrenheit, Celsius, and Kelvin temperature scales.
t Discuss the concept of apparent temperature and compare two basic indices that are expressions of this idea. 65
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The Atmosphere: An Introduction to Meteorology
For the Record: Air-Temperature Data ATMOSPHERE
Temperature Data and the Controls of Temperature ▸ Basic Temperature Data
Temperatures recorded daily at thousands of weather stations worldwide provide much of the temperature data compiled by meteorologists and climatologists (Figure 3–1). Hourly temperatures may be recorded by an observer or obtained from automated observing systems that continually monitor the atmosphere. At many locations only the maximum and minimum temperatures are obtained see (Box 3–1).
Basic Calculations The daily mean temperature is determined by averaging the 24 hourly readings or by adding the maximum and minimum temperatures for a 24-hour period and dividing by 2. From the maximum and minimum, the daily temperature range is computed by finding the difference between these figures. Other data involving longer periods are also compiled: r The monthly mean temperature is calculated by adding together the daily means for each day of the month and dividing by the number of days in the month. r The annual mean temperature is an average of the 12 monthly means. r The annual temperature range is computed by finding the difference between the warmest and coldest monthly mean temperatures. Mean temperatures are especially useful for making daily, monthly, and annual comparisons. It is common to hear a weather reporter state, “Last month was the warmest February on record” or “Today Omaha was 10° warmer than Chicago.” Temperature ranges are also useful statistics because they give an indication of extremes, a necessary part of understanding the weather and climate of a place or an area.
(a)
Isotherms To examine the distribution of air temperatures over large areas, isotherms are commonly used. An isotherm is a line that connects points on a map that have the same temperature (iso 5 ”equal,” therm 5 ”temperature”). Therefore, all points through which an isotherm passes have identical temperatures for the time period indicated. Generally, isotherms representing 5° or 10° temperature differences are used, but any interval may be chosen. Figure 3–2 illustrates how isotherms are drawn on a map. Notice that most isotherms do not pass directly through the observing stations because the station readings may not coincide with the values chosen for the isotherms. Only an occasional station temperature will be exactly the same as the value of the isotherm, so it is usually necessary to draw the lines by estimating the proper position between stations. Isothermal maps are valuable tools because they clearly make temperature distribution visible at a glance. Areas of low and high temperatures are easy to pick out. In addition, the amount of temperature change per unit of distance, called the temperature gradient, is easy to visualize. Closely spaced isotherms indicate a rapid rate of temperature change, whereas more widely spaced lines indicate a more gradual rate of change. For example, notice in Figure 3–2 that the isotherms are more closely spaced in Colorado and Utah (steeper temperature gradient), whereas the isotherms are spread farther apart in Texas (gentler temperature gradient). Without isotherms a map would be covered with numbers representing temperatures at tens or hundreds of places, which would make patterns difficult to see.
Concept Check 3.1 1 How are the following temperature data calculated: daily mean, daily range, monthly mean, annual mean, and annual range?
2 What are isotherms, and what is their purpose?
(b)
Figure 3–1 People living in the middle latitudes can experience a wide range of temperatures during a year. (a) Pedestrian navigating through a Chicago neighborhood during a February 2011 blizzard that dropped more than 50 centimeters (nearly 20 inches) of snow on the city. (Photo by Zuma Press/Newscom) (b) Beating the heat on a hot summer day. (Photo by mylife photos/Alamy).
Chapter 3 Temperature
Vancouver 49 2
2 Calgary 22
Portland 56 2
20 2 Missoula 36
50 40 Sacramento 63 2
60
30
Ely 42 2 Las Vegas 59 2
50
2 Tucson 76
Bismarck 19 2
2 Fargo 23
Albany 57 2
Boston 2 57
Milwaukee Cleveland 2 52 2 Philadelphia 2 62 60 Des Moines Peoria 64 Scottsbluff 42 2 2 552 32 Charleston Raleigh 72 2 Louisville Topeka 2 78 66 2 Denver 2 2 54 41 60
70
Little Rock 75 2 2 Lubbock 76
70
80
30s
40s
Toronto 59 2
2 Duluth 31
Bangor 2 47
Casper 242
Houston 83 2
50s 20s
Montreal 2 46
50
40
40s
10s
40
30 20
Gallup 57 2 Albuquerque 2 65
2 Palm Springs 72
50s
Regina 132
67
90
80
Myrtle Beach 2 81
Montgomery 2 83
Miami 2 85
Monterrey 2 93
60s 70s
70s 80s 90s
Figure 3–2 Map showing high temperatures for a spring day. Isotherms are lines that connect points of equal temperature. Showing temperature distribution in this way makes patterns easier to see. Notice that most isotherms do not pass directly through the observing stations. It is usually necessary to draw isotherms by estimating their proper position between stations. On television and in many newspapers, temperature maps are in color. Rather than labeling isotherms, the area between isotherms is labeled. For example, the zone between the 60° and 70° isotherms is labeled “60s.”
Students Sometimes Ask… What’s the hottest city in the United States? It depends on how you define “hottest.” If average annual temperature is used, then Key West, Florida, is the hottest, with an annual mean of 25.6°C (78°F) for the 30-year span 1971–2000. However, if we look at cities with the highest July maximums during the 1971–2000 span, then the desert community of Palm Springs, California, has the distinction of being hottest. Its average daily high in July is a blistering 42.4°C (108.3°F)! Yuma, Arizona (41.7°C/107°F), Phoenix, Arizona (41.4°C/106°F), and Las Vegas, Nevada (40°C/104.1°F), aren’t far behind.
Why Temperatures Vary: TheControls of Temperature ATMOSPHERE
Temperature Data and the Controls of Temperature ▸ Controls of Temperature
The controls of temperature are factors that cause temperatures to vary from place to place and from time to time. Chapter 2 examined the most important cause for temperature variation—differences in the receipt of solar radiation. Because variations in Sun angle and length of daylight
depend on latitude, they are responsible for warm temperatures in the tropics and colder temperatures poleward. Of course, seasonal temperature changes at a given latitude occur as the Sun’s vertical rays migrate toward and away from a place during the year. Figure 3–3 reminds us of the importance of latitude as a control of temperature. But latitude is not the only control of temperature. If it were, we would expect all places along the same parallel to have identical temperatures. This is clearly not the case. For instance, Eureka, California, and New York City are both coastal cities at about the same latitude, and both places have an annual mean temperature of 11°C (51.8°F). Yet New York City is 9.4°C (16.9°F) warmer than Eureka in July and 9.4°C (16.9°F) colder than Eureka in January. In another example, two cities in Ecuador—Quito and Guayaquil—are relatively close to one another, but the mean annual temperatures at these two cities differ by 12.2°C (22°F). To explain these situations and countless others, we must realize that factors other than latitude also exert a strong influence on temperature. In the next sections we examine these other controls, which include: r r r r r
Differential heating of land and water Ocean currents Altitude Geographic position Cloud cover and albedo
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The Atmosphere: An Introduction to Meteorology
40 36
Mbandaka, Rep. of Congo 0.01´ N
32
100
Miami, Florida 25˚ 45´ N
90
28
80
24
Temperature (˚C)
16
60
12 50 8 Auckland, New Zealand 37˚ 43´ S
4 0
40
Temperature (˚F)
70
20
30
–4 Moscow, Russia 55° 45´ N
–8
20
–12 –16
10 J
F
M
A
M
J
J
A
S
O
N
D
Month
Surface temperatures in the Pacific Ocean are much lower. The peaks of the Sierra Nevada, still capped with snow, form a cool blue line down the eastern side of California. In side-by-side bodies of land and water, such as is shown in Figure 3–4, land heats more rapidly and to higher temperatures than water, and it cools more rapidly and to lower temperatures than water. Variations in air temperatures, therefore, are much greater over land than over water. Why do land and water heat and cool differently? Several factors are responsible. 1. An important reason that the surface temperature of water rises and falls much more slowly than the surface temperature of land is that water is highly mobile. As water is heated, convection distributes the heat through a considerably larger mass. Daily temperature changes occur to depths of 6 meters (20 feet) or more below the surface, and yearly, oceans and deep lakes experience temperature variations through a layer between 200 and 600 meters (650 and 2000 feet) thick. In contrast, heat does not penetrate deeply into soil or rock; it remains near the surface. Obviously, no mixing can occur on land because it is not fluid. Instead, heat must be transferred by the slow process of
Figure 3–3 The data for these four cities reminds us that latitude (Earth–Sun relationship) is a major control of temperature.
Concept Check 3.2 1 List five controls of temperature other than latitude. 2 Provide two examples that illustrate that latitude is not the only temperature control.
Land and Water In Chapter 2 you saw that the heating of Earth’s surface controls the heating of the air above it. Therefore, to understand variations in air temperature, we must understand the variations in heating properties of the different surfaces that Earth presents to the Sun—soil, water, trees, ice, and so on. Different land surfaces reflect and absorb varying amounts of incoming solar energy, which in turn cause variations in the temperature of the air above. The greatest contrast, however, is not between different land surfaces but between land and water. Figure 3–4 illustrates this idea nicely. This satellite image shows surface temperatures in portions of Nevada, California, and the adjacent Pacific Ocean on the afternoon of May 2, 2004, during a spring heat wave. Land-surface temperatures are clearly much higher than water-surface temperatures. The image shows the extreme high surface temperatures in southern California and Nevada in dark red.* *Realize that air temperatures are cooler than surface temperatures. For example, the surface of a sandy beach can be painfully hot even though the temperature of the air above the sand is comfortable.
NEVADA
CALIFORNIA
PACIFIC OCEAN
–10
1
14
34
Temperature (°C) 12 23 34 53 73 93 Temperature (°F)
45
56
113
133
Figure 3–4 The differential heating of land and water is an important control of air temperatures. In this satellite image from the afternoon of May 2, 2004, water-surface temperatures in the Pacific Ocean are much lower than land-surface temperatures in California and Nevada. The narrow band of cool temperatures in the center of the image is associated with mountains (the Sierra Nevada). The cooler water temperatures immediately offshore are associated with the California Current (see Figure 3–9). (NASA)
Chapter 3 Temperature
Box 3–1
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North America’s Hottest and Coldest Places
and subtropical places such as Hawaii are known for being warm throughout the year, they seldom experience maximum temperatures that surpass the low to mid-30s Celsius (90s Fahrenheit). The highest accepted temperature record for the United States as well as the entire Western Hemisphere is 57°C (134°F). This long-standing record was set at Death Valley, California, on July 10, 1913. Summer temperatures at Death Valley are consistently Maximum Temperature Records among the highest in the Western Hemisphere. During June, July, and August, temSurprisingly, the state that ties Alaska for peratures exceeding 49°C (120°F) are to be the “lowest high” is Hawaii. Panala, on the expected. Fortunately, Death Valley has few south coast of the big island, recorded 38°C human summertime residents. on April 27, 1931. Although humid tropical Why are summer temperatures at Death Valley so high? In addition to having the lowest elevation in the Western Hemisphere (53 meters (174 feet) below sea level), Death Valley is a desert. Although it is only about 300 kilometers (less than 200 miles) from the Pacific Ocean, mountains cut off the valley from the ocean’s moderating influence and moisture. Clear skies allow a maximum of sunshine to strike the dry, FIGURE 3–A At 1886 meters (6288 feet), Mount Washington is the barren surface. Because highest peak in the White Mountains and in fact the entire Northeast. no energy is used to Some consider it to be “the home of the world’s worst weather” due to evaporate moisture as ocits extreme cold, heavy snows, high winds, frequent icing, and dense curs in humid regions, all fog. The observatory at its summit keeps detailed weather records. the energy is available to (Photo by Jim Salge/Mount Washington Observatory) Most people living in the United States have experienced temperatures of 38°C (100°F) or more. When statistics for the 50 states are examined for the past century or longer, we find that every state has a maximum temperature record of 38°C or higher. Even Alaska has recorded a temperature this high. Its record was set June 27, 1915, at Fort Yukon, a town along the Arctic Circle in the interior of the state.
conduction. Consequently, daily temperature changes are small below a depth of 10 centimeters (4 inches), although some change can occur to a depth of perhaps 1 meter (3 feet). Annual temperature variations usually reach depths of 15 meters (50 feet) or less. Thus, as a result of the mobility of water and the lack of mobility in the solid Earth, a relatively thick layer of water is heated to moderate temperatures during the summer. On land only a thin layer is heated but to much higher temperatures. During winter the shallow layer of rock and soil that was heated in summer cools rapidly. Water bodies, in contrast, cool slowly as they draw on the reserve of heat stored within. As the water surface cools, vertical motions are established. The chilled surface water, which
heat the ground. In addition, subsiding air that warms by compression as it descends is also common to the region and contributes to its high maximum temperatures. Minimum Temperature Records The temperature controls that produce truly frigid temperatures are predictable, and they should come as no surprise. We should expect extremely cold temperatures during winter in high-latitude places that lack the moderating influence of the ocean (Figure 3–A). Moreover, stations located on ice sheets and glaciers should be especially cold, as should stations positioned high in the mountains. All of these criteria apply to Greenland’s Northice Station (elevation 2307 meters (7567 feet)). Here on January 9, 1954, the temperature plunged to 66C (87F). If we exclude Greenland from consideration, Snag, in Canada’s Yukon Territory, holds the record for North America. This remote outpost experienced a temperature of 63C (81 F) on February 3, 1947. When only locations in the United States are considered, Prospect Creek, located north of the Arctic Circle in the Endicott Mountains of Alaska, came close to the North American record on January 23, 1971, when the temperature plunged to 62C (80F). In the lower 48 states the record of 57C (70F) was set in the mountains at Rogers Pass, Montana, on January 20, 1954. Remember that many other places have no doubt experienced equally low or even lower temperatures; they just were not recorded.
is dense, sinks and is replaced by warmer water from below, which is less dense. Consequently, a larger mass of water must cool before the temperature at the surface will drop appreciably. 2. Because land surfaces are opaque, heat is absorbed only at the surface. This fact is easily demonstrated at a beach on a hot summer afternoon by comparing the surface temperature of the sand to the temperature just a few centimeters beneath the surface. Water, being more transparent, allows some solar radiation to penetrate to a depth of several meters. 3. The specific heat (the amount of heat needed to raise the temperature of 1 gram of a substance 1°C) is more than three times greater for water than for land. Thus, water
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The Atmosphere: An Introduction to Meteorology
requires considerably more heat to raise its temperature the same amount as an equal volume of land. 4. Evaporation (a cooling process) from water bodies is greater than from land surfaces. Energy is required to evaporate water. When energy is used for evaporation, it is not available for heating.* All these factors collectively cause water to warm more slowly, store greater quantities of heat, and cool more slowly than land. Monthly temperature data for two cities will demonstrate the moderating influence of a large water body and the extremes associated with land (Figure 3–5). Vancouver, British Columbia, is located along the windward Pacific coast, whereas Winnipeg, Manitoba, is in a continental position far from the influence of water. Both cities are at about the same latitude and thus experience similar sun angles and lengths of daylight. Winnipeg, however, has a mean January temperature that is 20°C (36°F) lower than Vancouver’s. Conversely, Winnipeg’s July mean is 2.6°C (4.7°F) higher than Vancouver’s. Although their latitudes are nearly the same, Winnipeg, which has no water influence, experiences much greater temperature extremes than does Vancouver. The key to Vancouver’s moderate year-round climate is the Pacific Ocean. On a different scale, the moderating influence of water may also be demonstrated when temperature variations in
the Northern and Southern Hemispheres are compared. The views of Earth in Figure 3–6 show the uneven distribution of land and water over the globe. Water covers 61 percent of the Northern Hemisphere; land represents the remaining 39 percent. However, the figures for the Southern Hemisphere (81 percent water and 19 percent land) reveal why it is correctly called the water hemisphere. Between 45° north and 79° north latitude there is actually more land than water, whereas between 40° south and 65° south latitude there is almost no land to interrupt the oceanic and atmospheric circulation. Figure 3–7 portrays the considerably smaller annual temperature ranges in the water-dominated Southern Hemisphere compared with the Northern Hemisphere.
Northern Hemisphere
North Pole
24 70
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United States
0 J
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Figure 3–5 Mean monthly temperatures for Vancouver, British Columbia, and Winnipeg, Manitoba. Vancouver has a much smaller annual temperature range, due to the strong marine influence of the Pacific Ocean. Winnipeg illustrates the greater extremes associated with an interior location.
*Evaporation is an important process that is discussed more thoroughly in the section “Water’s Changes of State” in Chapter 4.
Southern Hemisphere
Figure 3–6 The uneven distribution of land and water between the Northern and Southern Hemispheres. Almost 81 percent of the Southern Hemisphere is covered by the oceans—20 percent more than the Northern Hemisphere.
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50 25 0 25 50 75 100 125 150 175 Millions of square kilometers per 5˚ latitude zone
m
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Mean Annual Temperature Range (°C)
Chapter 3 Temperature
31
Figure 3–7 The graph shows the amount of land and water in each 5° latitude belt. Mean annual temperature ranges are noted on the right. The moderating influence of water is demonstrated when the Northern and Southern Hemispheres are compared. Annual temperature ranges in the water-dominated midlatitudes (30°–60°) of the Southern Hemisphere are considerably smaller than in the landdominated midlatitudes of the Northern Hemisphere.
Concept Check 3.3 1 State the relationship between the heating and cooling of land and water.
2 List and briefly describe the factors that cause land and water to heat and cool differently.
Ocean Currents You probably have heard of the Gulf Stream, an important surface current in the Atlantic Ocean that flows northward along the East Coast of the United States (Figure 3–8). Surface currents like this one are set in motion by the wind. At the water surface, where the atmosphere and ocean meet, energy is passed from moving air to the water through friction. As a consequence, the drag exerted by winds blowing steadily across the ocean causes the surface layer of water to move. Thus, major horizontal movements of surface waters are closely related to the circulation of the atmosphere, which in turn is driven by the unequal heating of Earth by the Sun (Figure 3–9).* Surface ocean currents have an important effect on climate. It is known that for Earth as a whole, the gains in solar energy equal the losses to space of heat radiated from
*The relationship between global winds and surface ocean currents is examined in Chapter 7.
Figure 3–8 Satellite image of the Gulf Stream off the East Coast of the United States. Red represents higher water temperatures and blue cooler water temperatures. The current transports heat from the tropics far into the North Atlantic. (Johns Hopkins University Applied Physics Laboratory)
the surface. When most latitudes are considered individually, however, this is not the case. There is a net gain of energy in lower latitudes, and there is a net loss at higher latitudes. Because the tropics are not becoming progressively warmer, nor are the polar regions becoming colder, there must be a large-scale transfer of heat from areas of excess to areas of deficit. This is indeed the case. The transfer of heat by winds and ocean currents equalizes these latitudinal energy imbalances. Ocean water movements account for about onequarter of this total heat transport, and winds account for the remaining three-quarters. The moderating effect of poleward-moving warm ocean currents is well known. The North Atlantic Drift, an extension of the warm Gulf Stream, keeps wintertime temperatures in Great Britain and much of Western Europe warmer than would be expected for their latitudes (London is farther north than St. John’s, Newfoundland.) Because of the prevailing westerly winds, the moderating effects are carried far inland. For example, Berlin (52° north latitude) has a mean January temperature similar to that experienced in New York City, which lies 12° latitude farther south. The January mean in London (51° north latitude) is 4.5°C (8.1°F) higher than in New York City. In contrast to warm ocean currents such as the Gulf Stream, the effects of which are felt most during the winter, cold currents exert their greatest influence in the tropics or during the summer months in the middle latitudes. For example, the cool Benguela Current off the western coast of southern Africa moderates the tropical heat along this coast. Walvis Bay (23° south latitude), a
72
The Atmosphere: An Introduction to Meteorology 30˚
60°
30˚
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East Wind Drift
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Figure 3–9 Major surface ocean currents. Poleward-moving currents are warm, and equatorwardmoving currents are cold. Surface ocean currents are driven by global winds and play an important role in redistributing heat around the globe. Note that cities mentioned in the text discussion are shown on this map.
influence the temperatures of adjacent land areas?
Altitude Recall from Chapter 1 that temperatures decrease with an increase in altitude in the troposphere. As a result, some mountaintops are snow-covered year round. This can even
80
24
Rio de Janeiro
16
60 Arica
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30 Bra zil C.
Rio de Janeiro
–4 40°
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C.
1 What force drives ocean currents? 2 Are poleward-moving ocean currents warm or cold? 3 How do ocean currents that move toward the equator
28
Peru
Concept Check 3.4
32
Temperature (˚C)
town adjacent to the Benguela Current, is 5°C (9°F) cooler in summer than Durban, which is 6° latitude farther poleward but on the eastern side of South Africa, away from the influence of the current (Figure 3–9). The east and west coasts of South America provide another example. Figure 3–10 shows monthly mean temperatures for Rio de Janeiro, Brazil, which is influenced by the warm Brazil Current and Arica, Chile, which is adjacent to the cold Peru Current. Closer to home, because of the cold California current, summer temperatures in subtropical coastal southern California are lower by 6°C (10.8°F) or more compared to East Coast stations.
60°
A
40°
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J J Month
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Figure 3–10 Monthly mean temperatures for Rio de Janeiro, Brazil, and Arica, Chile. Both are coastal cities near sea level. Even though Arica is closer to the equator than Rio de Janeiro, its temperatures are cooler. Arica is influenced by the cold Peru Current, whereas Rio de Janeiro is adjacent to the warm Brazil Current.
73
Chapter 3 Temperature
Figure 3–11 Recall from Chapter 2 that temperatures decrease with an increase in altitude in the troposphere. As a result, some mountaintops are snow-covered all year. This can even occur in the tropics. Africa’s Mt. Kilimanjaro actually has a small glacier at its summit. (Photo by Corbis/ SuperStock)
36 32
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occur in the tropics if the mountains are high enough (Figure 3–11). The two cities in Ecuador mentioned earlier, Quito and Guayaquil, demonstrate the influence of altitude on mean temperature. Both cities are near the equator and relatively close to one another, but the annual mean temperature at Guayaquil is 25.5°C (77.9°F), compared with Quito’s mean of 13.3°C (55.9°F). The difference may be understood when the cities’ elevations are noted. Guayaquil is only 12 meters (39 feet) above sea level, whereas Quito is high in the Andes Mountains at 2800 meters (9200 feet). Figure 3–12 provides another example. In Chapter 1 you learned that temperatures drop an average of 6.5°C per kilometer (3.5°F per 1000 feet) in the troposphere. However, if this figure is applied, we would expect Quito to be about 18.2°C (32.7°F) cooler than Guayaquil, but the difference is only 12.2°C (22°F). The fact that high-altitude places, such as Quito, are warmer than the value calculated using the normal lapse rate results from the absorption and reradiation of solar energy by the ground surface. In addition to the effect of altitude on mean temperatures, the daily temperature range also changes with variations in height. Not only do temperatures drop with an increase in altitude but atmospheric pressure and density also diminish. Because of the reduced density at high altitudes, the overlying atmosphere absorbs and reflects a smaller portion of the incoming solar radiation. Consequently, with an increase in altitude, the intensity of solar radiation increases, resulting in relatively rapid and intense daytime heating. Conversely, rapid nighttime cooling is also the rule in high mountain locations. Therefore, stations located high in the mountains generally have a greater daily temperature range than do stations at lower elevations.
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La Paz J
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Figure 3–12 Graph comparing monthly mean temperatures at Concepción and La Paz, Bolivia. Both cities have nearly the same latitude (about 16° South). However, because La Paz is high in the Andes, at 4103 meters (13,461 feet), it experiences much cooler temperatures than Concepcieón, which is at an elevation of 490meters (1608 feet).
Concept Check 3.5 1 How does altitude influence average temperatures? Provide an example.
2 Where is daily temperature range usually greater: at the base or the top of a mountain? Explain.
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The Atmosphere: An Introduction to Meteorology
24 70
New York City (Leeward)
20 16
60 Eureka, California (Windward)
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Imagine being at this beach on a warm, sunny summer afternoon. (Photo by Tequilab/Shutterstock) Question 1 Describe the temperatures you would expect if you measured the surface of the beach and at a depth of 12 inches.
Figure 3–13 Monthly mean temperatures for Eureka, California, and New York City. Both cities are coastal and located at about the same latitude. Because Eureka is strongly influenced by prevailing winds from the ocean and New York City is not, the annual temperature range at Eureka is much smaller.
Question 2 If you stood waist deep in the water and measured the water’s surface temperature and the temperature at a depth of 12 inches, how would these measurements compare to those taken at the beach?
Geographic Position
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The geographic setting can greatly influence the temperatures experienced at a specific location. A coastal location where prevailing winds blow from the ocean onto the shore (a windward coast) experiences considerably different temperatures than does a coastal location where prevailing winds blow from the land toward the ocean (a leeward coast). In the first situation the windward coast will experience the full moderating influence of the ocean—cool summers and mild winters—compared to an inland station at the same latitude. A leeward coastal situation, however, will have a more continental temperature regime because the winds do not carry the ocean’s influence onshore. Eureka, California, and New York City, the two cities mentioned earlier, illustrate this aspect of geographic position (Figure 3–13). The annual temperature range in New York City is 19°C (34°F) greater than Eureka’s. Seattle and Spokane, both in the state of Washington, illustrate a second aspect of geographic position: mountains acting as barriers. Although Spokane is only about 360 kilometers (225 miles) east of Seattle, the towering Cascade Range separates the cities. Consequently, Seattle’s temperatures show a marked marine influence, but Spokane’s are more typically continental (Figure 3–14). Spokane is
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Figure 3–14 Monthly mean temperatures for Seattle and Spokane, Washington. Because the Cascade Mountains cut off Spokane from the moderating influence of the Pacific Ocean, its annual temperature range is greater than Seattle’s.
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Chapter 3 Temperature
7°C (12.6°F) cooler than Seattle in January and 4°C (7.2°F) warmer than Seattle in July. The annual range at Spokane is 11°C (nearly 20°F) greater than in Seattle. The Cascade Range effectively cuts off Spokane from the moderating influence of the Pacific Ocean.
One city is located along a windward coast, another in the center of the continent, and the third along a leeward coast. Compare the annual temperature ranges of these cities.
2 Why does Spokane, Washington, have a higher annual
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Cloud Cover and Albedo You may have noticed that clear days are often warmer than cloudy ones and that clear nights usually are cooler than cloudy ones. This demonstrates that cloud cover is another factor that influences temperature in the lower atmosphere. Studies using satellite images show that at any particular time, about half of our planet is covered by clouds. Cloud cover is important because many clouds have a high albedo and therefore reflect a significant proportion of the sunlight that strikes them back to space. By reducing the amount of incoming solar radiation, daytime temperatures will be lower than if the clouds were absent and the sky were clear (Figure 3–15). As was noted in Chapter 2, the albedo of clouds depends on the thickness of the cloud cover and can vary from 25 to 80 percent (see Figure 2–17, page 51). At night, clouds have the opposite effect as during daylight. They absorb outgoing Earth radiation and emit a portion of it toward the surface. Consequently, some of the heat that otherwise would have been lost remains near the ground. Thus, nighttime air temperatures do not drop as low as they would on a clear night. The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the nighttime minimum. This is illustrated nicely by the graph in Figure 3–15. The effect of cloud cover on reducing maximum temperatures can also be detected when monthly mean temperatures are examined for some stations. For example, each year much of southern Asia experiences an extended period of relative drought during the cooler low-Sun period; it is then followed by heavy monsoon rains.* The graph for Yangon, Myanmar (also known as Rangoon, Burma), illustrates this pattern (Figure 3–16). Notice that the highest monthly mean temperatures occur in April and May, before the summer solstice, rather than in July and August, as normally occurs at most stations in the Northern Hemisphere. Why?
*This pattern is associated with the monsoon circulation and is discussed in Chapter 7.
NOON 6 PM Hours of the day
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Figure 3–15 The graph shows the daily temperature cycle at Peoria, Illinois, for two July days—one clear and the other overcast. On a clear day the maximum temperature is higher, and the minimum temperature is lower than if the day had been overcast. On an overcast day, the daily temperature range is lower than if it had been cloud free.
Yangon, Myanmar 16°46´ N 96°09´ E
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Figure 3–16 Monthly mean temperatures (curve) and monthly mean precipitation (bar graph) for Yangon, Myanmar. The highest mean temperature occurs in April, just before the onset of heavy summer rains. The abundant cloud cover associated with the rainy period reflects back into space the solar energy that otherwise would strike the ground and raise summer temperatures.
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The Atmosphere: An Introduction to Meteorology
Heat Waves*
A
heat wave is a prolonged period of abnormally hot and usually humid weather that typically lasts from a few days to several weeks. The impact of heat waves on individuals varies greatly. The elderly are the most vulnerable because heat puts more stress on weak hearts and bodies. The poor, who often cannot afford air-conditioning, also suffer disproportionately. Studies also show that the temperature at which death rates increase varies from city to city. In Dallas, Texas, a temperature of 39°C (103°F) is required before the death rate climbs. In San Francisco the key temperature is just 29°C (84°F).
The 1936 North American heat wave is likely the continent’s most severe in modern history Heat waves do not elicit the same sense of fear or urgency as tornadoes, hurricanes, and flash floods. One reason is that it may take many days of oppressive temperatures for a heat wave to exact its toll rather than just a few minutes or a few hours. Another reason is that they cause far less property damage than other extreme weather events. Nevertheless, heat waves can be deadly and costly. The 1936 North American heat wave is likely the continent’s most severe in modern history. It took place during the economic hard times of the Great Depression and coin-
cided with a significant drought in many parts of the Great Plains and Midwest. The prolonged heat wave began in late June and did not end until early September. The estimated death toll exceeded 5000, and agricultural losses were catastrophic in many areas. Many records from this summer of exceptional temperatures still stand. In fact, current record high temperatures for 13 states are from July and August 1936 (Table 3–A). There are other extraordinary records as well. For example, Mt. Vernon, Illinois, experienced 18 straight days (August 12–29) when temperatures surpassed 38 °C (100 °F). A recent example occurred in the summer of 2003, when much of Europe experienced perhaps its worst heat wave in more than a century. The image in Figure 3–B relates to this deadly event. An estimate based on government records puts the death toll at between 20,000 and 35,000. The majority died during the first two weeks of August— the hottest span. France suffered the greatest number of heat-related fatalities—about 14,000.
The severity of heat waves is usually greatest in cities The dangerous impact of summer heat is also reinforced when examining Figure 3–C, which shows average annual weatherrelated deaths in the United States for the 10-year period 2001–2010. A comparison
The reason is that during the summer months, when we would usually expect temperatures to climb, the extensive cloud cover increases the albedo of the region, which reduces incoming solar radiation at the surface. As a result, the highest monthly mean temperatures occur in late spring, when the skies are still relatively clear. Cloudiness is not the only phenomenon that increases albedo and thereby reduces air temperature. We also recognize that snow- and ice-covered surfaces have high albedos (Figure 3–17). This is one reason mountain glaciers do not melt away in the summer and why snow may still be present on a mild spring day. In addition, during the winter when snow covers the ground, daytime maximums on a sunny day are cooler than they otherwise would be because energy that the land would have absorbed and used to heat the air is reflected and lost.
of values reveals that the number of heat deaths is among the highest. The severity of heat waves is usually greatest in cities because of the urban heat island (see Box 3–3). Large cities do not cool off as much at night during heat waves as rural areas do, and this can be a critical difference in the amount of heat stress within
*For a related discussion, see the section on heat stress later in this chapter.
TABLE 3–A State Temperature Records Remaining from 1936 State
Temperature (°F)
Date
Arkansas
120
August 10
Indiana
116
July 14
Kansas
121
July 24
Louisiana
114
August 10
Maryland
109
July 10
Michigan
112
July 13
Minnesota
114
July 13
Nebraska
118
July 24
New Jersey
110
July 10
North Dakota
121
July 6
Pennsylvania
111
July 10
West Virginia
112
July 10
Wisconsin
114
July 13
Concept Check 3.7 1 Contrast the daily temperature range on an overcast day with that on a cloudless sunny day.
2 Examine Figure 3–16 and explain why Yangon’s monthly mean temperature for April is higher than the July monthly mean.
World Distribution ofTemperatures Take a moment to study the two world isothermal maps in (Figures 3–18 and 3–19). From hot colors near the equator to cool colors toward the poles, these maps portray sea-level
Chapter 3 Temperature
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160 140 116 115
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71 56
60 39
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Land Surface Temperature difference (°C) –10
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FIGURE 3–B This image is derived from satellite data and shows the difference in daytime land surface temperatures during the 2003 European heat wave (July 20–August 20) as compared to the years 2000, 2001, 2002, and 2004. The zone of deep red shows where temperatures were 10°C (18°F) hotter than in the other years. France was hardest hit. (NASA)
the inner city. In addition, the stagnant atmospheric conditions usually associated with heat waves trap pollutants in urban areas and add the stresses of severe air pollution to the already dangerous stresses caused by the high temperatures.
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PACIFIC OCEAN
In July 1995, a brief but intense heat wave developed in the central United States. A total of 830 deaths were attributed to this severe 5-day event, the worst in 50 years in the northern Midwest. The greatest loss of life occurred in Chicago,
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FIGURE 3–C Average annual weather-related fatalities for the 10-year period 2001–2010. The figure for the hurricane category was dramatically affected by a single storm—Hurricane Katrina in 2005. Katrina was responsible for 1016 of the 1154 hurricane-related fatalities that occurred during this 10-year span. (Data from NOAA)
where there were 525 fatalities. This event provided a sobering lesson by focusing attention on the need for more effective warning and response plans, especially in major urban areas where heat stress is greatest.
Figure 3–17 Ice- and snowcovered surfaces have high albedos, thus keeping air temperatures lower than if the surface were not highly reflective. This view shows sea ice (frozen seawater) near Barrow, Alaska. When sea ice melts, as has occurred on the left side of the image, a bright reflective surface is replaced by a darker surface that absorbs a higher percentage of incoming solar radiation. (Photo by Michael Collier)
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Chapter 3 Temperature
temperatures in the seasonally extreme months of January and July. On these maps you can study global temperature patterns and the effects of the controls of temperature, especially latitude, the distribution of land and water, and ocean currents. Like most other isothermal maps of large regions, all temperatures on these world maps have been reduced to sea level to eliminate the complications caused by differences in altitude. On both maps the isotherms generally trend east and west and show a decrease in temperatures poleward from the tropics. They illustrate one of the most fundamental aspects of world temperature distribution: that the effectiveness of incoming solar radiation in heating Earth’s surface and the atmosphere above it is largely a function of latitude. Moreover, there is a latitudinal shifting of temperatures caused by the seasonal migration of the Sun’s vertical rays. To see this, compare the color bands by latitude on the two maps. If latitude were the only control of temperature distribution, our analysis could end here, but this is not the case. The added effect of the differential heating of land and water is clearly reflected on the January and July temperature maps. The warmest and coldest temperatures are found over land—note the coldest area, a purple oval in Siberia, and the hottest areas, the deep orange ovals—all over land. Consequently, because temperatures do not fluctuate as much over water as over land, the north–south migration of isotherms is greater over the continents than over the oceans. In addition, it is clear that the isotherms in the Southern Hemisphere, where there is little land and where
the oceans predominate, are much more regular than in the Northern Hemisphere, where they bend sharply northward in July and southward in January over the continents. Isotherms also reveal the presence of ocean currents. Warm currents cause isotherms to be deflected toward the poles, whereas cold currents cause an equatorward bending. The horizontal transport of water poleward warms the overlying air and results in air temperatures that are higher than would otherwise be expected for the latitude. Conversely, currents moving toward the equator produce cooler-than-expected air temperatures. Figures 3–18 and 3–19 show the seasonal extremes of temperature, and comparing them enables us to see the annual range of temperature from place to place. Comparing the two maps shows that a station near the equator has a very small annual range because it experiences little variation in the length of daylight, and it always has a relatively high Sun angle. A station in the middle latitudes, however, experiences wide variations in Sun angle and length of daylight and hence large variations in temperature. Therefore, we can state that the annual temperature range increases with an increase in latitude (see Box 3–2). Moreover, land and water also affect seasonal temperature variations, especially outside the tropics. A continental location must endure hotter summers and colder winters than a coastal location. Consequently, outside the tropics the annual range will increase with an increase in continentality. Figure 3–20, which shows the global distribution of annual temperature ranges, serves to summarize the pre-
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Figure 3–20 Global annual temperature ranges in Celsius (C) and Fahrenheit (F). Annual ranges are small near the equator and increase toward the poles. Outside the tropics, annual temperature ranges increase as we move away from the ocean and toward the interior of large landmasses. (After Robert W. Christopherson, Geosystems: An Introduction to Physical Geography, 8th ed., Pearson Prentice Hall, 2012)
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much smaller than in the Northern Hemisphere with its large continents.
Concept Check 3.8 1 Why do isotherms on the January and July temperature maps generally trend east–west?
2 On the January map (Figure 3–18), why do the isotherms bend in the North Atlantic?
3 Explain why the isotherms bend northward over North America on the July map (Figure 3–19).
4 Refer to Figure 3–20 to determine which area on Earth experiences the highest annual temperature range. Why is the annual range so high in this region?
Students Sometimes Ask . . . Where in the world would I experience the greatest contrast between summer and winter temperatures? Among places for which records exist, it appears as though Yakutsk, a station in the heart of Siberia, is the best candidate. The latitude of Yakutsk is 62° north, just a few degrees south of the Arctic Circle. Moreover, it is far from the influence of water. The January mean at Yakutsk is a frigid 43C (45F), whereas its July mean is a pleasant 20°C (68°F). The result is an average annual temperature range of 63°C (113°F), among the highest ranges anywhere on the globe.
This image shows a snow-covered area in the middle latitudes on a sunny day in late winter. Assume that one week after this photo was taken, conditions were essentially identical except that the snow was gone. (Photo by CoolR/Shutterstock) Question 1 Would you expect the air temperatures to be different on the two days? If so, which day would likely be warmer? Question 2 Suggest a reason for the different temperatures.
ceding two paragraphs. By examining this map, it is easy to see the influence of latitude and continentality on this temperature statistic. The tropics clearly experience small annual temperature variations. As expected, the highest values occur in the middle of large landmasses in the subpolar latitudes. It is also obvious that annual temperature ranges in the ocean-dominated Southern Hemisphere are
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Cycles of Air Temperature You know from experience that a rhythmic rise and fall of air temperature occurs almost every day. Your experience is confirmed by thermograph records like the one in Figure 3–21. (A thermograph is an instrument that continuously records temperature.) The temperature curve reaches a mini-
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Figure 3–21 Thermograph of temperatures in Peoria, Illinois, during a seven-day span in May 1992. The typical daily rhythm, with minimums around sunrise and maximums in mid- to late afternoon, occurred on most days. The obvious exception occurred on May 23, when the maximum was reached at midnight and temperatures dropped throughout the day.
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Box 3–2
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Latitude and Temperature Range Gregory J. Carbone*
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*Professor Carbone is a faculty member in the Department of Geography at the University of South Carolina.
FIGURE 3–D The annual temperature range at Winnipeg is much greater than at San Antonio.
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Latitude, because of its influence on Sun angle, is the most important temperature control. Figures 3–18 and 3–19 clearly show higher temperatures in tropical locations and lower temperatures in polar regions. The maps also show that higher latitudes experience a greater range of temperatures during the year than do lower latitudes. Notice also that the temperature gradient between the subtropics and the poles is greatest during the winter season. A look at two cities—San Antonio, Texas, and Winnipeg, Manitoba— illustrates how seasonal differences in Sun angle and day length account for these temperature patterns. Figure 3–D shows the annual march of temperatures for the two cities, and Figure 3–E illustrates the Sun angles for the June and December solstices. San Antonio and Winnipeg are a fixed distance apart (approximately 20.5° latitude), so the difference in Sun angles between the two cities is the same throughout the year. However, in December, when the Sun’s rays are least direct, this difference more strongly affects the intensity of solar radiation received at Earth’s surface. Therefore, we expect a greater difference in temperatures between the two stations in winter than in summer. Moreover, the seasonal difference in intensity (spreading out of the light beam) at Winnipeg is considerably greater than at San Antonio. This helps to explain the greater annual temperature range at the more northerly station. Table 2–2, Chapter 2, shows that seasonal contrasts in day length also contribute to different temperature patterns at the two cities.
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FIGURE 3–E A comparison of Sun angles (solar noon) for summer and winter solstices at San Antonio and Winnipeg. The space covered by a 90° angle is 1.00.
mum around sunrise (Figure 3–22). It then climbs steadily to a maximum between 2 PM and 5 PM. The temperature then declines until sunrise the following day.
Daily Temperature Variations The primary control of the daily cycle of air temperature is as obvious as the cycle itself: It is Earth’s daily rotation, which causes a location to move into daylight for part of each day and then into darkness. As the Sun’s angle in-
creases throughout the morning, the intensity of sunlight also rises, reaching a peak at local noon and gradually diminishing in the afternoon. Figure 3–23 shows the daily variation of incoming solar energy versus outgoing Earth radiation and the resulting temperature curve for a typical middle-latitude location at the time of an equinox. During the night the atmosphere and the surface of Earth cool as they radiate away heat that is not replaced by incoming solar energy. The minimum temperature, therefore, occurs about the time of sunrise,
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(a)
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Figure 3–22 The minimum daily temperature usually occurs near the time of sunrise. As the ground and air cool during the nighttime hours, familiar early-morning phenomena such as the frost in photo (a) and the ground fog in photo (b) may form. ((a): AP Photo/Daily Telegram/Jed Carlson; (b): Michael Collier) More radiation is received than is lost
Solar radiation
after which the Sun again heats the ground, which in turn heats the air. It is apparent that the time of highest temperature does not generally coincide with the time of maximum radiation. By comparing Figures 3–21 and 3–23, you can see that the curve for incoming solar energy is symmetrical with respect to noon, but the daily air temperature curves are not. The delay in the occurrence of the maximum until mid- to late afternoon is termed the lag of the maximum. Although the intensity of solar radiation drops in the afternoon, it still exceeds outgoing energy from Earth’s surface for a period of time. This produces an energy surplus for up to several hours in the afternoon and contributes substantially to the lag of the maximum. In other words, as long as the solar energy gained exceeds the rate of Earth radiation lost, the air temperature continues to rise. When the input of solar energy no longer exceeds the rate of energy lost by Earth, the temperature falls. The lag of the daily maximum is also a result of the process by which the atmosphere is heated. Recall that air is a poor absorber of most solar radiation; consequently, it is heated primarily by energy reradiated from Earth’s surface. The rate at which Earth supplies heat to the atmosphere through radiation, conduction, and other means, however, is not in balance with the rate at which the atmosphere radiates away heat. Generally, for a few hours after the period of maximum solar radiation, more heat is supplied to the atmosphere by Earth’s surface than is emitted by the atmosphere to space. As a result, most locations experience an increase in air temperature during the afternoon.
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In dry regions, particularly on cloud-free days, the amount of radiation absorbed by the surface will generally be high. Therefore, the time of the maximum temperature at these locales will often occur quite late in the afternoon. Humid locations, in contrast, will frequently experience a shorter time lag in the occurrence of their temperature maximum.
Chapter 3 Temperature
For more than a decade scientists have used the Moderate Resolution Imaging Spectroradiometer (MODIS, for short) aboard NASA’s Aqua and Terra satellites to gather surface temperature data from around the globe. This image shows average landsurface temperatures for the month of February over a 10-year span (2001-2010). (NASA)
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°C –25 Question 1 What are the approximate temperatures for southern Great Britain and northern Newfoundland (white arrows)?
Magnitude of Daily Temperature Changes The magnitude of daily temperature changes is variable and may be influenced by locational factors or local weather conditions or both (see Box 3–3). Four common examples illustrate this point. The first two relate to location, and the second two pertain to local weather conditions: 1. Variations in Sun angle are relatively great during the day in the middle and low latitudes. However, points near the poles experience a low Sun angle all day. Consequently, the temperature change experienced during a day in the high latitudes is small. 2. A windward coast is likely to experience only modest variations in the daily cycle. During a typical 24-hour period the ocean warms less than 1°C. As a result, the air above it shows a correspondingly slight change in temperature. For example, Eureka, California, a windward coastal station, consistently has a lower daily temperature range than Des Moines, Iowa, an inland city at about the same latitude. Annually the daily
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Question 2 Both are coastal areas at the same latitude yet average February temperatures are quite different. Suggest a cause for this disparity.
range at Des Moines averages 10.9°C (19.6°F) compared with 6.1°C (11°F) at Eureka, a difference of 4.8°C (8.6°F). 3. As mentioned earlier, an overcast day is responsible for a flattened daily temperature curve (see Figure 3–15). By day, clouds block incoming solar radiation and so reduce daytime heating. At night the clouds retard the loss of radiation by the ground and air. Therefore, nighttime temperatures are warmer than they otherwise would have been. 4. The amount of water vapor in the air influences daily temperature range because water vapor is one of the atmosphere’s important heat-absorbing gases. When the air is clear and dry, heat readily escapes at night, and the temperature falls rapidly. When the air is humid, absorption of outgoing long-wavelength radiation by water vapor slows nighttime cooling, and the temperature does not fall to as low a value. Thus, dry conditions are associated with a higher daily temperature range because of greater nighttime cooling.
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Box 3–3
How Cities Influence Temperature: The Urban Heat Island
One of the most apparent human impacts on climate is the modification of the atmospheric environment by the building of cities (Figure 3–F). The construction of every factory, road, office building, and house destroys microclimates and creates new ones of great complexity. The most studied and well-documented urban climatic effect is the urban heat island. The term refers to the fact that temperatures within cities are generally higher than in rural areas. The heat island is evident when temperature data such as those that appear in Table 3–B are examined. As is typical, the data for Philadelphia show the heat island is most pronounced when minimum temperatures are examined. The magnitude of the temperature differences shown by Table 3–B is probably even greater than the figures indicate because temperatures observed at suburban airports are usually higher than those in truly rural environments. Figure 3–G, which shows the distribution of average minimum temperatures in the Washington, D.C., metropolitan area for the three-month winter period (December through February) over a five-year span, also illustrates a well-developed heat island. The warmest winter temperatures occurred in the heart of the city, whereas the suburbs and surrounding countryside experienced average minimum temperatures that were as much as 3.3°C (6°F) lower. Remember that these temperatures are averages. On many clear, calm nights the temperature difference between the city center and the countryside was considerably greater, often 11°C (20°F) or more. Conversely, on many overcast or windy nights the temperature differential approached 0°. One effect of an urban heat island is toinfluence the biosphere by extending
FIGURE 3–F The radical change in the surface that occurs when rural areas are transformed into cities leads to the creation of an urban heat island. (Photo by Grin Maria/Shutterstock)
theplant cycle.* After examining data for 70cities in eastern North America, researchers found that the growing cycle in cities was about 15 days longer than in surrounding rural areas. Plants began the growth cycle an average of about 7 days earlier in spring and continued growing an average of 8 days longer in fall. Why are cities warmer than rural areas? The radical change in the surface that results when rural areas are transformed into cities is a significant cause of an urban heat island (Figure 3–H). First, the tall buildings and the concrete and asphalt of the city absorb and store greater quantities of solar radiation than do the vegetation and soil typical
TABLE 3–B Average Temperatures (°C) for Suburban Philadelphia Airport and Downtown Philadelphia (10-year averages) Airport
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Source: After H. Neuberger and J. Cahir, Principles of Climatology (New York: Holt, Rinehart and Winston, 1969), 128.
of rural areas. In addition, because the city surface is impermeable, the runoff of water following a rain is rapid, resulting in a significant reduction in the evaporation rate. Hence, heat that once would have been used to convert liquid water to a gas now goes to increase further the surface temperature. At night, as both the city and countryside cool by radiative losses, the stonelike surface of the city gradually releases the additional heat accumulated during the day, keeping the urban air warmer than that of the outlying areas. A portion of the urban temperature rise is also attributable to waste heat from sources such as home heating and air-conditioning, power generation, industry, and transportation. In addition, the “blanket” of pollutants over a city contributes to a heat island by absorbing a portion of the upward-directed longwave radiation emitted by the surface and reemitting some of it back to the ground.
*
Reported in Weatherwise, 57, no. 6 (November/ December 2004), 20.
Chapter 3 Temperature
FIGURE 3–G The heat island of Washington, D.C., as shown by the average minimum temperatures (°C) during the winter season (December through February). The city center had an average minimum that was nearly 4°C higher than some outlying areas. (After Clarence A. Woolum, “Notes from the Study of the Microclimatology of the Washington, D.C., area for the Winter and Spring Seasons,” Weatherwise, 17, no. 6 (1964), 264, 267.)
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FIGURE 3–H This pair of satellite images provides two views of Atlanta, Georgia, on September 28, 2000. The urban core is in the center of the images. The top image is a photo-like view in which vegetation is green, roads and dense development appear gray, and bare ground is tan or brown. The bottom image is a land-surface temperature map in which cooler temperatures are yellow and hotter temperatures are red. It is clear that where development is densest, the land-surface temperatures are highest. (NASA)
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Although the rise and fall of daily temperatures usually reflects the general rise and fall of incoming solar radiation, such is not always the case. For example, a glance back at Figure 3–21 shows that on May 23 the maximum temperature occurred at midnight, after which temperatures fell throughout the day. If records for a station are examined for a period of several weeks, apparently random variations are seen. Obviously these are not Sun controlled. Such irregularities are caused primarily by the passage of atmospheric disturbances (weather systems) that are often accompanied by variable cloudiness and winds that bring air having contrasting temperatures. Under these circumstances, the maximum and minimum temperatures may occur at any time of the day or night.
Annual Temperature Variations In most years the months with the highest and lowest mean temperatures do not coincide with the periods of maximum and minimum incoming solar radiation. North of the tropics the greatest intensity of solar radiation occurs at the time of the summer solstice in June, yet the months of July and August are generally the warmest of the year in the Northern Hemisphere. Conversely, a minimum of solar energy is received in December at the time of the winter solstice, but January and February are usually colder. The fact that the occurrence of annual maximum and minimum radiation does not coincide with the times of temperature maximums and minimums indicates that the amount of solar radiation received is not the only factor determining the temperature at a particular location. Recall from Chapter 2 that places equatorward of about 38° north and 38° south receive more solar radiation than is lost to space and that the opposite is true of more poleward regions. Based on this imbalance between incoming and outgoing radiation, any location in the southern United States, for example, should continue to get warmer late into autumn. But this does not occur because more poleward locations begin experiencing a negative radiation balance shortly after the summer solstice. As the temperature contrasts become greater, the atmosphere and ocean currents “work harder” to transport heat from lower latitudes poleward.
Concept Check 3.9 1 Although the intensity of incoming solar radiation is greatest at local noon, the warmest part of the day is most often midafternoon. Why? Use Figure 3–23 to explain.
2 List at least three factors that might cause the daily temperature range to vary significantly from place to place and from time to time.
Temperature Measurement ATMOSPHERE
Temperature Data and the Controls of Temperature ▸ Basic Temperature Data
Thermometers are “meters of therms”: They measure temperature (Figure 3–24). Thermometers measure temperature either mechanically or electrically.
Mechanical Thermometers Most substances expand when heated and contract when cooled, and many common thermometers operate using this property. More precisely, they rely on the fact that different substances react to temperature changes differently. The liquid-in-glass thermometer shown in Figure 3–25 is a simple instrument that provides relatively accurate readings over a wide temperature range. Its design has remained essentially unchanged since it was developed in the late 1600s. When temperature rises, the molecules of fluid grow more active and spread out (the fluid expands). Expansion of the fluid in the bulb is much greater than the expansion of the enclosing glass. As a consequence, a thin “thread” of fluid is forced up the capillary tube. Conversely, when temperature falls, the liquid contracts, and the thread of fluid moves back down the tube toward the bulb. The movement of the end of this thread (known as the meniscus) is calibrated against an established scale to indicate the temperature. The highest and lowest temperatures that occur each day are of considerable importance and are often obtained by using specially designed liquid-in-glass thermometers. Mercury is the liquid used in the maximum thermometer, which has a narrowed passage called a constriction in the bore of the glass tube just above the bulb (Figure 3–26a). As the temperature rises, the mercury expands
Figure 3–24 The design of this thermometer is based on an instrument called a thermoscope that was invented by Galileo in the late 1500s. Today such devices, which are fairly accurate, are used mostly for decoration. The instrument is made of a sealed glass cylinder containing a clear liquid and a series of glass bulbs of different densities that can “float” up and down. (Photo by Frank Labua/Pearson Education)
Chapter 3 Temperature
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and is forced through the constricMaximum Thermometer tion. When the temperature falls, Current temperature (75°F) Maximum temperature (93°F) Constriction the constriction prevents a return of –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90 100 110 120 mercury to the bulb. As a result, the top of the mercury column remains at the highest point (maximum temMercury (a) perature attained during the measurement period). The instrument is Minimum Thermometer reset by shaking or by whirling it to Minimum temperature (53°F) Current temperature (75°F) force the mercury through the con–40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90 100 110 120 striction back into the bulb. Once the thermometer is reset, it indicates the current air temperature. Index Alcohol In contrast to a maximum ther(b) mometer that contains mercury, a Figure 3–26 (a) Maximum thermometer and (b) minimum thermometer. minimum thermometer contains a liquid of low density, such as alcohol. mounted horizontally; otherwise, the index will fall to the Within the alcohol, and resting at the top of the column, is bottom. a small dumbbell-shaped index (Figure 3–26b). As the air Another commonly used mechanical thermometer is temperature drops, the column shortens, and the index is the bimetal strip. As the name indicates, this thermometer pulled toward the bulb by the effect of surface tension with consists of two thin strips of metal that are bonded together the meniscus. When the temperature subsequently rises, the and have widely different expansion properties. When the alcohol flows past the index, leaving it at the lowest temtemperature changes, both metals expand or contract, but perature reached. To return the index to the top of the alcothey do so unequally, causing the strips to curl. This change hol column, the thermometer is simply tilted. Because the corresponds to the change in temperature. index is free to move, the minimum thermometer must be The primary meteorological use of the bimetal strip is in the construction of a thermograph, an instrument that continuously records temperature. The changes in the curvature of the strip can be used to move a pen arm that records the temperature on a calibrated chart that is attached to a clock-driven, rotating drum (Figure 3–27). Although very Capillary convenient, thermograph records are generally less accutube rate than readings obtained from a mercury-in-glass thermometer. To obtain the most reliable values, it is necessary 75 Meniscus to check and correct the thermograph periodically by comparing it with an accurate, similarly exposed thermometer. 70
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Figure 3–27 A common use of the bimetal strip is in the construction of a thermograph, an instrument that continuously records temperatures.
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Electrical Thermometers
Instrument Shelters
Some thermometers do not rely on differential expansion but instead measure temperature electrically. A resistor is a small electronic part that resists the flow of electrical current. A thermistor (thermal resistor) is similar, but its resistance to current flow varies with temperature. As temperature increases, so does the resistance of the thermistor, reducing the flow of current. As temperature drops, so does the resistance of the thermistor, allowing more current to flow. The current operates a meter or digital display that is calibrated in degrees of temperature. The thermistor thus is used as a temperature sensor—an electrical thermometer. Thermistors are rapid-response instruments that quickly register temperature changes. Therefore, they are commonly used in radiosondes where rapid temperature changes are often encountered. The National Weather Service also uses a thermistor system for ground-level readings. The sensor is mounted inside a shield made of louvered plastic rings, and a digital readout is placed indoors (Figure 3–28).
How accurate are thermometer readings? Accuracy depends not only on the design and quality of the instruments but also on where they are placed. Placing a thermometer in direct sunlight will give a grossly excessive reading because the instrument itself absorbs solar energy much more efficiently than does the air. Placing a thermometer near a heat-radiating surface, such as a building or the ground, also yields inaccurate readings. Another way to assure false readings is to prevent air from moving freely around the thermometer. So where should a thermometer be placed to read air temperature accurately? The ideal location is an instrument shelter (Figure 3–29). An instrument shelter is a white box that has louvered sides to permit the free movement of air through it, while shielding the instruments from direct sunshine, heat from the ground, and precipitation. Furthermore, the shelter is placed over grass whenever possible and as far away from buildings as circumstances permit. Finally, the shelter must conform to a standardized height so that the thermometers will be mounted at 1.5 meters (5 feet) above the ground (Box 3–4).
Students Sometimes Ask… What are the highest and lowest temperatures ever recorded at Earth’s surface? The world’s record-high temperature is nearly 58°C (136°F)! It was recorded on September 13, 1922, at Azizia, Libya, in North Africa’s Sahara Desert. The lowest recorded temperature is 89C (129F). This incredibly frigid temperature was recorded in Antarctica, at the Russian Vostok Station, on July 21, 1983.
Figure 3–28 This modern shelter contains an electrical thermometer called a thermistor. (Photo by Bobbé Christopherson)
Figure 3–29 This traditional standard instrument shelter is white (for high albedo) and louvered (for ventilation). It protects instruments from direct sunlight and allows for the free flow of air. (Courtesy of Qualimetrics, Inc.)
Chapter 3 Temperature
Concept Check 3.10
The Celsius–Fahrenheit relationship also is shown by the following formulas: °F 5 1 1.8 3 °C 2 1 32
1 Describe how each of the following thermometers works: liquid-in-glass, maximum, minimum, bimetal strip, and thermistor.
2 What is a thermograph? Which type of mechanical thermometer is commonly used in the construction of a thermograph?
3 In addition to having an accurate thermometer, which other factors must be considered to obtain a meaningful air temperature reading?
Temperature Scales In the United States, TV weather reporters give temperatures in degrees Fahrenheit. But scientists as well as most people outside the United States use degrees Celsius. Scientists sometimes also use the Kelvin, or absolute, scale. What are the differences among these three temperature scales? To make quantitative measurement of temperature possible, it was necessary to establish scales. Such temperature scales are based on the use of reference points, sometimes called fixed points. In 1714, Gabriel Daniel Fahrenheit, a German physicist, devised the Fahrenheit scale. He constructed a mercury-in-glass thermometer in which the zero point was the lowest temperature he could attain with a mixture of ice, water, and common salt. For his second fixed point he chose human body temperature, which he arbitrarily set at 96°. On this scale, he determined that the melting point of ice (the ice point) was 32° and the boiling point of water (the steam point) was 212°. Because Fahrenheit’s original reference points were difficult to reproduce accurately, his scale is now defined by using the ice point and the steam point. As thermometers improved, average human body temperature was later changed to 98.6°F.* In 1742, 28 years after Fahrenheit invented his scale, Anders Celsius, a Swedish astronomer, devised a decimal scale on which the melting point of ice was set at 0° and the boiling point of water at 100°.† For many years it was called the centigrade scale, but it is now known as the Celsius scale, after its inventor. Because the interval between the melting point of ice and the boiling point of water is 100° on the Celsius scale and 180° on the Fahrenheit scale, a Celsius degree (°C) is larger than a Fahrenheit degree (°F) by a factor of 180/100, or 1.8. So, to convert from one system to the other, allowance must be made for this difference in the size of the degrees. Also, conversions must be adjusted because the ice point on the Celsius scale is at 0° rather than at 32°. This relationship is shown graphically in Figure 3–30. *This traditional value for “normal body temperature” was established in 1868. A recent assessment places the value at 98.2°F, with a range of 4.8°F. †
The boiling point referred to in the Celsius and Fahrenheit scales pertains to pure water at standard sea-level pressure. It is necessary to remember this fact because the boiling point of water gradually decreases with altitude.
89
and: °F 2 32 1.8 You can see that the formulas adjust for degree size with the 1.8 factor and adjust for the different 0° points with the 632 factor. For some scientific purposes a third temperature scale is used, the Kelvin, or absolute, scale. On this scale, degrees Kelvin are called Kelvins (abbreviated K). This scale is similar to the Celsius scale because its divisions are exactly the same; there are 100° separating the melting point of ice and the boiling point of water. However, on the Kelvin scale, the ice point is set at 273 K, and the steam point is at 373 K (Figure 3–30). The reason is that the zero point represents the temperature at which all molecular motion is presumed to cease (called absolute zero). Thus, unlike with the Celsius and Fahrenheit scales, it is not possible to have a negative °C 5
Boiling point of water (steam point) (100°C, 212°F)
°F
°C
K
210
100
373
90
363
80
353
70
343
60
333
50
323
40
313
30
303
20
293
10
283
273
–10
263
–20
253
200 190 180 170 160 150
Record world high temperature (58°C, 136°F)
140 130 120 110 100 90
Comfortable room temperature (22°C, 72°F)
80 70 60 50
Melting point of ice (ice point) (0°C, 32°F)
40 30 20 10 0 10
Figure 3–30 The three temperature scales compared.
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The Atmosphere: An Introduction to Meteorology
Box 3–4
Applying Temperature Data heat a building in Chicago (nearly 6500 total heating degree-days) than to heat a similar building in Los Angeles (almost 1300 heating degree-days). This statement is true, however, only if we assume that building construction and living habits in these areas are the same. Each day, the previous day’s accumulation is reported along with the total thus far in the season. For reporting purposes the heating season is defined as the period
To make weather data more useful to people, many different applications have been developed. This box focuses on three indices that all have the term degree-days as part of their name: heating degree-days, cooling degree-days, and growing degree-days. The first two are relative measures that allow us to evaluate the weather-produced needs and costs of heating and cooling. The third is a simple index used by farmers to estimate the maturity of crops.
portional to the total heating degree-days. This linear relationship means that doubling the heating degree-days usually doubles the fuel consumption. Consequently, a fuel bill will generally be twice as high for a month with 1000 heating degree-days as for a month with 500. When seasonal totals are compared for different places, we can estimate differences in seasonal fuel consumption (Table 3–C). For example, more than five times as much fuel is required to
Heating Degree-Days Heating degree-days represent a practical method for evaluating energy demand and consumption. This index starts from the assumption that heating is not required in a building when the daily mean temperature is 65°F (18.3°C) or higher.* SimpIy, each degree of temperature below 65°F is counted as 1 heating degree-day. Therefore, heating degree-days are determined each day by subtracting the daily mean below 65°F from 65°F. Thus, a day with a mean temperature of 50°F has 15 heating degree-days (65 – 50 = 15), and one with an average temperature of 65°F or higher has none. The amount of heat required to maintain a certain temperature in a building is pro-
TABLE 3–C Average Annual Heating and Cooling Degree-Days for Selected Cities*
*Because the National Weather Service and the news media in the United States still compute and report degree-day information in Fahrenheit degrees, we will use the Fahrenheit scale throughout this discussion.
City
Cooling degree-days
10,470
3
3807
1774
Boston, MA
5630
777
Chicago, IL
6498
830
Denver, CO
6128
695
Detroit, MI
6422
736
Great Falls, MT
7828
288
Baltimore, MD
International Falls, MN
10,269
233
Las Vegas, NV
2239
3214
Los Angeles, CA
1274
679
Miami, FL
149
4361
New York City, NY
4754
1151
Phoenix. AZ
1125
4189
San Antonio, TX
1573
3038
Seattle, WA
4797
173
*
Source: NOAA, National Climatic Data Center.
value when using the Kelvin scale, for there is no temperature lower than absolute zero. The relationship between the Kelvin and Celsius scales is easily written as follows: °C 5 K 2 273 or: K 5 °C 1 273
Concept Check 3.11 1 What is meant by the terms steam point and ice point? What values are given to these points on each of the three temperature scales presented here?
2 Why is it not possible to have a negative value when using the Kelvin scale?
Heating Degree-Days
Anchorage, AK
Students Sometimes Ask… Which countries use the Fahrenheit scale? The United States and Belize (a small country in Central America) continue to use the Fahrenheit scale for everyday applications. The rest of the world has adopted the Celsius scale. The Celsius and Kelvin scales are used universally in the scientific community.
Heat Stress and Windchill: Indices ofHuman Discomfort Summertime weather reports often try to make people aware of potential harmful effects of high humidity coupled with high temperatures. In winter we are reminded about the effect of strong winds combined with low temperatures.
Chapter 3 Temperature
91
number represents the accumulation since January 1 of that year. Although indices that are more sophisticated than heating and cooling degree-days have been proposed to take into account the effects of wind speed, solar radiation, and humidity, degree-days continue to be widely used.
FIGURE 3–I Growing degree-days are used to determine the approximate date when crops will be ready for harvest. (Photo by Kletr/Shutterstock)
from July I through June 30. These reports often include a comparison with the total up to this date last year or with the long-term average for this date or both, and so it is a relatively simple matter to judge whether the season thus far is above, below, or near normal. Cooling Degree-Days Just as fuel needs for heating can be estimated and compared by using heating degree-days, the amount of power required to cool a building can be estimated by using a similar index called the cooling degreeday. Because the 65°F base temperature is
also used in calculating this index, cooling degree-days are determined each day by subtracting 65°F from the daily mean. Thus, if the mean temperature for a given day is 80°F, 15 cooling degree-days would be accumulated. Mean annual totals of cooling degree-days for selected cities are shown in Table 3–C. By comparing the totals for Baltimore and Miami, we can see that the fuel requirements for cooling a building in Miami are almost 2½ times as great as for a similar building in Baltimore. The “cooling season” is conventionally measured from January1 through December 31. Therefore, when cooling degree-day totals are reported, the
In the first instance, we are cautioned about heat stress and the possibility of heat stroke, and in the second case we are warned about windchill and the potential dangers of frostbite. These indices are expressions of apparent temperature—the temperature that a person perceives. Heat stress and windchill are based on the fact that our sensation of temperature is often different from the actual air temperature recorded by a thermometer. The human body is a heat generator that continually releases energy. Anything that influences the rate of heat loss from the body also influences our sensation of temperature, thereby affecting our feeling of comfort. Several factors control the thermal comfort of the human body, and certainly air temperature is a major one. Other environmental conditions are also significant, such as relative humidity, wind, and sunshine.
Growing Degree-Days Another practical application of temperature data is used in agriculture to determine the approximate date when crops will be ready for harvest. This simple index is called the growing degree-day. The number of growing degree-days for a particular crop on any day is the difference between the daily mean temperature and the base temperature of the crop, which is the minimum temperature required for it to grow. For example, the base temperature for sweet corn is 50°F, and for peas it is 40°F. Thus, on a day when the mean temperature is 75°F, the number of growing degree-days for sweet corn is 25 and the number for peas is 35. Starting with the onset of the growth season, the daily growing degree-day values are added. Thus, if 2000 growing degreedays are needed for a crop to mature, it should be ready to harvest when the accumulation reaches 2000 (Figure 3–I). Although many factors important to plant growth are not included in the index, such as moisture conditions and sunlight, this system nevertheless serves as a simple and widely used tool in determining approximate dates of crop maturity.
Heat Stress—High Temperatures PlusHigh Humidities High humidity contributes significantly to the discomfort people feel during a heat wave. Why are hot, muggy days so uncomfortable? Humans, like other mammals, are warmblooded creatures who maintain a constant body temperature, regardless of the temperature of the environment. One of the ways the body prevents overheating is by perspiring. However, this process does little to cool the body unless the perspiration can evaporate. It is the cooling created by the evaporation of perspiration that reduces body temperature. Because high humidities retard evaporation, people are more uncomfortable on a hot and humid day than on a hot and dry day. Generally, temperature and humidity are the most important elements influencing summertime human comfort.
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The Atmosphere: An Introduction to Meteorology
Heat Index Relative Humidity (%) 40
45
50
55
60
65
70
75
80
85
90
95
100
110 136
With prolonged exposure and/or physical activity Extreme danger Heat stroke or sunstroke highly likely
108 130 137 106 124 130 137 104 119 124 131 137 Air Temperature (°F)
102 114 119 124 130 137
Danger Sunstroke, muscle cramps, and/or heat exhaustion likely
100 109 114 118 124 129 136 98
105 109 113 117 123 128 134
96
101 104 108 112 116 121 126 132
94
97
100 102 106 110 114 119 124 129 135
92
94
96
99
101 105 108 112 116 121 126 131
90
91
93
95
97
88
88
89
91
93
Extreme caution Sunstroke, muscle cramps, and/or heat 100 103 106 109 113 117 122 127 132 exhaustion possible 95 98 100 103 106 110 113 117 121
86
85
87
88
89
91
93
95
97
100 102 105 108 112
84
83
84
85
86
88
89
90
92
94
96
98
100 103
82
81
82
83
84
84
85
86
88
89
90
91
93
95
80
80
80
81
81
82
82
83
84
84
85
86
86
87
One index widely used by the National Weather Service that combines these factors to establish the degree of comfort or discomfort is called the heat stress index, or simply the heat index. If you examine Figure 3–31, you will see that as relative humidity increases, the apparent temperature, and thus heat stress, increases as well. Further, when the relative humidity is low, the apparent temperature can have a value that is less than the actual air temperature. It is important to note that factors such as the length of exposure to direct sunlight, the wind speed, and the general health of the individual greatly affect the amount of stress a person will experience. In addition, while a period of hot, humid weather in New Orleans might be reasonably well tolerated by its residents, a similar event in a northern city such
Caution Fatigue possible
Wind (mph)
10 0 –5 –10 –15 –20 –25 –30 –35 –40 –45 1 –5 –11 –16 –22 –28 –34 –40 –46 –52 –57 –63 –4 –10 –16 –22 –28 –35 –41 –47 –53 –59 –66 –72 –7 –13 –19 –26 –32 –39 –45 –51 –58 –64 –71 –77 –9 –15 –22 –29 –35 –42 –48 –55 –61 –68 –74 –81 5
Calm 40
35
30
25
20 15
5
36
31
25
19
13
7
10
34
27
21
15
9
3
15
32
25
19
13
6
20
30
24
17
11
4
–2
25
29
23
16
9
3
30
28
22
15
8
1
35
28
21
14
7
–4 –11 –17 –24 –31 –37 –44 –51 –58 –64 –71 –78 –84 –5 –12 –19 –26 –33 –39 –46 –53 –60 –67 –73 –80 –87 –7 –14 –21 –27 –34 –41 –48 –55 –62 –69 –76 –82 –89
40
27
20
13
6
45
26
19
12
5
50
26
19
12
4
0 –1 –8 –15 –22 –29 –36 –43 –50 –57 –64 –71 –78 –84 –91 –2 –9 –16 –23 –30 –37 –44 –51 –58 –65 –72 –79 –86 –93 –3 –10 –17 –24 –31 –38 –45 –52 –60 –67 –74 –81 –88 –95
55
25
18
11
4
60
25
17
10
3
–3 –11 –18 –25 –32 –39 –46 –54 –61 –68 –75 –82 –89 –97 –4 –11 –19 –26 –33 –40 –48 –55 –62 –69 –76 –84 –91 –98
30 minutes
10 minutes
as Minneapolis would not be well tolerated. This occurs because hot and humid weather is more taxing on people who live where these conditions are relatively rare than it is on people who live where prolonged periods of heat and high humidity are the rule.
Windchill—The Cooling Power ofMovingAir
Most everyone is familiar with the wintertime cooling power of moving air. When the wind blows on a cold day, we realize that comfort would improve if the wind would stop. A stiff breeze penetrates ordinary clothing and reduces its capacity to retain body heat while causing exposed parts of the body to chill rapidly. Not only is cooling by evaporation heightened in this situation but the wind is also acting to carry heat away from the body by constantly replacing warmer air next to the body with colder air. The U.S. National Weather Service and the Meteorological Services of Canada use a Wind Chill Temperature (WCT) index that is designed to calculate how the wind and cold feel on human skin (Figure 3–32). The index accounts for wind effects at face level and takes into account body heat-loss estimates. It was tested on human subjects in a chilled wind tunnel. The results of those trials were used to validate and improve the accuracy of the formula. The wind
Temperature (˚F)
Frostbite Times
Figure 3–31 Heat index is a measure of apparent temperature. For example, if the air temperature is 90°F and the relative humidity is 65%, it would “feel like” 103°F. As relative humidity increases, apparent temperature increases as well.
5 minutes
Figure 3–32 Wind chill chart. Fahrenheit temperatures are used because this is how the National Weather Service and the news media in the United States commonly report wind chill information. The shaded areas on the chart indicate frostbite danger. Each shaded zone shows how long a person can be exposed before frostbite develops. For example, a temperature of 0°F and a wind speed of 15 miles per hour will produce a windchill temperature of −19°F. Under these conditions, exposed skin can freeze in 30 minutes. (After NOAA, National Weather Service)
Chapter 3 Temperature
93
Captain Ryan J. Harris, Military Meteorologist Before any United States military mission gets a green light, commanders always check the weather. Weather is a crucial element in virtually every task, from flying a bombing mission, to delivering supplies, to conducting covert reconnaissance. The approach of a storm, the percentage of cloud cover, and even magnetic storms on the Sun can influence whether deployed troops will return safe and sound. As a military meteorologist, “my role is to know exactly what the weather is and how it’s going to provide battlefield commanders with an advantage over our adversaries,” says Captain Ryan J. Harris. As a Weather Flight Commander in the United States Air Force, Captain Harris leads a unit of 45 Airmen devoted to weather forecasting. He works out of Offutt Air Force Base near Omaha, Nebraska. “We’re not your typical weatherman on TV. We’re providing similar information, but tailored to military operations,” Captain Harris says. Commanders want to know, “Can I move a battalion through this pass, or will it be too muddy because of heavy snowmelt? Is there enough nasty weather to shroud the actions of my special ops mission? Can I fly this helicopter in these winds?” To serve the needs of both the Army and the Air Force, weather teams like Captain Harris’s operate 24 hours a day. Different weather units, at bases around the world, track atmospheric models on local, regional, and global scales. Their predictions have shaped events ranging from the invasion of Normandy during World War II, to the Navy SEALS mission that tracked down Osama bin Laden. Captain Harris has watched the skies for almost as long as he can remember. As a kindergartner, he often preferred the Weather Channel to cartoons on TV. He wrote so often about weather in his first grade journal that his teacher suggested he become a weatherman. His plans still hadn’t changed by high school. There, a counselor
mentioned that the Air Force always needs meteorologists and may even help pay for a college degree. He applied and was accepted to the Reserve Officer Training Corps (ROTC), which supported his bachelor’s degree in meteorology, and earned him an officer commission in the Air Force.
Captain Harris’s Air Force assignments have taken him around the world. Captain Harris’s Air Force assignments have taken him around the world. In Illinois, he learned to make global weather forecasts for aerial refueling and aeromedical evacuation missions. In Germany, he learned to tailor forecasts to meet fighter aircraft commanders’ specific needs. He recently returned to school to earn a master’s degree at the Naval Postgraduate School in Monterey, California. There, he specialized in satellite meteorology. The military, Captain Harris says, “has niches for everyone in meteorology. Those who want to do research and understand the theory, and those who want to take the science and apply it.” Captain Harris puts himself in the latter category. “The idea of applying meteorology to aviation and military operations certainly intrigued me and drew me in. It’s been a great experience.”
to improve forecasting models, and provides snow analyses for Army and Special Operations missions, among other duties. As sophisticated as this work can be, Captain Harris still applies the meteorology basics he learned in college. “I don’t analyze weather charts so often anymore. But I still have to retain the knowledge from my college days to see that this part of this model is not right, and use theoretical physics and meteorology as a critical foundation when my commander asks why we need this model or instrument over that,” he says. The art and science of forecasting weather has improved dramatically over the last century since the first numerical models were developed. Nowhere are these advances more important to get the weather right than in the military. “For us in military weather, the lives of our fellow Airmen, Soldiers, Sailors, and Marines depend on an accurate forecast. All of us take our jobs very seriously,” Captain Harris says.
“The idea of applying meteorology to aviation and military operations certainly intrigued me and drew me in.” Captain Harris now has a unique job in the Air Force. “The best way to describe it is, anything that doesn’t fit in the scope of normal terrestrial weather falls under us,” he says. His purview includes tracking airborne ash from volcanoes, delivering global cloud analyses and forecasts, and analyzing meteorological satellite imagery. He also works with scientists
chill chart includes a frostbite indicator that shows where temperature, wind speed, and exposure time will produce frostbite (Figure 3–32). It is worth pointing out that in contrast to a cold and windy day, a calm and sunny day in winter feels warmer than the thermometer reading. In this situation the warm
CAPTAIN RYAN J. HARRIS leads a 45-person forecasting team. (Photo courtesy of USAF)
feeling is caused by the absorption of direct solar radiation by the body. The index does not take into account any offsetting effect for windchill due to solar radiation. Such a factor may be added in the future. It is important to remember that the windchill temperature is only an estimate of human discomfort. The degree
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The Atmosphere: An Introduction to Meteorology
of discomfort felt by different people will vary because it is influenced by many factors. Even if clothing is assumed to be the same, individuals vary widely in their responses because of such factors as age, physical condition, state of health, and level of activity. Nevertheless, as a relative measure, the WCT index is useful because it allows people to make more informed judgments regarding the potential harmful effects of wind and cold.
Concept Check 3.12 1 What is apparent temperature? 2 Why does high humidity contribute to summertime discomfort?
3 Why do strong winds make apparent temperatures in winter feel lower than the thermometer reading?
4 List several reasons heat stress and wind chill do not affect everyone the same.
Give It Some Thought 1. If you were asked to identify the coldest city in the
United States (or any other designated region), what statistics could you use? Can you list at least three different ways of selecting the coldest city? 2. The accompanying graph shows monthly high
temperatures for Urbana, Illinois, and San Francisco, California. Although both cities are located at about the same latitude, the temperatures they experience are quite different. Which line on the graph represents Urbana and which represents San Francisco? How did you figure this out?
3. On which summer day would you expect the greatest
temperature range? Which would have the smallest range in temperature? Explain your choices. a. Cloudy skies during the day and clear skies at night b. Clear skies during the day and cloudy skies at night c. Clear skies during the day and clear skies at night d. Cloudy skies during the day and cloudy skies at night 4. The accompanying scene (Photo by Chad Ehlers/ shows an island near the Photolibrary)
equator in the Indian Ocean. Describe how latitude, altitude, and the differential heating of land and water influence the climate of this place. 5. The accompanying sketch map represents a hypothetical continent in the Northern Hemisphere. One isotherm has been placed on the map.
a. Is the temperature higher at city A or city B? Explain. b. Is the season winter or summer? How are you able to determine this? c. Describe (or sketch) the position of this isotherm six months later. 6. The data below are mean monthly temperatures in °C for an inland location that lacks any significant ocean influence. Based on the annual temperature range, what is the approximate latitude of this place? Are these temperatures what you would normally expect for this latitude? If not, what control would explain these temperatures? J F M A M J J A S O N D 6.1 6.6 6.6 6.6 6.6 6.1 6.1 6.1 6.1 6.1 6.6 6.6 7. Refer to Figure 3–18. What causes the bend or kink in
the isotherms in the North Atlantic?
Chapter 3 Temperature
95
TEMPERATURE IN REVIEW ●
●
●
●
Temperature is one of the basic elements of weather and climate. Commonly used temperature data include daily mean temperature, daily temperature range, monthly mean temperature, annual mean temperature, and annual temperature range. Such data are commonly depicted on maps using isotherms, lines of equal temperature. The controls of temperature—factors that cause temperature to vary from place to place and from time to time—include latitude, differential heating of land and water, ocean currents, altitude, geographic position, and cloud cover and albedo. On world maps showing January and July mean temperatures, isotherms generally trend east–west and show a decrease in temperature moving poleward from the equator. When the two maps are compared, a latitudinal shifting of temperatures is easily seen. Bending isotherms reveal the locations of ocean currents. Annual temperature range is small near the equator and increases with an increase in latitude. Outside the tropics, annual temperature range also increases as marine influence diminishes.
●
●
●
● ●
The primary control of the daily cycle of air temperature is Earth’s rotation. The magnitude of daily changes is variable and influenced by locational factors, local weather conditions, or both. As a consequence of the mechanism by which Earth’s atmosphere is heated, the months of the highest and lowest temperatures do not coincide with the periods of maximum and minimum incoming solar radiation. Thermometers measure temperature either mechanically or electrically. Most mechanical thermometers are based on the ability of a material to expand when heated and contract when cooled. Electrical thermometers use a thermistor (a thermal resistor) to measure temperature. Temperature scales are established using reference points, called fixed points. Three common scales are the Fahrenheit scale, the Celsius scale, and the Kelvin, or absolute, scale. Heat stress and wind chill are two familiar uses of temperature data that relate to apparent temperature—the temperature people perceive.
VOCABULARY REVIEW absolute zero (p. 89) apparent temperature (p. 91) annual mean temperature (p. 66) annual temperature range (p. 66) bimetal strip (p. 87) Celsius scale (p. 89) controls of temperature (p. 67) daily mean temperature (p. 66)
daily temperature range (p. 66) Fahrenheit scale (p. 89) fixed points (p. 89) ice point (p. 89) isotherm (p. 66) Kelvin, or absolute, scale (p. 89) liquid-in-glass thermometer (p. 86) maximum thermometer (p. 86)
minimum thermometer (p. 87) monthly mean temperature (p. 66) specific heat (p. 69) steam point (p. 89) temperature gradient (p. 66) thermistor (p. 88) thermograph (p. 87) thermometer (p. 86)
PROBLEMS 1. Examine the number of hurricane-related deaths in Figure 3–C (p. 77). Be sure to read the entire caption. If Hurricane Katrina had not occurred, what number would have been plotted on the graph for hurricane fatalities? 2. Refer to the thermograph record in Figure 3–21. Determine the maximum and minimum temperature for each day of the week. Use these data to calculate the daily mean and daily range for each day. 3. By referring to the world maps of temperature distribution for January and July (Figures 3–18 and 3–19), determine the approximate January mean, July mean, and annual temperature range for a place located at 60° north latitude, 80° east longitude, and a place located at 60° south latitude, 80° east longitude. 4. Calculate the annual temperature range for three cities in Appendix G. Try to choose cities with different ranges and explain these differences in terms of the controls of temperature.
5. Referring to Figure 3–32, determine wind chill temperatures under the following circumstances: a. Temperature 5 5°F, wind speed 5 15 mph. b. Temperature 5 5°F, wind speed 5 30 mph. 6. The mean temperature is 55°F on a particular day. The following day the mean drops to 45°F. Calculate the number of heating degree-days for each day. How much more fuel would be needed to heat a building on the second day compared with the first day? 7. Use the appropriate formula to convert the following temperatures: 20°C 5 ____°F 225°C 5 ____K 59°F 5 ____°C
Log in to www.mymeterologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Temperature.
Moisture and Atmospheric Stability Water vapor is an odorless, colorless gas that mixes freely with the other gases of the atmosphere. Unlike oxygen and nitrogen—the two most abundant components of the atmosphere—water can change from one state of matter to another (solid, liquid, or gas) at the temperatures and pressures experienced on Earth. (By comparison, nitrogen in the atmosphere condenses to a liquid at a temperature of –l96°C [–371°F].) Because of this unique property, water leaves the oceans as a gas and returns again as a liquid.
An afternoon rain storm near Muddy Creek, Utah. (Photo by Michael Collier)
After completing this chapter, you should be able to:
t 4LFUDIBOEEFTDSJCFUIFNPWFNFOUPGXBUFSUISPVHI
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DIBOHFTJOOBUVSF t $PNQBSFBOEDPOUSBTUSFMBUJWFIVNJEJUZBOEEFX QPJOUUFNQFSBUVSF t %FTDSJCFBEJBCBUJDUFNQFSBUVSFDIBOHFTBOE FYQMBJOXIZUIFXFUBEJBCBUJDSBUFPGDPPMJOHJTMFTT UIBOUIFESZBEJBCBUJDSBUF t -JTUBOEEFTDSJCFGPVSNFDIBOJTNTUIBUDBVTFBJSUPSJTF
t %FGJOFvapor pressureBOEEFTDSJCFUIFSFMBUJPOTIJQ
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97
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The Atmosphere: An Introduction to Meteorology
Movement of Water Through the Atmosphere Water is found everywhere on Earth—in the oceans, glaciers, rivers, lakes, air, soil, and living tissue (Figure 4-1). The vast majority of the water on or close to Earth’s surface (over 97 percent) is saltwater found in the oceans. Much of the remaining 3 percent is stored in the ice sheets of Antarctica and Greenland. Only a meager 0.001 percent is found in the atmosphere, and most of this is in the form of watervapor. The continuous exchange of water among the oceans, the atmosphere, and the continents is called the hydrologic cycle (Figure 4-2). Water from the oceans and, to a lesser extent, from land areas, evaporates into the atmosphere. Winds transport this moisture-laden air, often over great distances, until conditions cause the moisture to condense into cloud droplets. The process of cloud formation may result in precipitation. The precipitation that falls into the ocean has ended its cycle and is ready to begin another. A portion of the water that falls on the land soaks into the ground, some of it moving downward and then laterally, where it eventually seeps into lakes and streams. Much of the water that soaks in or runs off returns to the atmosphere through evaporation. In addition, some of the water that infiltrates the ground is absorbed by plants through their roots. They then release it into the atmosphere, a process called transpiration. Because we cannot clearly distinguish between the amount of water that evaporates from that which is transpired by plants, the term evapotranspiration is often used to describe the combined process. The total amount of water vapor in the atmosphere remains fairly constant. Therefore, the average annual precipitation over Earth must be roughly equal to the quantity of water lost through evaporation. However, over the continents, precipitation exceeds evaporation. Evidence for the
roughly balanced hydrological cycle is found in the fact that the level of the world’s ocean is not dropping. In summary, the hydrologic cycle depicts the continuous movement of Earth’s water from the oceans to the atmosphere, from the atmosphere to the land, and from the land back to the sea. The movement of water through the cycle holds the key to the distribution of moisture over the surface of our planet and is intricately related to all atmospheric phenomena.
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2 5IFRVBOUJUZPGXBUFSMPTUUPFWBQPSBUJPOPWFSUIFPDFBOT JTOPUFRVBMFECZQSFDJQJUBUJPO8IZ UIFO EPFTTFBMFWFM OPUESPQ
3 /BNFBOEEFTDSJCFUIFUXPQSPDFTTFTUIBUPDDVSEVSJOH FWBQPUSBOTQJSBUJPO
Water: A Unique Substance Water has several unique properties that set it apart from most other substances. For instance, (1) water is the only liquid found at Earth’s surface in large quantities; (2) water is readily converted from one state of matter to another (solid, liquid, gas); (3) water’s solid phase, ice, is less dense than liquid water; and (4) water has a high heat capacity— meaning it requires considerable energy to change its temperature. All these properties influence Earth’s weather and climate and are favorable to life as we know it. These unique properties are largely a consequence of water’s ability to form hydrogen bonds. To better grasp the nature of hydrogen bonds, let’s examine a water molecule (Figure 4-3a). A water molecule (H2O) consists of two hydrogen
Figure 4-1 5IJTMBLFJO"MBCBNBJT FYQFSJFODJOHTUFBNGPHPOBDPPMBVUVNO NPSOJOH 1IPUPCZ1BU$IVDL#MBDLMFZ "MBNZ
Chapter 4 Moisture and Atmospheric Stability
99
o
ec
Precipitation 96,000 km3
tion
Precipitation 284,000 km3
ita
ap
ra
Pr
n tio
ip
Evaporation 320,000 km3
Ev
Hydrologi c C ycl e
Figure 4-2 &BSUITIZESPMPHJDDZDMF
Evaporation/Transpiration 60,000 km3
36,000 km3
Run
off Infiltration
Oceans
Hydrogen bonds
Hydrogen bonds (a)
(c)
(b)
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The Atmosphere: An Introduction to Meteorology
atoms that are strongly bonded to an oxygen atom. Because oxygen atoms have a greater affinity for the bonding electrons (negatively charged subatomic particles) than hydrogen atoms, the oxygen end of a water molecule acquires a partial negative charge. For the same reason, both hydrogen atoms of a water molecule acquire a partial positive charge. Because oppositely charged particles attract, a hydrogen atom on one water molecule is attracted to an oxygen atom on another water molecule. Hydrogen bonds are the attractive forces that exist between hydrogen atoms in one water molecule and oxygen atoms of any other water molecule (Figure 4-3a). Hydrogen bonds are what hold water molecules together to form the solid we call ice. In ice, hydrogen bonds produce a rigid hexagonal network, as shown in Figure 43b. The resulting molecular configuration is very open (lots of empty space). When ice is heated sufficiently, it melts. Melting causes some, but not all, of the hydrogen bonds to break. As a result, the water molecules in liquid water display a more compact arrangement (Figure 4-3c). This accounts for the fact that water in its liquid phase is denser than it is in the solid phase. Because ice is less dense than the liquid water beneath it, a water body freezes from the top down (Figure 4-4). This has far-reaching effects, both for our daily weather and aquatic life. When ice forms on a water body, it insulates the underlying liquid and slows the rate of freezing at depth. If a water body froze from the bottom, imagine the consequences. Many lakes would freeze solid during the winter, killing the aquatic life. In addition, deep bodies of Figure 4-4 "TBCPEZPGXBUFSGSFF[FT JDFGPSNTPOUPQCFDBVTF JDFJTMFTTEFOTFUIBOMJRVJEXBUFS5IJTJTBWFSZVOJRVFQSPQFSUZ UIFTPMJEGPSNTPGNPTUPUIFSTVCTUBODFTBSFEFOTFSUIBOUIFMJRVJE GPSNT 1IPUPCZ+BO.BSUJO8JMM4IVUUFSTUPDL
water, such as the Arctic Ocean, would never be ice covered. This would alter Earth’s heat budget, which in turn would modify global atmospheric and oceanic circulations. Water’s heat capacity is also related to hydrogen bonding. When water is heated, some of the energy is used to break hydrogen bonds rather than to increase molecular motion. (Recall that an increase in average molecular motion corresponds to an increase in temperature.) Thus, under similar conditions, water heats up and cools down more slowly than most other common substances. As a result, large water bodies tend to moderate temperatures by remaining warmer than adjacent landmasses in winter and cooler in summer.
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2 &YQMBJOXIBUIBQQFOTBTJDFNFMUTUPCFDPNFMJRVJEXBUFS 3 8IBUQSPQFSUZPGXBUFSDBVTFTMBSHFXBUFSCPEJFTUPSFNBJO XBSNFSUIBOBEKBDFOUMBOENBTTFTJOXJOUFSCVUDPPMFSJO TVNNFS
Water’s Changes of State ATMOSPHERE
Moisture and Cloud Formation ▸ Water’s Changes of State
Water is the only substance that exists on Earth as a solid (ice), liquid, and gas (water vapor). Recall that all forms of water are composed of hydrogen and oxygen atoms that are bonded together to form water molecules (H2O). The primary difference among liquid water, ice, and water vapor is the arrangement of these water molecules.
Ice, Liquid Water, and Water Vapor Ice is composed of water molecules that have low kinetic energies (motion) and are held together by mutual molecular attractions (hydrogen bonds). The molecules of ice form a tight, orderly network, as shown in Figure 4-5. As a consequence, these water molecules are not free to move relative to each other but rather vibrate about fixed sites. When ice is heated, the molecules oscillate more rapidly. When the rate of molecular movement increases sufficiently, the hydrogen bonds between some of the water molecules are broken, resulting in melting. In the liquid state, water molecules are still tightly packed but are moving fast enough that they are able to easily slide past one another. As a result, liquid water is fluid and will take the shape of the container that holds it. When liquid water gains heat from the environment, some of the molecules acquire enough energy to break their hydrogen bonds and escape from the surface, becoming water vapor. Water-vapor molecules are widely spaced
Chapter 4 Moisture and Atmospheric Stability
Subl
imatio
101
n
sed) cal relea ut 680 o b a ( When ice melts, some of the bonds are broken and the water molecules become more closely packed. Water molecules in ice are bonded together in a configuration that has lots of empty space, which accounts for ice being less dense than water.
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GAS (water vapor) When liquid water evaporates all of the hydrogen bonds are broken and the water molecules move about freely.
LIQUID (water)
sitio
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compared to liquid water and exhibit very energetic and random motion. To summarize, when water changes state, hydrogen bonds either form or are broken.
Latent Heat Whenever water changes state, heat is exchanged between water and its surroundings. For example, heat is required to evaporate water. Meteorologists measure the heat involved when water changes state in units called calories. One calorie is the amount of heat required to raise the temperature of 1 gram of water 1°C (l.8°F). Thus, when 10 calories of heat are absorbed by 1 gram of water, the molecules vibrate faster, and a 10°C (18°F) temperature increase occurs. Under certain conditions, heat may be added to a substance without an accompanying rise in temperature. For example, when the ice in a glass of ice water melts, the temperature of the mixture remains a constant 0°C (32°F) until all the ice has melted. If the added energy does not raise the temperature of ice water, where does this energy go? In this case, the added energy breaks the hydrogen bonds that once bound the water molecules into a crystalline structure. Because the heat used to melt ice does not produce a temperature change, it is referred to as latent heat. (Latent
means hidden, like the latent fingerprints hidden at a crime scene.) This energy is essentially stored in liquid water and released to its surroundings as heat when the liquid returns to the solid state. It requires 80 calories to melt 1 gram of ice, an amount referred to as latent heat of melting. Freezing, the reverse process, releases these 80 calories per gram to the environment as the latent heat of fusion. We will consider the importance of latent heat of fusion in Chapter 5, in the section on frost prevention. Evaporation and Condensation Latent heat is also involved during evaporation, the process of converting a liquid to a gas (vapor). The energy absorbed by water molecules during evaporation is used to give them the motion needed to escape the surface of the liquid and become a gas. This energy is referred to as the latent heat of vaporization and varies from about 600 calories per gram for water at 0°C to 540 calories per gram at 100°C. (Notice from Figure 4-5 that it takes much more energy to evaporate 1gram of water than it does to melt the same amount of ice.) During the process of evaporation, the higher-temperature (faster-moving) molecules escape the surface. As a result, the average molecular motion (temperature) of the remaining water is lowered—hence the expression “Evaporation is a cooling process.” You have undoubtedly experienced
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The Atmosphere: An Introduction to Meteorology
this coolingeffect if you have stepped, dripping wet, out of a swimming pool or bathtub full of water. In this situation the energy used to evaporate water comes from your skin— hence, you feel cool. Condensation, the reverse process, occurs when water vapor changes to the liquid state. During condensation, water-vapor molecules release energy (latent heat of condensation) in an amount equivalent to what was absorbed during evaporation. When condensation occurs in the atmosphere, it results in the formation of fog and clouds (Figure 4-6). Latent heat plays an important role in many atmospheric processes. In particular, when water vapor condenses to form cloud droplets, latent heat is released, which warms the surrounding air and gives it buoyancy. When the moisture content of air is high, this process can spur the growth of towering storm clouds. In addition, the evaporation of water over the tropical oceans and the subsequent condensation at higher latitudes results in significant energy transfer from equatorial to more poleward locations. Sublimation and Deposition You are probably least familiar with the last two processes illustrated in Figure 4-5— sublimation and deposition. Sublimation is the conversion of a solid directly to a gas, without passing through the liquid state. Examples you may have observed include the gradual shrinkage of unused ice cubes in a freezer and the rapid conversion of dry ice (frozen carbon dioxide) to wispy clouds that quickly disappear. Deposition refers to the reverse process: the conversion of a vapor directly to a solid. This change occurs, for example, when water vapor is deposited as ice on solid objects
such as grass or windows (Figure 4-7). These deposits are called white frost or simply frost. A household example of the process of deposition is the “frost” that accumulates in a freezer. As shown in Figure 4-5, deposition releases an amount of energy equal to that which is released by condensation and freezing.
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Students Sometimes Ask... What is “freezer burn”? Freezer burn is what happens to poorly wrapped food stored in the freezer of a frost-free refrigerator for an extended time. Because modern refrigerators are designed to remove moisture from the freezer compartment, the air within them is relatively dry. As a result, the moisture in food sublimates—turns from ice to water vapor—and escapes. Thus, the food is not actually burned; it is simply dried out.
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Chapter 4 Moisture and Atmospheric Stability
103
similar in that both are expressed as the quantity of water vapor contained in a specific amount of air. Absolute humidity is the mass of water vapor in a given volume of air (usually as grams per cubic meter): Absolute humidity =
As air moves from one place to another, changes in pressure and temperature cause changes in its volume. When such volume changes occur, the absolute humidity also changes, even if no water vapor is added or removed. Consequently, it is difficult to monitor the water-vapor content of a moving mass of air when using the absolute humidity index. Therefore, meteorologists generally prefer to use mixing ratio to express the water-vapor content of air. The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air:
Figure 4-7 8IJUFGSPTUPOBXJOEPXQBOF 1IPUPCZ 4UPDLYQFSU5IJOLTUPDL
Humidity: Water Vapor in the Air ATMOSPHERE
Mixing ratio =
Mass of water vapor (grams) Mass of dry air (kilograms)
Because it is measured in units of mass (usually grams per kilogram), the mixing ratio is not affected by changes in pressure or temperature (Figure 4-9).*
Moisture and Cloud Formation ▸ Humidity: Water Vapor in the Air
Water vapor constitutes only a small fraction of the atmosphere, varying from as little as. 0.1 percent up to about 4 percent by volume. But the importance of water in the air is far greater than these small percentages indicate. In fact, water vapor is the most important gas in the atmosphere when it comes to atmospheric processes. Humidity is the general term used to describe the amount of water vapor in the air (Figure 4-8). Meteorologists employ several methods to express the water-vapor content of the air, including (1) absolute humidity, (2) mixing ratio, (3) vapor pressure, (4) relative humidity, and (5) dew point. Two of these methods, absolute humidity and mixing ratio, are Parcel of air
Mass of water vapor (grams) Volume of air (cubic meters)
Size of parcel = 2m3 Mass of parcel (dry) = 1 kg
Mass of water vapor = 20g Absolute humidity = 10g/m3 Mixing ratio = 20g/kg
Nitrogen
Size of parcel = 1m3 Mass of parcel (dry) = 1 kg
Water vapor Oxygen
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Mass of water vapor = 20g Absolute humidity = 20g/m3 Mixing ratio = 20g/kg
Figure 4-9 $PNQBSJTPOPGBCTPMVUFIVNJEJUZBOENJYJOHSBUJPGPS BSJTJOHQBSDFMPGBJS/PUFUIBUUIFNJYJOHSBUJPJTOPUBGGFDUFECZ DIBOHFTJOQSFTTVSFBTUIFQBSDFMPGBJSSJTFTBOEFYQBOET *Another commonly used expression is “specific humidity,” which is the mass of water vapor in a unit mass of air, including the water vapor. Because the amount of water vapor in the air rarely exceeds a few percent of the total mass of the air, the specific humidity of air is equivalent to its mixing ratio for all practical purposes.
The Atmosphere: An Introduction to Meteorology
Neither the absolute humidity nor the mixing ratio, however, can be easily determined by direct sampling. Therefore, other methods are also used to express the moisture content of the air.
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Vapor Pressure and Saturation
Pressure gauge
Gauge shows rising vapor pressure 20°C
20°C
Moist air
Evapo ratio n
n atio ens nd Co
Dry air
More H2O molecules are evaporating than are condensing Water
Water (b)
(a)
Gauge records saturation vapor pressure for 30°C
Gauge records saturation vapor pressure for 20°C
30°C
20°C
Saturated air
Saturated air
Evap orat ion
The same number of H2O molecules are evaporating as are condensing
The same number of H2O molecules are evaporating as are condensing Water
Water (c)
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n atio ens nd Co
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The moisture content of the air can be obtained from the pressure exerted by water vapor. To understand how water vapor exerts pressure, imagine a closed flask containing pure water and overlain by dry air, as shown in Figure 4-10a. Almost immediately some of the water molecules begin to leave the water surface and evaporate into the dry air above. The addition of water vapor into the air can be detected by a small increase in pressure (Figure 4-10b). This increase in pressure is the result of the motion of the water-vapor molecules that were added to the air through evaporation. In the atmosphere, this pressure is called vapor pressure and is defined as the part of the total atmospheric pressure attributable to its water-vapor content. Initially, many more molecules will leave the water surface (evaporate) than will return (condense). However, as more and more molecules evaporate from the water surface, the steadily increasing vapor pressure in the air above forces more and more water molecules to return to the liquid. Eventually a balance is reached in which the number of water molecules returning to the surface equals the number leaving. At that point the air is said to have reached an equilibrium called saturation (Figure 4-10c). When air is
saturated, the pressure exerted by the motion of the watervapor molecules is called the saturation vapor pressure. Now suppose we were to disrupt the equilibrium by heating the water in our closed container, as illustrated in Figure 4-10d. The added energy would increase the rate at which the water molecules would evaporate from the surface. This in turn would cause the vapor pressure in the air above to increase until a new equilibrium was reached between evaporation and condensation. Thus, we conclude that the saturation vapor pressure is temperature dependent,
Evap orat ion
104
(d)
Chapter 4 Moisture and Atmospheric Stability
105
Water is everywhere on Earth—in the oceans, glaciers, rivers, lakes, air, and living tissue. In addition, water can change from one state of matter to another at the temperatures and pressures experienced on Earth. Refer to this image, taken near Vega Island, Antarctica, to answer thefollowing questions. (Photo by Michael Collier) Question 1 8IBUUXPGFBUVSFTJOUIJTQIPUP BSFDPNQPTFEPGXBUFSJOUIFTPMJETUBUF Question 2 8IBUUISFFEJGGFSFOUGFBUVSFTBSF DPNQPTFEPGMJRVJEXBUFS Question 3 8IFSFJTXBUFSWBQPSGPVOEJO UIJTJNBHF
such that at higher temperatures it takes more water vapor to saturate air (Figure 4-11). The amount of water vapor required for the saturation of 1 kilogram (2.2 pounds) of dry air at various temperatures is shown in Table 4-1. Note that for every 10°C (18°F) increase in temperature, the amount
TABLE 4-1
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Temperature, °C (°F)
240 (240)
0.1
230 (222)
0.3
220 (24)
0.75
210 (14) 50 45 40
Water vapor (grams/kilogram)
35 Saturation mixing ratio
30 25 20 15 10 5
–40
–30
–20
–10 0 10 Temperature (°C)
20
30
40
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Saturation Mixing Ratio, g/kg
2
0 (32)
3.5
5 (41)
5
10 (50)
7
15 (59)
10
20 (68)
14
25 (77)
20
30 (86)
26.5
35 (95)
35
40 (104)
47
of water vapor needed for saturation almost doubles. Thus, roughly four times more water vapor is needed to saturate 30°C (86°F) air than 10°C (50°F) air. The atmosphere behaves in much the same manner as our closed container. In nature, gravity, rather than a lid, prevents water vapor (and other gases) from escaping into space. Also as with our container, water molecules are constantly evaporating from liquid surfaces (such as lakes or cloud droplets), and other water vapor molecules are condensing. However, in nature a balance is not always achieved. More often than not, more water molecules are leaving the surface of a water puddle than are arriving, causing what meteorologists call net evaporation. By contrast, during the formation of fog, more water molecules are condensing than are evaporating from the tiny fog droplets, resulting in net condensation.
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The Atmosphere: An Introduction to Meteorology
What determines whether the rate of evaporation exceeds the rate of condensation (net evaporation) or vice versa? One of the major factors is the temperature of the surface water, which in turn determines how much motion (kinetic energy) the water molecules possess. At higher temperatures the molecules have more energy and can more readily escape. Thus, under otherwise similar conditions, because hot water has more energy, it evaporates faster than cold water. The other major factor determining whether evaporation or condensation will dominate is the vapor pressure in the air around the liquid. Recall from our example of a closed container that vapor pressure determines the rate at which the water molecules return to the surface (condense). When the air is dry (low vapor pressure), the rate at which water molecules return to the liquid phase is low. However, when the air around a liquid reaches the saturation vapor pressure, the rate of condensation will be equal to the rate of evaporation. Thus, at saturation there is neither a net condensation nor a net evaporation. Therefore, all else being equal, net evaporation is greater when the air is dry (low vapor pressure) than when the air is humid (high vapor pressure).
Students Sometimes Ask... Why do the sizes of snow piles seem to shrink a few days after a snowfall, even when the temperatures remain below freezing? On clear, cold days following a snowfall, the air can be very dry. This fact, plus solar heating, causes the ice crystals to sublimate— turn from a solid to a gas. Thus, even without any appreciable melting, these accumulations of snow gradually get smaller.
Concept Check 4.5 1 %FGJOFvapor pressureBOEEFTDSJCFUIFSFMBUJPOTIJQCFUXFFO WBQPSQSFTTVSFBOETBUVSBUJPO Hint:4FF'JHVSF
2 "GUFSSFWJFXJOH5BCMF XSJUFBHFOFSBMJ[BUJPOSFMBUJOH BJSUFNQFSBUVSFBOEUIFBNPVOUPGXBUFSWBQPSOFFEFEUP TBUVSBUFBJS
Relative Humidity ATMOSPHERE
Moisture and Cloud Formation ▸ Humidity: Water Vapor in the Air
The most familiar and, unfortunately, the most misunderstood term used to describe the moisture content of air is relative humidity. Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure). Thus, relative humidity indicates how near the air is to saturation rather than the actual quantity of water vapor in the air (Box 4-1). To illustrate, we see from Table 4-1 that at 25°C, air is saturated when it contains 20 grams of water vapor per kilogram of air. Thus, if the air contains 10 grams per kilogram on a 25°C day, the relative humidity is expressed as 10/20, or 50 percent. Further, if air with a temperature of 25°C had a water-vapor content of 20 grams per kilogram, the relative humidity would be expressed as 20/20 or 100 percent. On occasions when the relative humidity reaches 100 percent, the air is said to be saturated.
How Relative Humidity Changes Temperature 25˚C
25˚C
25˚C
1 kg air 10 grams H 2O vapor
20 grams H 2O vapor
Evaporation
Evaporation
1. Saturation mixing ratio at 25˚ C = 20 grams
1. Saturation mixing ratio at 25˚ C = 20 grams
1. Saturation mixing ratio at 25˚ C = 20 grams
2. H2 O vapor content = 5 grams
2. H2 O vapor content = 10 grams
2. H2 O vapor content = 20 grams
3. Relative humidity = 5/20 = 25%
3. Relative humidity = 10/20 = 50%
3. Relative humidity = 20/20 = 100%
5 grams H 2O vapor
(a) Initial condition: 5 grams of water vapor
(b) Addition of 5 grams of water vapor = 10 grams
(c) Addition of 10 grams of water vapor = 20 grams
Because relative humidity is based on the air’s water-vapor content, as well as the amount of moisture required for saturation, it can change in one of two ways. First, relative humidity changes when water vapor is added to or removed from the atmosphere. Second, because the amount of moisture required for saturation is a function of air temperature, relative humidity varies with temperature. Adding or Subtracting Moisture Notice in Figure 4-12 that when water vapor is added to a parcel of air through Figure 4-12 "UBDPOTUBOUUFNQFSBUVSF JOUIJTFYBNQMFJUJT$
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Chapter 4 Moisture and Atmospheric Stability
Box 4-1
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Dry Air at 100 Percent Relative Humidity?
A common misconception relating to meteorology is the notion that air with a high relative humidity must have a greater water-vapor content than air with a lower relative humidity. This is not always the case ('JHVSFø"). To illustrate, let us compare a typical January day at International Falls, Minnesota, to one in the desert near Phoenix, Arizona. On this hypothetical day the temperature in International Falls is a cold –10°C (14°F) and the relative humidity is 100 percent. By referring to Table 4-1, we can see that saturated –10°C (14°F) air has a water-vapor content (mixing ratio) of 2 grams per kilogram (g/kg). By contrast, the desert air at Phoenix on this January day is a warm 25°C (77°F), and the relative humidity is just 20 percent. A look at Table 4-1 reveals that 25°C (77°F) air has a saturation mixing ratio of 20 g/kg. Therefore, with a relative humidity of 20percent, the air at Phoenix has a water-vapor content of 4 g/kg (20 grams × 20 percent). Consequently, the “dry” air at Phoenix actually contains twice the water vapor as the air at International Falls, with a relative humidity of 100percent. This example illustrates that places that are very cold are also very dry. The low water-vapor content of frigid air (even when saturated) helps to explain why many arctic areas receive only meager amounts of precipitation and are referred to as “polar deserts.” This also helps us understand why people frequently experience dry skin and chapped lips during the winter months. The water-vapor content of cold air is low, even compared to some hot, arid regions.
'*(63&"Moisture content of hot air versus frigid air. Hot desert air with a low relative humidity generally has a higher water-vapor content than frigid air with a high relative humidity. (Top photo by Matt Duvall; bottom photo by E. J. Tarbuck)
evaporation, the relative humidity of the air increases until saturation occurs (100 percent relative humidity). What if even more moisture is added to this parcel of saturated air? Does the relative humidity exceed 100 percent? Normally, this situation does not occur. Instead, the excess water vapor condenses to form liquid water. You may have experienced such a situation while taking a hot shower. The water leaving the shower is composed of very energetic (hot) molecules, which means that the rate of evaporation is high. As long as you run the shower, the process of evaporation continually adds water vapor to the unsaturated air in the bathroom. If you stay in a hot shower long enough, the air eventually becomes saturated, and the excess water vapor begins to condense on the mirror, window, tile, and other surfaces in the room. In nature, moisture is added to the air mainly via evaporation from the oceans. However, plants, soil, and smaller bodies of water also make substantial contributions. Unlike with your shower, however, the natural processes that add water vapor to the air generally do not operate at rates fast enough to cause saturation to occur directly. One exception is what happens when you exhale on a cold winter day and “see
your breath”—the warm, moist air from your lungs mixes with the cold outside air. Your breath has enough moisture to saturate a small quantity of cold outside air, which results in a miniature “cloud.” Almost as fast as the “cloud” forms, it mixes with the surrounding dry air and evaporates. Changes with Temperature The second condition that affects relative humidity is air temperature (Box 4-2). Examine Figure 4-13a carefully and note that when air at 20°C contains 7grams of water vapor per kilogram, it has a relative humidity of 50 percent. When the flask in Figure 4-13a is cooled from 20° to 10°C, as shown in Figure 4-13b, the relative humidity increases from 50 to 100percent. We can conclude that when the water-vapor content remains constant, a decrease in temperature results in an increase in relative humidity. But there is no reason to assume that cooling would cease the moment the air reached saturation. What happens when the air is cooled below the temperature at which saturation occurs? Figure 4-13c illustrates this situation. Notice from Table 4-1 that when the flask is cooled to 0°C, the air is saturated, at 3.5 grams of water vapor per kilogram of
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The Atmosphere: An Introduction to Meteorology
Box 4-2
Humidifiers and Dehumidifiers and eastern United States. When hot and humid air enters a home, some of it circulates into the cool basement. As a result, the temperature of this air drops, and its relative humidity increases. The result is a damp, musty-smelling basement. The homeowner may install a dehumidifier to alleviate the problem. As air is drawn over the cold coils of the dehumidifier, water vapor condenses and collects in a bucket or flows down a drain. This process reduces the relative humidity and makes for a drier, more comfortable basement.
In summer, stores sell dehumidifiers. As winter rolls around, these same retailers feature humidifiers. Why do you suppose so many homes are equipped with both appliances? The answer lies in the relationship between temperature and relative humidity. Recall that if the water-vapor content of air remains at a constant level, an increase in temperature lowers the relative humidity and a lowering of temperature increases the relative humidity. During the summer months, warm, moist air frequently dominates the weather of the central
air. Because this flask originally contained 7 grams of water vapor, 3.5 grams of water vapor will condense to form liquid droplets that collect on the walls of the container. In the meantime, the relative humidity of the air inside remains at 100 percent. This raises an important concept. When air aloft is cooled below its saturation level, some of the water vapor condenses to form clouds. Since clouds are made of liquid droplets, this moisture is no longer part of the water-vapor content of the air. Review Figure 4-13 and see what would happen if the flask in Figure 4-13a were heated to 35°C. From Table 4-1 we see that at 35°C, saturation occurs at 35 grams of water vapor per kilogram of air. Consequently, by heating the air from 20° to 35°C, the relative humidity would drop from 7/14 (50 percent) to 7/35 (20 percent).
We can summarize the effects of temperature on relative humidity as follows: When the water-vapor content of air remains constant, a decrease in air temperature results in an increase in relative humidity, and, conversely, an increase in temperature causes a decrease in relative humidity.
Natural Changes in Relative Humidity In nature there are three major ways that air temperatures change (over relatively short time spans) to cause corresponding changes in relative humidity: 1. Daily changes in temperatures (daylight versus nighttime temperatures) 2. Temperature changes that result as air moves horizontally from one location to another 3. Temperature changes caused as air moves vertically in the atmosphere
Temperature 20˚C 10˚C 0˚C
7 grams H 2O vapor
3.5 grams H 2O vapor
1. Saturation mixing ratio at 20˚ C = 14 grams
1. Saturation mixing ratio at 10˚ C = 7 grams
1. Saturation mixing ratio at 0˚ C = 3.5 grams
2. H2 O vapor content = 7 grams
2. H2 O vapor content = 7 grams
2. H2 O vapor content = 3.5 grams
3. Relative humidity = 7/14 = 50%
3. Relative humidity = 7/7 = 100%
3. Relative humidity = 3.5 /3.5 = 100%
(a) Initial condition: 20°C
The effect of the first of these three processes (daily changes) is shown in Figure 4-14. Notice that during the midafternoon, relative humidity reaches
3.5 grams H 2O liquid
1 kg air
7 grams H 2O vapor
By contrast, during the winter months, outside air is cool and dry. When this air is drawn into the home, it is heated to room temperature. This process in turn causes the relative humidity to plunge, often to uncomfortable levels of 25 percent or lower. Living with dry air can mean static electrical shocks, dry skin, sinus headaches, and even nosebleeds. Consequently, the homeowner may install a humidifier, which adds water to the air and increases the relative humidity to a more comfortable level.
(b) Cooled to 10°C
(c) Cooled to 0°C
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Chapter 4 Moisture and Atmospheric Stability
16 (61)
Temperature °C (°F)
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Highest temperature
14 (57)
Why do my lips get chapped in the winter? During the winter months, outside air is comparatively cool and dry. When this air is drawn into the home, it is heated, which causes the relative humidity to plunge. Unless your home is equipped with a humidifier, you are likely to experience chapped lips and dry skin at that time of year.
12 (54) 10 (50) 8 (46) 6 (43) 4 (39) 2 (36) 0 (32)
Lowest temperature
Dew-Point Temperature
–2 (29) 12
6 A.M.
Noon
6 P.M.
12 ATMOSPHERE
80% Highest relative humidity Relative humidity
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70%
60%
Lowest relative humidity
50% 12
6 A.M.
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Moisture and Cloud Formation ▸ Humidity: Water Vapor in the Air
Another important measure of humidity is dew-point temperature. The dew-point temperature, or simply the dew point, is the temperature to which air needs to be cooled to reach saturation. For example, in Figure 4-13 the unsaturated air in the flask had to be cooled to 10°C before saturation occurred. Therefore, 10°C is the dew-point temperature for this air. In nature, cooling below the dew point causes water vapor to condense, typically as dew, fog, or clouds (Figure4-15). The term dew point stems from the fact that during nighttime hours, objects near the ground often cool below the dew-point temperature and become coated with dew. Unlike relative humidity, which is a measure of how near the air is to being saturated, dew-point temperature is a measure of the actual moisture content of a parcel of air. Recall that
its lowest level, whereas the cooler evening hours are associated with higher relative humidity. In this example, the actual water-vapor content (mixing ratio) of the air remains unchanged; only the relative humidity varies. In summary, relative humidity indicates how near the air is to being saturated, whereas the air’s mixing ratio denotes the actual quantity of water vapor contained in that air.
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The Atmosphere: An Introduction to Meteorology
the Southeast is dominated by humid conditions (dew points above 65°F), most of the remainder of the country is experiencing comparatively drier air.
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TABLE 4-2
Dew-Point Temperature
Threshold
< 10°F
Significant snowfall is inhibited
$ 55°F
Minimum for severe thunderstorms to form
$ 65°F
Considered humid by most people
$ 70°F
Typical of the rainy tropics
$ 75°F
Considered oppressive by most
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the saturation vapor pressure is temperature dependent and that for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation doubles. Therefore, cold saturated air (0°C [32°F]) contains about half the water vapor of saturated air having a temperature of 10°C (50°F) and roughly one-fourth that of saturated air with a temperature of 20°C (68°F). Because the dew point is the temperature at which saturation occurs, we can conclude that high dew-point temperatures equate to moist air and, conversely, low dew-point temperatures indicate dry air (Table 4-2). More precisely, based on what we have learned about vapor pressure and saturation, we can state that for every 10°C (18°F) increase in the dew-point temperature, air contains about twice as much water vapor. Therefore, we know that when the dew-point temperature is 25°C (77°F), air contains about twice the water vapor as when the dew point is 15°C (59°F) and four times that of air with a dew point of 5°C (41°F). Because the dew-point temperature is a good measure of the amount of water vapor in the air, it commonly appears on weather maps. When the dew point exceeds 65°F (18°C), most people consider the air to feel humid; air with a dew point of 75°F (24°C) or higher is considered oppressive. Notice on the map in Figure 4-16 that much of the area near the warm Gulf of Mexico has dew-point temperatures that exceed 70°F (21°C). Also notice in Figure 4-16 that although
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Concept Check 4.7
Students Sometimes Ask... Is frost just frozen dew? Contrary to popular belief, frost is not frozen dew. Rather, white frost (hoar frost) forms when saturation occurs at a temperature of 0°C (32°F) or below (a temperature called the frost point). Thus, frost forms when water vapor changes directly from a gas into a solid (ice), without entering the liquid state. This process, called deposition, produces delicate patterns of ice crystals that often decorate windows during winter.
How Is Humidity Measured?
Instruments called hygrometers are used to measure the moisture content of the air. One of the simplest hygrometers, a psychrometer, consists of two identical thermometers mounted side by side (Figure 4-17a). One thermometer is dry and measures air temperature, and the other, called the wet bulb, has a thin cloth wick tied at the bottom. To use the psychrometer, the cloth wick is saturated with water, and a continuous current of air is passed over the wick, either by swinging the instrument or by using an electric fan to move air past the instruments (Figure 4-17b,c). As a result, water evaporates from the wick, absorbing heat energy 45 35 from the wet-bulb thermometer, which causes its temperature to drop. The amount 40 55 of cooling that takes place is directly proportional to the dryness of the air. The drier the air, the greater the cooling. There65 fore, the larger the difference between the wet- and dry-bulb temperatures, the lower the relative humidity. By contrast, 60 if the air is saturated, no evaporation will occur, and the two thermometers will have 70 identical readings. By using a psychrometer and the tables provided in Appendix C, 65 the relative humidity and the dew-point 65 temperature can be easily determined. 65 70
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Chapter 4 Moisture and Atmospheric Stability
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calibration. It is also slow in responding to changes in humidity, especially at low temperatures. A variety of electric hygrometers are also used to measure humidity. One relatively accurate type of electric hygrometer uses a chilled mirror and a mechanism that detects condensation on the mirror. Thus, a chilled mirror hydrometer accurately measures the dew-point temperature of the air. From the dew point and air temperature, the relative humidity can be easily calculated. In addition to being used in meteorology, hygrometers are used in greenhouses, humidors, museums, and numerous industrial settings that are sensitive to humidity, such as places where paint is applied to vehicles. (c)
Room temperature water
(b)
(a)
Students Sometimes Ask... What are the most humid cities in the United States? As you might expect, the most humid cities in the United States are located near the ocean, in regions that experience frequent onshore breezes. The record belongs to Quillayute, Washington, with an average relative humidity of 83 percent. However, many coastal cities in Oregon, Texas, Louisiana, and Florida also have average relative humidities that exceed 75 percent. Coastal cities in the Northeast tend to be somewhat less humid because they often experience air masses that originate over the drier continentalinterior.
Another instrument used for measuring relative humidity, the hair hygrometer, can be read directly without using tables. The hair hygrometer operates on the principle that hair changes length in proportion to changes in relative humidity. Hair lengthens as relative humidity increases and shrinks as relative humidity drops. People with naturally curly hair experience this phenomenon, for in humid weather their hair lengthens and hence becomes curlier. The hair hygrometer uses a bundle of hairs linked mechanically to an indicator that is calibrated between 0 and 100 percent. Thus, we need only glance at the dial to read directly the relative humidity. As you might expect, the hair hygrometer is less accurate than a psychrometer and requires frequent
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Adiabatic Temperature Changes ATMOSPHERE
Moisture and Cloud Formation ▸ The Basics of Cloud Formation: Adiabatic Cooling
Recall that condensation occurs when sufficient water vapor is added to the air or, more commonly, when the air is cooled to its dew-point temperature. Condensation may produce dew, fog, or clouds. Heat near Earth’s surface is readily exchanged between the ground and the air directly above. As the ground loses heat in the evening (radiation cooling), dew may condense on the grass, while fog may form slightly above Earth’s surface. Thus, surface cooling
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that occurs after sunset produces some condensation. Cloud formation, however, often takes place during the warmest part of the day—an indication that another mechanism must operate aloft that cools air sufficiently to generate clouds. The process that generates most clouds is easily visualized. Have you ever pumped up a bicycle tire with a hand pump and noticed that the pump barrel became very warm? When you applied energy to compress the air, the motion of the gas molecules increased, and the temperature of the air rose. Conversely, if you allow air to escape from a bicycle tire, the air expands; the gas molecules move less rapidly, and the air cools. You have probably felt the cooling effect of the propellant gas expanding as you applied hair spray or spray deodorant. The temperature changes just described, in which heat was neither added nor subtracted, are called adiabatic temperature changes. In summary, when air is compressed, it warms and when air is allowed to expand, it cools.
Adiabatic Cooling and Condensation To simplify the discussion of adiabatic cooling, imagine a volume of air enclosed in a thin balloon-like bubble. Meteorologists call this imaginary volume of air a parcel. Typically, we consider a parcel to be a few hundred cubic meters in volume, and we assume that it acts independently of the surrounding air. It is also assumed that no heat is transferred into, or out of, the parcel. Although highly idealized, over short time spans, a parcel of air behaves much like an actual volume of air moving up or down in the atmosphere. In nature, sometimes the surrounding air infiltrates a rising or descending column of air, a process called entrainment.
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For the following discussion, however, we assume that no mixing of this type occurs. Any time a parcel of air moves upward, it passes through regions of successively lower pressure. As a result, ascending air expands and cools adiabatically. Unsaturated air cools at a constant rate of 10°C for every 1000 meters of ascent (5.5°F per 1000 feet). Conversely, descending air comes under increasing pressure and is compressed and heated 10°C for every 1000 meters of descent (Figure 4-18). This rate of cooling or healing applies only to unsaturated air and is known as the dry adiabatic rate (“dry” because the air is unsaturated). If an air parcel rises high enough, it will eventually cool to its dew point and trigger the process of condensation. The altitude at which a parcel reaches saturation and cloud formation begins is called the lifting condensation level. At the lifting condensation level an important change occurs: The latent heat that was absorbed by the water vapor when it evaporated is released as sensible heat—heat we can measure with a thermometer. Although the parcel will continue to cool adiabatically, the release of latent heat slows the rate of cooling. In other words, when a parcel of air ascends above the lifting condensation level, the rate at which it cools is reduced. This slower rate of cooling is called the wet adiabatic rate (“wet” because the air is saturated). Because the amount of latent heat released depends on the quantity of moisture present in the air (generally between 0 and 4 percent), the wet adiabatic rate varies from 5°C per 1000 meters for air with a high moisture content to 9°C per 1000 meters for air with a low moisture content. Figure 4-19 illustrates the role of adiabatic cooling in the formation of clouds. To summarize, rising air cools at the faster dry adiabatic rate from the surface up to the lifting condensation level, at which point the slower Lower air wet adiabatic rate commences. pressure
2°C
Height (m)
Concept Check 4.9 2000
Rising air parcel expands and cools
12°C
Sinking air parcel is compressed and warms
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Chapter 4 Moisture and Atmospheric Stability
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–8°C
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Processes That Lift Air ATMOSPHERE
Moisture and Cloud Formation ▸ Processes That Lift Air
Wet adiabatic rate (temperature of rising air drops at 5°C/1000 meters) Condensation level
Dry adiabatic rate (temperature of rising air drops at 10°C/1000 meters)
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Four mechanisms cause air to rise: 1. Orographic lifting, in which air is forced to rise over a mountainous barrier 2. Frontal wedging, in which warmer, less dense air is forced over cooler, denser air 3. Convergence, which is a pileup of horizontal air flow that results in upward movement 4. Localized convective lifting, in which unequal surface heating causes localized pockets of air to rise because of their buoyancy
In later chapters, we will consider other mechanisms that cause air to rise.
Orographic Lifting
Orographic lifting occurs when elevated terrains, such as mountains, act as barriers to the flow of air (Figure 4-20). As Why does air rise on some occasions but not on others? air ascends a mountain slope, adiabatic cooling often generGenerally, the tendency is for air to resist vertical moveates clouds and copious precipitation. In fact, many of the ment. Therefore, air located near the surface “wants“ to rainiest places in the world are located on windward mounstay near the surface, and air aloft tends to remain aloft. tain slopes (Box 4-3). Because clouds are a common occurrence, we are left to conBy the time air reaches the leeward side of a mountain, clude that some processes must be at work that cause the air much of its moisture has been lost. If the air descends, it to rise. warms adiabatically, making condensation and precipitaSierra Nevada Range tion even less likely. As shown Rainshadow Coast Range in Figure 4-20, the result can be a rain shadow desert (Box 4-4). The Great Basin Leeward Great Windward (dry) Desert of the western United Basin (wet) Wind States lies only a few hundred kilometers from the Pacific Ocean, but it is effectively cut off from the ocean’s moisture by the imposing Sierra Nevada (Figure 4-20). The Gobi Desert Figure 4-20 0SPHSBQIJDMJGUJOH MFBETUPQSFDJQJUBUJPOPOXJOEXBSE TMPQFT#ZUIFUJNFBJSSFBDIFTUIF MFFXBSETJEFPGUIFNPVOUBJOT NVDIPGUIFNPJTUVSFIBTCFFOMPTU 5IF(SFBU#BTJOEFTFSUJTBSBJO TIBEPXEFTFSUUIBUDPWFSTOFBSMZ BMMPG/FWBEBBOEQPSUJPOTPG BEKBDFOUTUBUFT 1IPUPPOMFGUCZ %FBO1FOOBMB4IVUUFSTUPDL QIPUP POSJHIUCZ%FOOJT5BTB
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The Atmosphere: An Introduction to Meteorology
Warm air Cold air
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of Mongolia, the Takla Makan of China, and the Patagonia Desert of Argentina are other examples of deserts that exist because they are on the leeward sides of large mountain systems.
Frontal Wedging If orographic lifting were the only mechanism that forced air aloft, the relatively flat central portion of North America would be an expansive desert rather than the area known as “the nation’s breadbasket.” Fortunately, this is not the case. In central North America, masses of warm and cold air collide, producing fronts. Along a front the cooler, denser air acts as a barrier over which the warmer, less dense air rises. This process, called frontal wedging, is illustrated in Figure 4-21. It should be noted that weather-producing fronts are associated with storm systems called middle-latitude cyclones. Because these storms are responsible for producing a high percentage of the precipitation in the middle latitudes, we will examine them in detail in Chapter 9.
Convergence We saw that the collision of contrasting air masses forces air to rise. In a more general sense, whenever air in the lower troposphere flows together, lifting results—a phenomenon called convergence (Figure 4-22). Convergence can also occur when an obstacle slows or restricts horizontal air flow (wind). For example, when air moves from a relatively smooth surface, such as the ocean, onto an irregular landscape, increased friction reduces its speed. The result is a pileup of air (convergence). Imagine what happens when large numbers of people leave a concert or sporting event—pileups occur at the exits. When air converges, there is a net upward flow of air molecules
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Chapter 4 Moisture and Atmospheric Stability
Box 4-3
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Precipitation Records and Mountainous Terrain
Many of the rainiest places in the world are located on windward mountain slopes. Typically, these areas are rainy because the mountains act as barriers to Earth’s natural circulation. The prevailing winds are forced to ascend the sloping terrain, thereby generating clouds and often abundant precipitation. Mount Waialeale, Hawaii, for example, records the highest average annual rainfall in the world, some 1234 centimeters
(486 inches). The station is located on the windward (northeast) coast of the island of Kauai, at an elevation of 1569 meters (5148 feet). By contrast, only 31 kilometers (19 miles) away lies sunny Barking Sands, with annual precipitation that averages less than 50 centimeters (20 inches). The largest recorded rainfall for a 12-month period occurred at Cherrapunji, India, where an astounding 2647 centimeters
'*(63&#This heavy snowpack is at Gotthard Pass in the Swiss Alps. (Photo by agefotostock/ SuperStock)
rather than a simple squeezing together of molecules (as happens with people exiting a crowded building). The Florida peninsula provides an excellent example of the role that convergence can play in initiating cloud development and precipitation. On warm days, the airflow is from the ocean to the land along both coasts of Florida. This leads to a pileup of air along the coasts and general convergence over the peninsula. This pattern of air movement and the uplift that results are aided by intense solar heating of the land. As a result, Florida’s peninsula experiences the greatest frequency of midafternoon thunderstorms in the United States (Figure 4-22b). Convergence as a mechanism of forceful lifting is also a major contributor to the severe weather associated with middle-latitude cyclones and hurricanes. These important weather producers will be covered in more detail later, but for now remember that convergence near the surface results in a general upward flow.
(1042 inches), over 86 feet, fell from August 1860 through July 1861. Most of this rainfall occurred in the summer, particularly during the month of July, when a record 930 centimeters (366 inches) fell. By comparison, 10 times more rain fell in a month at Cherrapunji, India, than falls in Chicago in an average year. Cherrapunji’s elevation of 1293 meters (4309 feet) lies just north of the Bay of Bengal, making it an ideal location to receive the full effect of India’s wet summer monsoon. Because mountains can be sites of abundant precipitation, they are typically very important sources of water, especially for many dry locations in the western United States. The snow pack, which accumulates high in the mountains during the winter, is a major source of water for the summer season, when precipitation is light and demand for water is great ('JHVSF#). The record for greatest annual snowfall in the United States goes to the Mount Baker ski area north of Seattle, Washington, where 2896 centimeters (1140 inches) of snow fell during the winter of 1998–1999. In addition to providing lift, mountains remove additional moisture in other ways. By slowing the horizontal flow of air, they cause convergence and impede the passage of storm systems. Moreover, the irregular topography of mountains enhances the differential heating that causes some localized convective lifting. These combined effects account for the generally higher precipitation associated with mountainous regions as compared to surrounding lowlands.
Localized Convective Lifting On warm summer days, unequal heating of Earth’s surface may cause pockets of air to be warmed more than the surrounding air (Figure 4-23). For instance, air above a plowed field will be warmed more than the air above adjacent fields of crops. Consequently, the parcel of air above the field, which is warmer (less dense) than the surrounding air, will be buoyed upward. These rising parcels of warmer air are called thermals. Birds such as hawks and eagles use thermals to carry them to great heights, where they can gaze down on unsuspecting prey (Figure 4-24). People have learned to employ these rising parcels to use hang gliders as a way to “fly.” The phenomenon that produces rising thermals is called localized convective lifting. When these warm parcels of air rise above the lifting condensation level, clouds form and on occasion produce midafternoon rain
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Solar heating Lifting condensation level
Rising thermal
TIME
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showers. The height of clouds produced in this fashion is somewhat limited, because the buoyancy caused solely by unequal surface heating is confined to, at most, the first few kilometers of the atmosphere. Also, the accompanying rains, although occasionally heavy, are of short duration and widely scattered, a phenomenon described as sun showers.
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The Critical Weathermaker: Atmospheric Stability (Michael Collier) The Navajo Generating Station, located on the Navajo Indian Reservation near Page, Arizona, has three 236-meter (560-feet) stacks. Question 1 8IBUGVFMJTUIJTQMBOUCVSOJOHUPHFOFSBUFFMFDUSJDJUZ Hint: -PPLEJSFDUMZCFIJOEUIFGBDJMJUZ
Question 2 8IZEPQPXFSHFOFSBUJOHGBDJMJUJFTTVDIBTUIJTPOFIBWF TVDIUBMMTUBDLT Question 3 $BOZPVFYQMBJOXIZUIFiTNPLFwDIBOHFTDPMPSGSPNCSJHIU XIJUFUPQBMFZFMMPXXIFOJUSFBDIFTBIFJHIUPGBCPVUGFFUBCPWFUIF TUBDLT
ATMOSPHERE
Moisture and Cloud Formation ▸ The Critical Weathermaker: Atmospheric Stability
Why do clouds vary so much in size, and why does the resulting precipitation vary so widely? The answer is closely tied to the stability of the air. Recall that when a parcel is forced to rise, its temperature will decrease because of expansion (adiabatic cooling). By comparing the parcel’s temperature to that of the surrounding air, we can determine its stability. If the parcel is cooler than the surrounding environment, it will be more dense; and if allowed to do so, it will sink to its original
Chapter 4 Moisture and Atmospheric Stability
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Figure 4-24 #JSET TVDIBTFBHMFTBOEIBXLT HMJEFBDSPTTUIF TLZVTJOHUIFSNBMT 1IPUPCZ5FFLBZHFF%SFBNTUJNF
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Environmental lapse rate 5°/1000 m
2000 m
15°C
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Expands and cools to 5°C
1000 m
20°C
Rising air 5° cooler than environment
Expands and cools to 15°C
Tendency
position. Air of this type, called stable air, resists vertical movement. If, however, our imaginary rising parcel is warmer and hence less dense than the surrounding air, it will continue to rise until it reaches an altitude where its temperature equals that of its surroundings. This type of air is classified as unstable air. Unstable air is like a hot-air balloon—it will rise as long as the air in the balloon is sufficiently warmer and less dense than the surrounding air (Figure 4-25). In summary, stability is a property of air that describes its tendency to remain in its original position (stable) or to rise (unstable).
Types of Stability The stability of the atmosphere is determined by measuring the air temperature at various heights. This measure, called the environmental lapse rate, should not be confused with adiabatic temperature changes. The environmental lapse rate is the actual temperature of the atmosphere, as determined from observations made by radiosondes and aircraft. Adiabatic temperature changes, on the other hand, are the changes in temperature that a parcel of air experiences as it moves vertically through the atmosphere. Figure 4-26 illustrates how the stability of the atmosphere is determined. In the example, the air at 1000 meters is 5°C cooler than the air at the surface, whereas the air at 2000 meters is 10°C cooler, and so forth. Thus, the prevailing environmental lapse rate is 5°C per 1000 meters. The air at the surface appears to be less dense than the air at 1000 meters because it is 5°C warmer. However, if the surface air were forced to rise to 1000 meters, it would expand
Air parcel forced upward 25°C Surface
25°C
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and cool at the dry adiabatic rate of 10°C per 1000 meters. Therefore, on reaching 1000 meters, the temperature of the rising parcel would have dropped from 25°C to 15°C. Because the rising air would be 5°C cooler than its environment, it would be more dense and, if allowed to do so, it would sink to its original position. Thus, we say that the air near the surface is potentially cooler than the air aloft, and therefore, it will not rise unless forced to do so. (For example, air can be forced to rise over a mountainous terrain.) The air just described is referred to as stable and resists vertical movement. We will now look at three fundamental conditions of the atmosphere: absolute stability, absolute instability, and conditional instability. Absolute Stability Stated quantitatively, absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. Figure 4-27 depicts this situation by using an environmental lapse rate of 5°C per 1000 meters and a wet adiabatic rate of 6°C per 1000 meters. Note that at 1000 meters the temperature of the rising parcel is 5°C cooler than its environment, which makes it denser. Even if this stable air were forced above the lifting condensation level, it would remain cooler and denser than its environment and would have a tendency to return to the surface.
Environmental lapse rate 5°C/1000 m
Absolute Instability At the other extreme, a layer of air is said to exhibit absolute instability when the environmental lapse rate is greater than the dry adiabatic rate. As shown in Figure 4-28, the ascending parcel of air is always warmer than its environment and will continue to rise because of its own buoyancy. Absolute instability occurs most often during the warmest months and on clear days, when solar heating is intense. Under these conditions the lowermost layer of the atmosphere is heated to a much higher temperature than the air aloft. This results in a steep environmental lapse rate and a very unstable atmosphere. Conditional Instability The most common type of atmospheric instability is called conditional instability. This situation prevails when moist air has an environmental lapse rate between the dry and wet adiabatic rates (between about 5° and 10°C per 1000 meters). Simply stated, the
ABSOLUTE STABILITY
Rising air 13° cooler
5000 m –5°C Rising air 12° cooler
Wet rate 6°C/1000 m
Tendency
4000 m 0°C
3000 m 5°C
The most stable conditions occur when the temperature in a layer of air increases with altitude, rather than decreases. When this type of environmental lapse rate occurs, it is called a temperature inversion. Many processes, such as radiation cooling at Earth’s surface on clear nights, can create temperature inversions. Under these conditions an inversion is created because the surface and the air directly above it cool more rapidly than the air aloft.
Rising air 11° cooler Rising air 10° cooler
2000 m 10°C
Environmental lapse rate – 18°C
– 12°C
Wet adiabatic rate
– 6°C
Lifting condensation level
0°C Cold air
Rising Dry rate air 5° 10°C 1000 m 15°C 10°C/1000 m cooler
Dry adiabatic rate
20°C Surface 20°C (a)
Figure 4-27 Absolute stabilityQSFWBJMTXIFOUIFFOWJSPONFOUBMMBQTFSBUFJTMFTTUIBOUIFXFU BEJBCBUJDSBUF B 5IFSJTJOHQBSDFMPGBJSJTBMXBZTDPPMFSBOEIFBWJFSUIBOUIFTVSSPVOEJOHBJS QSPEVDJOHTUBCJMJUZ C (SBQIJDSFQSFTFOUBUJPOPGUIFDPOEJUJPOTTIPXOJOQBSU B
– 20 –10 0 10 20(°C) Temperature
(b)
Chapter 4 Moisture and Atmospheric Stability
Environmental lapse rate 12°C/1000 m
4000 m
ABSOLUTE INSTABILITY
–8°C
4°C
2000 m
16°C
1000 m
Dry rate 28°C 10°C/1000 m
Surface
40°C
Tendency
Wet rate 6°C/1000 m 3000 m
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Rising air 16° warmer than environment
8°C
Rising air 10° warmer than environment
14°C
Wet adiabatic rate
Rising air 4° warmer than environment
20°C
Rising air 2° warmer than environment
30°C
Solar heating
Lifting condensation level
Dry adiabatic rate Environmental lapse rate
40°C – 10 0 10 20 30 40(°C) (b) Temperature
(a)
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atmosphere is said to be conditionally unstable when it is stable with respect to an unsaturated parcel of air but unstable with respect to a saturated parcel of air. Notice in Figure 4-29 that the rising parcel of air is cooler than the surrounding air for nearly 3000 meters. However, because of the release of latent heat that occurs above the lifting condensation level, the parcel eventually becomes warmer than the surrounding air. From this point along its ascent, the parcel will continue to rise without an outside force. The word conditional is used because the air must be forced upward before it reaches the level where it becomes unstable and rises on its own. In summary, the stability of air is determined by measuring the temperature of the atmosphere at various heights (environmental lapse rate). A column of air is deemed unstable when the air near the bottom of this layer is significantly warmer (less dense) than the air aloft. Conversely, the air is considered to be stable when the temperature decreases gradually with increasing altitude. The most stable conditions occur during a temperature inversion, when the temperature actually increases with height. Under these conditions, there is little vertical air movement.
Stability and Daily Weather How does stability manifest itself in our daily weather? When stable air is forced aloft, the clouds that form are widespread and have little vertical thickness compared with their
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3)PXJTUIFTUBCJMJUZPGBJSEFUFSNJOFE 48SJUFBTUBUFNFOUSFMBUJOHUIFFOWJSPONFOUBMMBQTFSBUFUP TUBCJMJUZ
5%FTDSJCFDPOEJUJPOBMJOTUBCJMJUZ horizontal dimension. Precipitation, if any, is light to moderate. In contrast, clouds associated with unstable air are towering and are usually accompanied by heavy precipitation. Thus, we can conclude that on a dreary and overcast day with light drizzle, stable air is forced aloft. Conversely, on a day when towering clouds are forming, we can be relatively certain that the atmosphere is unstable (Figure 4-30). As noted earlier, the most stable conditions occur during a temperature inversion, when temperature increases with height. In this situation, the air near the surface is cooler and heavier than the air aloft, and therefore little vertical mixing occurs between the layers. Because pollutants are generally added to the air from below, a temperature inversion confines them to the lowermost layer, where their concentration will continue to increase until the temperature inversion
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Environmental lapse rate 9°C/1000 m
CONDITIONAL INSTABILITY Environmental lapse rate Rising air 7° warmer
4000 m 4°C Wet rate 6°C/1000 m
Tendency
5000 m –5°C
8°C
Rising air 1° warmer
Tendency
2000 m 22°C
Wet adiabatic rate
Rising air 4° warmer
3000 m 13°C
Rising air 2° cooler
Rising Dry rate air 1° 1000 m 31°C 10°C/1000 m cooler
2°C
14°C
20°C
Lifting condensation level Cold air
Lifting condensation level
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30°C
40°C Surface 40°C (a)
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(b)
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dissipates. Widespread fog is another sign of stability. Fog generally forms when there is a lack of mixing between the air near the surface and the air aloft.
How Stability Changes Any factor that causes air near the surface to become warmed in relation to the air aloft increases instability. By contrast, any factor that causes the surface air to be chilled results in the air becoming more stable. Instability is enhanced by the following: 1. Intense solar heating warming the lowermost layer of the atmosphere 2. The heating of an air mass from below as it passes over a warm surface 3. General upward movement of air caused by processes such as orographic lifting, frontal wedging, and convergence 4. Radiation cooling from cloud tops Stability is enhanced by the following: 1. Radiation cooling of Earth’s surface after sunset 2. The cooling of an air mass from below as it traverses a cold surface 3. General subsidence within an air column Note that most processes that alter stability result from temperature changes caused by horizontal or vertical air movement, although daily temperature changes are important too. In general, any factor that increases the environmental lapse rate renders the air more unstable, whereas any factor that reduces the environmental lapse rate increases the air’s stability.
Temperature Changes and Stability On clear days when there is abundant surface heating, the lower atmosphere often becomes warmed sufficiently to cause parcels of air to rise. After the Sun sets, surface cooling generally renders the air stable again. Similar changes in stability occur as air moves horizontally over a surface having markedly different temperatures. In the winter, warm air from the Gulf of Mexico moves northward over the cold, snow-covered Midwest. Because the air is cooled from below, it becomes more stable, often producing widespread fog. The opposite occurs in the winter, when polar air moves southward over the open waters of the Great Lakes. Although we would die of hypothermia in a few minutes if we fell into these cold waters (perhaps 5°C [41°F]), these waters are warm compared to a polar air mass. Because the water of the Great Lakes is comparatively warm (high vapor pressure) and because the polar air is dry (low vapor pressure), the rate of evaporation will be high. The moisture and heat added to the frigid polar air from the water below render the polar air sufficiently unstable to produce heavy snow-
(NASA)
When Earth is viewed from space, the most striking feature of the planet is water. It is found as a liquid in the global oceans, as a solid in the polar ice caps, and as clouds and water vapor in the atmosphere. Although only onethousandth of 1 percent of the water on Earth exists as water vapor, it has a huge influence on our planet’s weather and climate. Question 1 8IBUSPMFEPFTXBUFSWBQPSQMBZJOIFBUJOH&BSUITTVSGBDF Question 2 )PXEPFTXBUFSWBQPSBDUUPUSBOTGFSIFBUGSPN&BSUIT MBOEoTFBTVSGBDFUPUIFBUNPTQIFSF
falls on the downwind shores of the Great Lakes—called “lake-effect snows” (see Chapter 8). Radiation Cooling from Clouds On a smaller scale, the loss of heat by radiation from cloud tops during evening hours adds to their instability and growth. Unlike air, which is a poor radiator of heat, cloud droplets emit considerable energy to space. Towering clouds that owe their growth to surface heating lose that source of energy at sunset. After sunset, however, radiation cooling at their tops steepens the lapse rate near the tops of these clouds and can lead to additional upward flow of warmer air from below. This process is responsible for producing nocturnal thunderstorms from clouds whose growth temporarily ceased at sunset.
Vertical Air Movement and Stability Vertical movements of air influence stability. When there is a general downward airflow, called subsidence, the upper portion of the subsiding layer is heated by compression,
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Box 4-4
Orographic Effects: Windward Precipitation and Leeward Rain Shadows
Orographic lifting is a significant factor in the development of windward precipitation and leeward rain shadows. A simplified hypothetical situation, illustrated in 'JHVSF $, shows prevailing winds forcing warm moist air over a 3000-meter-high mountain range. As the unsaturated air ascends the windward side of the range, it cools at a rate of 10°C per 1000 meters (dry adiabatic rate) until it reaches the dew-point temperature of 20°C. Because the dew-point temperature is reached at 1000 meters, we can say that this height represents the lifting condensation
level and the height of the cloud base. Notice in that above the lifting condensation level, latent heat is released, which results in a slower rate of cooling, called the wet adiabatic rate. From the cloud base to the top of the mountain, water vapor within the rising air is condensing to form more and more cloud droplets. As a result, the windward side of the mountain range experiences abundant precipitation. For simplicity, we will assume that the air that was forced to the top of the mountain is
4000 m 3000 m 2000 m 1000 m Sea level
Cooling at wet adiabatic rate 15°C
10°C
10°C Heating at dry adiabatic 20°C rate
20°C Starting temp. 30°C Cooling at dry adiabatic rate Dew point at condensation level = 20°C Dry adiabatic rate = 10°C/1000 m Wet adiabatic rate = 5°C/1000 m
more so than the lower portion. Usually the air near the surface is not involved in the subsidence, and so its temperature remains unchanged. The net effect is to stabilize the air, because the air aloft is warmed more than the surface air. The warming effect of a few hundred meters of subsidence is enough to evaporate the clouds. Thus, one sign of subsiding air is a deep blue, cloudless sky. Upward movement of air generally enhances instability, particularly when the lower portion of the rising layer has a higher moisture content than the upper portion, which is
30°C
Final temp. 40°C Rain shadow desert
cooler than the surrounding air and hence begins to flow down the leeward slope of the mountain. As the air descends, it is compressed and heated at the dry adiabatic rate. Upon reaching the base of the mountain range, the temperature of the descending air has risen to 40°C, or 10°C warmer than the temperature at the base of the mountain on the windward side. The higher temperature on the leeward side is a result of the latent heat that was released during condensation as the air ascended the windward slope of the mountain range. Two factors account for the rain shadow commonly observed on leeward mountain slopes. First, water is extracted from air in the form of precipitation on the windward side. Second, the air on the leeward side is warmer than the air on the windward side. (Recall that an increase in temperature results in a drop in relative humidity.) A classic example of windward precipitation and leeward rain shadows is found in western Washington State. As moist Pacific air flows inland over the Olympic and Cascade Mountains, orographic precipitation is abundant ('JHVSF%). By contrast, precipitation data for Sequim and Yakima indicate the presence of rain shadows on the leeward sides of these highlands.
'*(63&$Orographic lifting and the formation of rain shadow deserts.
usually the situation. As the air moves upward, the lower portion becomes saturated first and cools at the lesser wet adiabatic rate. The net effect is to increase the environmental lapse rate within the rising layer. This process is especially important in producing the instability associated with thunderstorms. In addition, recall that conditionally unstable air can become unstable if lifted sufficiently. In summary, the role of stability in determining our daily weather cannot be overemphasized. The air’s stability, or lack of it, determines to a large degree whether clouds develop and pro-
50.0 (20)
Quinault Ranger Station (windward)
ANNUAL PRECIPITATION cm in.
45.0 (18) 40.0 (16)
310 cm/yr
>500 405–499 250–404 150–249 50–149 25–49 200 160–199 100–159 60–99 20–59 10–19 0°C)
Colder (less than –5°C) Fog
Road
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'*(63&$ Distribution of fog when the temperature is above freezing and when the temperature falls below –5°C.
Two processes are responsible for the formation of precipitation: the Bergeron process and the collision–coalescence process.
low freezing. In the winter, even low clouds are cold enough to trigger precipitation.
Students Sometimes Ask… Precipitation from Cold Clouds: The Bergeron Process You have seen TV coverage during which mountain climbers brave an intensely cold and ferocious snowstorm as they scale an ice-covered peak. Very similar conditions exist in the upper portions of towering cumulonimbus clouds, even on sweltering summer days. (In fact, in the upper troposphere where commercial aircraft cruise, the temperature typically approaches –50°C [–58°F] or lower.) Frigid conditions high in the troposphere provide an ideal environment to initiate precipitation. In fact, in the middle latitudes much of the rain that falls begins with the birth of snowflakes high in the cloud tops, where temperatures are considerably be-
Why does it often seem like the roads are slippery when it rains after a long dry period? It appears that a buildup of debris on roads during dry weather can cause slippery conditions after a rainfall. One traffic study indicates that if it rains today, there will be no increase in risk of fatal crashes if it also rained yesterday. However, if it has been two days since the last rain, then the risk for a deadly accident increases by 3.7 percent. If it has been 21 days since the last rain, the risk increases by 9.2 percent.
The process that generates much of the precipitation in the middle latitudes is named the Bergeron process for its discoverer, the highly respected Swedish meteorologist Tor Bergeron (Box 5-2). The Bergeron process depends on
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This satellite image shows a layer of low-lying clouds moving eastward into the bays and waterways of the northwestern United States. Lining the coast are the forested Coast Ranges and further inland is the Cascade Range, composed of several large volcanoes. Question 1 8IZEPZPVUIJOLUIFDMPVETGPSNFE PWFSUIFPDFBO XIJMFUIFMBOEBSFBTBSFDMPVE GSFF
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the coexistence of water vapor, liquid cloud droplets, and ice crystals. To understand how this mechanism operates, we must first examine two important properties of water. First, contrary to what you might expect, cloud droplets do not freeze at 0°C (32°F). In fact, pure water suspended in air does not freeze until it reaches a temperature of nearly –40°C (–40°F). Water in the liquid state below 0°C (32°F) is referred to as supercooled. Supercooled water readily freezes if it impacts an object, which explains why airplanes collect ice when they pass through a liquid cloud made up of supercooled droplets. This also explains why freezing rain, or glaze, falls as a liquid but then turns to a sheet of ice when it strikes the pavement, tree branches, or car windshields. In the atmosphere, supercooled droplets freeze on contact with solid particles that have a shape that closely resembles that of ice (silver iodide, for example). These materials are called freezing nuclei. The need for freezing nuclei to initiate freezing is similar to the role of condensation nuclei in the process of condensation. In contrast to condensation nuclei, however, freezing nuclei are sparse in the atmosphere and do not generally become active until the temperature reaches –10°C (14°F) or below. Thus, at temperatures between 0 and –10°C, clouds consist mainly of supercooled water droplets. Between –10 and –20°C, liquid droplets coexist with ice crystals, and below –20°C (–4°F), clouds are generally composed entirely of ice crystals—for example, high-altitude cirrus clouds. This brings us to a second important property of water: The saturation vapor pressure above ice crystals is slightly lower than above supercooled liquid droplets. This occurs because
ice crystals are solid, so the individual water molecules are held together more tightly than those forming a liquid droplet. As a result, it is easier for water molecules to escape (evaporate) from the supercooled liquid droplets. Consequently, when air is saturated (100 percent relative humidity) with respect to liquid droplets, it is supersaturated with respect to ice crystals. Table 5-2, for example, shows that at –10°C (14°F), when the relative humidity is 100 percent with respect to water, the relative humidity with respect to ice is about 110 percent. With these facts in mind, we can now explain how precipitation is produced via the Bergeron process. Visualize a cloud at a temperature of –10°C (14°F), where each ice crystal (snow crystal) is surrounded by many thousands of liquid droplets (Figure 5-14). Because the air is saturated (100 percent) with respect to liquid water, it will be supersaturated (over 100 percent) with respect to the newly formed ice crystals. As a result of this supersaturated condition, the ice crys-
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TABLE 5-2
Temperature (°C)
Relative Humidity with Respect To: Water Ice
100%
100%
–5
100%
105%
–10
100%
110%
–15
100%
115%
–20
100%
121%
Chapter 5 Forms of Condensation and Precipitation
Cloud droplet
Water molecule
Ice crystal Cloud droplet
Snow crystal
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tals collect water molecules, lowering the relative humidity of the air. In response, water droplets shrink (evaporate) to replenish the lost water vapor. Thus the growth of ice crystals is fed by the continued evaporation of liquid droplets. When ice crystals become sufficiently large, they begin to fall. During their descent, the ice crystals grow as they intercept cloud droplets that freeze on them. Air movement will sometimes break up these delicate crystals, and the fragments will serve as freezing nuclei for other liquid droplets. A chain reaction ensues and produces many snow crystals, which, by accretion, form into larger masses called snowflakes. Large snowflakes may consist of many individual ice crystals. The Bergeron process can produce precipitation throughout the year in the middle latitudes, provided that at least the upper portions of clouds are cold enough to generate ice crystals. The type of precipitation (snow, sleet, rain, or freezing rain) that reaches the ground depends on the temperature profile in the lower few kilometers of the atmosphere. When the surface temperature is above 4°C (39°F), snowflakes usually melt before they reach the ground and continue their descent as rain. Even on a hot summer day, a heavy rainfall may have begun as a snowstorm high in the clouds overhead.
143
Precipitation from Warm Clouds: The Collision–Coalescence Process A few decades ago meteorologists believed that the Bergeron process was responsible for the formation of most precipitation except for light drizzle. Later it was discovered that copious rainfall is often associated with clouds located well below the freezing level (called warm clouds), especially in the tropics. This second mechanism for triggering precipitation is called the collision–coalescence process. Research has shown that clouds made entirely of liquid droplets often contain some droplets larger than 20 micrometers (0.02 millimeter). These large droplets form when “giant” condensation nuclei are present or when hygroscopic particles exist (such as sea salt). Hygroscopic particles begin to remove water vapor from the air at relative humidities under 100 percent. Because the rate at which drops fall is dependent on their size, these “giant” droplets fall most rapidly. Table 5-3 summarizes drop size and falling velocities. As the larger droplets fall through a cloud, they collide with smaller, slower droplets and coalesce. They become larger in the process, and fall even more rapidly (or, in an updraft, they rise more slowly) increasing their chances of collision and rate of growth (Figure 5-15a). After a million or so cloud droplets coalesce, a rain drop is are large enough to fall to the surface without evaporating. Because of the huge number of collisions required for growth to raindrop size, clouds that have great vertical thickness and contain large cloud droplets have the best chance of producing precipitation. Updrafts also aid this process because the droplets can traverse the cloud repeatedly, which results in more collisions. As raindrops grow in size, their fall velocity increases. This in turn increases the frictional resistance of the air, which causes the drop’s “bottom” to flatten out (Figure5-15b). As a drop approaches 4 millimeters in diameter, it develops a depression, as shown in Figure 5-15c. Raindrops can grow to a maximum of 5 millimeters when they fall at the rate of 33 kilometers (20 miles) per hour. At this size, the water’s surface tension, which holds the drop together, is surpassed by the frictional drag of the air. The depression grows almost explosively, forming a donutlike ring that immediately breaks apart. The resulting breakup of a large rain-
TABLE 5-3
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Types
Diameter (millimeters)
Fall Velocity (km/hr) (mi/hr)
Small cloud droplets
0.01
0.01
0.006
Typical cloud droplets
0.02
0.04
0.03
Large cloud droplets
0.05
0.3
0.2
Drizzle droplets
0.5
7
4
Typical rain drops
2.0
23
14
Large rain drops
5.0
33
20
Data from Smithsonian Meteorological Tables.
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The Atmosphere: An Introduction to Meteorology
Typical cloud droplets (0.02 millimeter)
Large cloud droplet (0.05 millimeter) (a)
Typical cloud droplets
Typical rain drop (2 millimeters)
(b)
Large rain drop (4 millimeters)
From the preceding discussion, it should be apparent that the collision–coalescence mechanism is most efficient in environments where large cloud droplets are plentiful. The air over the tropics, particularly the tropical oceans, is an ideal setting: Very humid and relatively clean air results in fewer condensation nuclei compared to the air over more populated regions. With fewer condensation nuclei to compete for available water vapor (which is plentiful), condensation is fast paced and produces comparatively few large cloud droplets. Within developing cumulus clouds, the largest drops quickly gather smaller droplets to generate the warm afternoon showers associated with tropical climates. In the middle latitudes the collision–coalescence process may contribute to the precipitation from a large cumulonimbus cloud by working in tandem with the Bergeron process—particularly during the hot, humid summer months. High in these towers the Bergeron process generates snow that melts as it passes below the freezing level. Melting generates relatively large drops with fast fall velocities. As these large drops descend, they overtake and coalesce with the slower and smaller cloud droplets that comprise much of the lower regions of the cloud. The result can be a heavy downpour.
Students Sometimes Ask…
(c)
What is the largest annual rainfall ever recorded? Large rain drop breaks into several small drops which grow by accretion
(d)
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drop produces numerous smaller drops that begin anew the task of sweeping up cloud droplets (Figure5-15d). The collision–coalescence process is not that simple, however. First, as the larger droplets descend, they produce an airstream around them similar to that produced by an automobile when driven rapidly down the highway. The airstream repels objects, especially small ones. Imagine driving on a summer night along a country road. The bugs in the air are like cloud droplets—most are pushed aside through the air. However, larger cloud droplets (bugs) have an increased chance of colliding with the giant droplet (car). Further, collision does not guarantee coalescence. Experimentation has indicated that the presence of atmospheric electricity may be the key to what holds these droplets together once they collide. If a droplet with a negative charge collides with a positively charged droplet, their electrical attraction may bind them together.
The greatest recorded rainfall for a single 12-month period occurred at Cherrapunji, India, where an astounding 2647 centimeters (1042 inches)—over 86 feet—fell. Cherrapunji is located at an elevation of 1293 meters (4309 feet), where orographic lifting of the onshore monsoon winds greatly contributed to the total.
In summary, two mechanisms are known to generate precipitation: the Bergeron process and the collision–coalescence process. The Bergeron process is dominant in the middle latitudes, where cold clouds (or cold cloud tops) are the rule. In the tropics, abundant water vapor and comparatively few condensation nuclei are more typical. This leads to the formation of fewer, larger drops with fast fall velocities that grow by collision and coalescence. Regardless of which process initiates precipitation, further growth in drop size occurs through collision–coalescence.
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Chapter 5 Forms of Condensation and Precipitation
Forms of Precipitation ATMOSPHERE
nimbostratus clouds or in towering cumulonimbus clouds that are capable of producing unusually heavy rainfalls known as cloudbursts. Fine, uniform droplets of water having a diameter less than 0.5 millimeter are called drizzle. Drizzle and small raindrops generally are produced in stratus or nimbostratus clouds, where precipitation may be continuous for several hours or for days on rare occasions. As rain enters the unsaturated air below the cloud, it begins to evaporate. Depending on the humidity of the air and the size of the drops, the rain may completely evaporate before reaching the ground. This phenomenon produces virga, which appear as streaks of precipitation falling from a cloud that extend toward Earth’s surface without reaching it (Figure 5-17). Similar to virga, ice crystals may sublimate when they enter the dry air below. These wisps of ice particles are called fallstreaks. Precipitation containing the very smallest droplets able to reach the ground is called mist. Mist can be so fine that the tiny droplets appear to float, and their impact is almost imperceptible.
Forms of Condensation and Precipitation ▸ Forms of Precipitation
Atmospheric conditions vary greatly both geographically and seasonally, resulting in several different types of precipitation (Figure 5-16). Rain and snow are the most common and familiar forms, but others, as listed in Table 5-4, are important as well. Sleet, freezing rain (glaze), and hail often produce hazardous weather. Although limited in occurrence and sporadic in both time and space, these forms, especially freezing rain and hail, may on occasion cause considerable damage.
Rain In meteorology the term rain is restricted to drops of water that fall from a cloud and have a diameter of at least 0.5 millimeter. (Drizzle and mist have smaller droplets, and are therefore not considered rain.) Most rain originates in either
Temperature profile
Snow
Snow melts
Rain
(a) Rain
Temperature profile
–10°C (14°F)
Snow
–20°C (–4°F)
0°C (32°F)
Snow
15°C (59°F) –20°C (–4°F)
0°C (32°F)
20°C (68°F)
0°C (32°F) –20°C (–4°F)
(b) Snow
Temperature profile
Snow
Snow melts
Snow
0°C (32°F) Snow melts
Rain Rain freezes
0°C (32°F)
Rain
Sleet
–10°C (14°F)
Glaze
(c) Sleet
–20°C (–4°F)
0°C (32°F)
0°C (32°F)
20°C (68°F)
Temperature profile
–10°C (14°F)
20°C (68°F)
145
(d) Glaze
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–10°C (14°F)
0°C (32°F)
0°C (32°F) –5°C (23°F) –20°C (–4°F)
0°C (32°F)
20°C (68°F)
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TABLE 5-4
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Type
Approximate Size
State of Water
Mist
0.005–0.05 mm
Liquid
Droplets large enough to be felt on the face when air is moving 1 meter/second. Associated with stratus clouds.
Drizzle
< 0.5 mm
Liquid
Small, uniform droplets that fall from stratus clouds, generally for several hours.
Rain
0.5–5 mm
Liquid
Generally produced by nimbostratus or cumulonimbus clouds. When heavy, size can be highly variable from one place to another.
Sleet
0.5–5 mm
Solid
Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer of subfreezing air. Because the ice particles are small, any damage is generally minor. Sleet can make travel hazardous.
Freezing Rain (Glaze)
Layers 1 mm–2 cm thick
Solid
Produced when supercooled raindrops freeze on contact with solid objects. Glaze can form a thick coating of ice that has sufficient weight to seriously damage trees and power lines.
Rime
Variable accumulations
Solid
Deposits usually consisting of ice feathers that point into the wind. These delicate frostlike accumulations form as supercooled cloud or fog droplets encounter objects and freeze on contact.
Snow
1 mm–2 cm
Solid
The crystalline nature of snow allows it to assume many shapes, including six-sided crystals, plates, and needles. Produced in supercooled clouds where water vapor is deposited as ice crystals that remain frozen during their descent.
Hail
5–10 cm or larger
Solid
Precipitation in the form of hard, rounded pellets or irregular lumps of ice. Produced in large convective, cumulonimbus clouds, where frozen ice particles and supercooled water coexist.
Graupel
2–5 mm
Solid
“Soft hail” that forms as rime collects on snow crystals to produce irregular masses of “soft” ice. Because these particles are softer than hailstones, they normally flatten out upon impact.
Description
Snow Snow is precipitation in the form of ice crystals or, more often, aggregates of ice crystals. The size, shape, and concentration of snowflakes depend to a great extent on the temperature at which they form.
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Recall that at very low temperatures, the moisture content of air is low. The result is the generation of very light and fluffy snow made up of individual six-sided ice crystals (Figure 5-18). This is the “powder” that downhill skiers covet. By contrast, at temperatures warmer than about −5°C (23°F), the ice crystals join together into larger clumps consisting of tangled aggregates of crystals. Snowfalls consisting of these composite snowflakes are generally heavy and have a high moisture content, which makes them ideal for making snowballs.
Students Sometimes Ask… What is the snowiest city in the United States? According to National Weather Service records, Rochester, New York, which averages nearly 239 centimeters (94 inches) of snow annually, is the snowiest city in the United States. However, Buffalo, New York, is a close runner-up.
Sleet and Freezing Rain or Glaze Sleet is a wintertime phenomenon that involves the fall of clear to translucent particles of ice. Figure 5-19 illustrates how sleet is produced: An above-freezing air layer must overlie a subfreezing layer near the ground. When the raindrops, which
Chapter 5 Forms of Condensation and Precipitation
147
In January 1998 an ice storm of historic proportions caused enormous damage in New England and southeastern Canada. Five days of freezing rain deposited a heavy layer of ice on exposed surfaces from eastern Ontario to the Atlantic coast. The 8 centimeters (3 inches) of precipitation caused trees, power lines, and high-voltage towers to collapse, leaving over 1 million households without power—many for nearly a month following the storm (Figure 5-20). At least 40 deaths were blamed on the storm, which caused damages in excess of $3 billion. Much of the damage was to the electrical grid, which one Canadian climatologist summed up this way: “What it took human beings a half-century to construct, took nature a matter of hours to knock down.”
Hail
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are often melted snow, leave the warmer air and encounter the colder air below, they freeze and reach the ground as small pellets of ice roughly the size of the raindrops from which they formed. Occasionally, the distribution of temperatures in a column of air is such that freezing rain, or glaze, results (Figure 5-16d). In these situations, the raindrops become supercooled because the subfreezing air near the ground is not thick enough to cause them to freeze. Upon striking objects on Earth’s surface, these supercooled raindrops instantly turn to ice. The result can be a thick coating of glaze that has sufficient weight to break tree limbs, down power lines, and make walking and driving extremely hazardous.
Cold air Temperature less than 0°C (32°F) Snow
Hail is precipitation in the form of hard, rounded pellets or irregular lumps of ice. Hail is produced only in large cumulonimbus clouds where updrafts can sometimes reach speeds approaching 160 kilometers (100 miles) per hour and where there is an abundant supply of supercooled water. Figure 5-21a illustrates this process. Hailstones begin as small embryonic ice pellets (graupel) that grow by collecting supercooled droplets as they fall through the cloud. If they encounter a strong updraft, they may be carried upward again and begin the return downward journey. Each trip through the supercooled portion of the cloud results in an additional layer of ice. Hailstones can also form from a single descent through an updraft. Either way, the process continues until the hailstone grows too heavy to remain suspended by the thunderstorm’s updraft or encounters a downdraft. Hailstones may contain several layers that alternate between clear and milky ice (Figure 5-21b). High in the clouds, rapid freezing of small supercooled water droplets traps air bubbles, which cause the milky appearance. By contrast, clear ice is produced in the lower and warmer regions of the clouds, where colliding droplets wet the surface of the hailstones. As these droplets slowly freeze, they produce relatively bubble-free clear ice. Most hailstones have diameters between 1 centimeter (pea size) and 5 centimeters (golf ball size), although some can be as big as an orange or larger. Occasionally, hailstones weighing a pound or more have been reported; most of these are composites of several stones frozen together. The record for the largest hailstone ever found in the United States was set on July 23, 2010, in Vivian, South Dakota. The stone was over 20 centimeters (8 inches)
Rain Warm air Temperature greater than 0°C (32°F)
Sleet (ice pellets)
Cold air Temperature less than 0°C (32°F)
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The Atmosphere: An Introduction to Meteorology
Students Sometimes Ask… What is the difference between a winter storm warning and a blizzard warning? A winter storm warning is usually issued when heavy snow exceeding 6 inches in 12 hours or possible icing conditions are likely. It is interesting to note that in Upper Michigan and mountainous areas where snowfall is abundant, winter storm warnings are issued only if 8 or more inches of snow is expected in 12 hours. By contrast, blizzard warnings are issued for periods in which considerable falling and/or blowing snow will be accompanied by winds of 35 or more miles per hour. Thus, a blizzard is a type of winter storm in which winds are the determining factor, not the amount of snowfall.
Ice nucleus (graupel) Path of hailstones 0°C (32°F) Downdrafts
Updrafts Hail shower
(a)
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(b)
in diameter and weighed nearly 900 grams (2 pounds). The stone that held the previous record of 766 grams (1.69 pounds) fell in Coffeyville, Kansas, in 1970 (Figure 5-21b). The diameter of the stone found in South Dakota also surpassed the previous record of a 17.8-centimeter (7-inch) stone that fell in Aurora, Nebraska, in 2003. Even larger hailstones have reportedly been recorded in Bangladesh, where a 1987 hailstorm killed more than 90 people. It is estimated that large hailstones hit the ground at speeds exceeding 160 kilometers (100 miles) per hour.
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Chapter 5 Forms of Condensation and Precipitation
149
Worst Winter Weather Blizzard A winter storm charYUSFNFT, whether they be the acterized by winds of at least 56 tallest building or the record kilometers (35 miles) per hour for low temperature for a locaat least three hours. The storm must tion, fascinate us. When it also be accompanied by low temcomes to weather, some places take peratures and considerable falling pride in claiming to have the worst and/or blowing snow that reduces winters on record. In fact, Fraser, visibility to one-quarter mile or less. Colorado, and International Falls, Minnesota, have both proclaimed Severe blizzard A storm with themselves the “ice box of the winds of at least 72 kilometers (45 nation.” Although Fraser recorded miles) per hour, a great amount of the lowest temperature for the 48 falling or drifting snow, and temcontiguous states 23 times in 1989, peratures –12°C (10°F) or lower. its neighbor, Gunnison, Colorado, recorded the lowest temperature Heavy snow warning 62 times, far more than any other A snowfall in which at least 4 location. inches (10 centimeters) in12 hours Such facts do not impress the or 6 inches (15 centimeters) in 24 residents of Hibbing, Minnesota, hours is expected. where the temperature dropped to –38°C (–37°F) during the first Freezing rain Rain falling in week of March 1989. But this a liquid form through a shallow is mild stuff, say the old-timers in subfreezing layer of air near the Parshall, North Dakota, where the ground. The rain (or drizzle) freezes temperature fell to –51°C (–60°F) on impact with the ground or other on February 15, 1936. Not to be objects, resulting in a clear coating left out, Browning, Montana, holds of ice known as glaze. the record for the most dramatic 24-hour temperature drop. Here Sleet Also called ice pellets. the temperature plummeted 56°C Sleet is formed when raindrops or (100°F), from a cool 7°C (44°F) '*(63&% "XJOUFSCMJ[[BSEPGIJTUPSJDQSPQPSUJPOTTUSVDL melted snowflakes freeze as they to a frosty –49°C (–56°F) during a $IJDBHP*MMJOPJT PO'FCSVBSZ (AP Photo/Kiichiro Sato) pass through a subfreezing layer of January evening in1916. air near Earth’s surface. Sleet does So, determining which location has the Although impressive, the temnot stick to trees and wires, and it usually worst winter weather depends on how you perature extremes cited here represent only bounces when it hits the ground. An accumumeasure it. Most snowfall in a season? Lonone aspect of winter weather. What about lation of sleet sometimes has the consistency gest cold spell? Coldest temperature? Most snowfall (Figure 5-D)? Cooke City holds the of dry sand. disruptive storm? seasonal snowfall record for Montana, with 1062 centimeters (418.1 inches) during the Travelers’ advisory An alert issued Winter Weather Events winter of 1977–1978. But what about cities to inform the public of hazardous driving like Sault Ste. Marie, Michigan, and Buffalo, Here are the meanings of some common conditions caused by snow, sleet, freezing New York? The winter snowfalls associated terms that the National Weather Service uses precipitation, fog, wind, or dust. with the Great Lakes are legendary. Even for winter weather events. larger snowfalls occur in many sparsely Cold wave A rapid fall of temperature inhabited mountainous areas. in a 24-hour period, usually signifying the Snow flurries Snow falling for short duraTry telling residents of the eastern United beginning of a spell of very cold weather. tions at intermittent periods and resulting in States that heavy snowfall alone makes for generally little or no accumulation. Wind chill A measure of apparent the worst weather. A blizzard in March temperature that uses the effects of wind 1993 produced heavy snowfall along Blowing snow Snow lifted from the surand temperature on the human body by with hurricane-force winds and record low face by the wind and blown about to such a translating the cooling power of wind to a temperatures that immobilized much of the degree that horizontal visibility is reduced. temperature under calm conditions. It is an region from Alabama to the Maritime Provapproximation only for the human body Drifting snow Significant accumulations inces of eastern Canada. This event quickly and has no meaning for cars, buildings, of falling or loose snow caused by strong earned the well-deserved title Storm of the or other objects. wind. Century.
&
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The Atmosphere: An Introduction to Meteorology
The destructive effects of large hailstones are well known, especially to farmers whose crops have been devastated in a few minutes and to people whose windows, roofs, and cars have been damaged (Figure 5-22). In the United States, hail damage each year can run into the hundreds of millions of dollars. One of the costliest hailstorms to occur in North America took place June 11, 1990, in Denver, Colorado, with total damage estimated to exceed $625 million.
Rime
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Rime is a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing. When rime forms on trees, it adorns them with its characteristic ice feathers, which can be spectacular to observe (Figure 5-23). In these situations, objects such as pine needles act as freezing nuclei, causing the supercooled droplets to freeze on contact. On occasions when the wind is blowing, only the windward surfaces of objects will accumulate the layer of rime.
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Precipitation Measurement The most common form of precipitation, rain, is probably the easiest to measure. Any open container that has a consistent cross section throughout can be a rain gauge (Figure 5-24a). In general practice, however, more sophisticated devices are used to measure small amounts of rainfall more accurately and to reduce loss from evaporation.
Standard Instruments
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The standard rain gauge (Figure 5-24b) has a diameter of about 20centimeters (8 inches) at the top. Once the water is caught, a funnel conducts the rain through a narrow opening into a cylindrical measuring tube that has a cross-sectional area only one-tenth as large as the receiver. Consequently, rainfall depth is magnified 10 times, which allows for accurate measurements to the nearest 0.025 centimeter
Chapter 5 Forms of Condensation and Precipitation
151
1 inch of rain Heated collecting funnel
Collecting funnel
1 inch of rain
2.0
Tipping buckets
Measuring scale 1.5
Measuring tube (1/10 area of funnel)
1.0
0.5
10 inches Recorder
1 inch
(a) Simple rain gauge
0.01 inch (0.025 cm) of rain (b) Standard rain gauge
(c) Tipping–bucket gauge
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(0.01 inch). When the amount of rain is less than 0.025centimeter (0.01 inch), it is generally reported as being a trace of precipitation. In addition to the standard rain gauge, several types of recording gauges are routinely used. These instruments not only record the amount of rain but also its time of occurrence and intensity (amount per unit of time). Two of the most common gauges are the tipping-bucket gauge and the weighing gauge. As Figure 5-24c illustrates, the tipping-bucket gauge consists of two compartments, each one capable of holding 0.025 centimeter (0.01 inch) of rain, situated at the base of a funnel. When one “bucket” fills, it tips and empties its water. Meanwhile, the other “bucket” takes its place at the mouth of the funnel. Each time a compartment tips, an electrical circuit is closed, and 0.025 centimeter (0.01 inch) of precipitation is automatically recorded on a graph. A weighing gauge collects precipitation in a cylinder that rests on a spring balance. As the cylinder fills, the movement is transmitted to a pen that records the data.
Measuring Snowfall When snow records are kept, two measurements are normally taken—depth and water equivalent (Figure 5-25). Usually, the depth of snow is measured with a calibrated stick. The actual measurement is not difficult, but choosing a representative spot can be. Even when winds are light or moderate, snow drifts freely. As a rule, it is best to take several measurements in an open place, away from trees and obstructions, and then average them. To obtain the water equivalent, samples may be melted and then weighed or measured as rain.
This image shows a phenomenon called a punch hole cloud that was produced when a jet aircraft ascended through a cloud deck composed of supercooled water droplets. As the aircraft passed though the clouds, tiny particles in the jet- engine exhaust interacted with some of the supercooled water droplets—which froze instantly. The dark area in the center of the image is actually white and consists of large ice crystals that formed within the area of the cloud occupied by the punch hole and began to fall as precipitation. (Photo by H. Raab) Question 18IBUJTUIFOBNFPGUIFQSPDFTTFTXIFSFCZTPNFDMPVE ESPQMFUTGSFF[FBOEHSPXJOTJ[FBUUIFFYQFOTFPGUIFSFNBJOJOHMJRVJE DMPVEESPQMFUT Question 2&YQMBJOXIZPOMZDMPVETUIBUBSFDPNQPTFEPGTVQFSDPPMFE ESPQMFUTDBOEFWFMPQiQVODIIPMFTw Question 3%FTDSJCFUIFSPMFUIBUBKFUBJSDSBGUQMBZTJOUIFGPSNBUJPOPG QVODIIPMFDMPVET
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The Atmosphere: An Introduction to Meteorology Figure 5-25"WFSBHFBOOVBMTOPXGBMMJO UIFDPOUJHVPVT6OJUFE4UBUFT
Precipitation Measurement by Weather Radar
The quantity of water in a given volume of snow is not constant. A general ratio of 10 units of snow to 1 unit of water is often used when exact information is not available. You may have heard TV weathercasters use this ratio, saying, “Every 10 inches of snow equals 1 inch of rain.” But the actual water content of snow may deviate widely from this figure. It may take as much as 30 centimeters of light and fluffy dry snow (30:1) or as little as 4 centimeters of wet snow (4:1) to produce 1 centimeter of water.
The National Weather Service produces weather maps, like the one in Figure 5-26, based on images produced by weather radar. The development of Total Inches Centimeters weather radar has given meteorolo>96 >243.8 gists an important tool to track storm 64.1–96 162.7–243.8 32.1–64 81.4–162.6 systems and the precipitation patterns 16.1–32 40.7–81.3 they produce, even when the storms 8.1–16 20.4–40.6 0–8 0–20.3 are as far as a few hundred kilometers away. A radar unit has a transmitter that sends out short pulses of radio waves. The specific wavelengths that are used depend on the objects the user wants to detect. When radar is used to monitor precipitation, wavelengths between 3 and 10 centimeters are employed. These radio waves are able to penetrate small cloud droplets but are reflected by larger raindrops, ice crystals, and hailstones. The reflected signal, called an echo, is received and displayed on a TV monitor. Because the echo is “brighter” when the precipitation is more intense, modern radar is able to depict both the regional extent and rate of precipitation. Figure 5-26 is a typical radar display in which colors show precipitation intensity. As you will see in Chapters 10 and 12, weather radar can also measure the rate and direction of storm movement.
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Chapter 5 Forms of Condensation and Precipitation
Intentional Weather Modification Intentional weather modification is deliberate human intervention to influence atmospheric processes that constitute the weather—that is, to alter the weather for human purposes. The desire to change or enhance certain weather phenomena dates back to ancient history, when people used prayer, wizardry, dances, and even black magic in attempts to alter the weather. Weather-modification strategies fall into three broad categories. The first employs energy to forcefully alter the weather. Examples are the use of intense heat sources or the mechanical mixing of air (such as by helicopters) to disperse fog at some airports or keep fruit trees from freezing. The second category involves modifying land and water surfaces to change their natural interaction with the lower atmosphere. One often discussed but untried example is the blanketing of a land area with a dark substance. The additional solar energy absorbed by this dark surface would warm the layer of air near the surface and encourage the development of updrafts that might aid cloud formation and ultimately produce precipitation. The third category involves triggering, intensifying, or redirecting atmospheric processes. The seeding of clouds with agents such as dry ice (frozen carbon dioxide) and silver iodide to stimulate precipitation is the primary example. Because cloud seeding has shown some promising results and is a relatively inexpensive technique, it has been a primary focus of modern weather-modification technology.
Snow and Rain Making The first breakthrough in weather modification came in 1946, when Vincent J. Schaefer discovered that dry ice, dropped into a supercooled cloud, spurred the growth of ice crystals. Recall that once ice crystals form in a supercooled cloud, they grow larger, at the expense of the remaining liquid cloud droplets and, upon reaching a sufficient size, fall as precipitation. Later it was learned that silver iodide crystals could also be used for cloud seeding. Unlike dry ice, which simply chills the air, silver iodide crystals act as freezing nuclei. Because silver iodide can be easily delivered to clouds from burners on the ground or from aircraft, it is a more cost-effective alternative than dry ice (Figure 5-27). For cloud seeding to trigger precipitation, certain atmospheric conditions must exist. Most importantly, a portion of the cloud must be supercooled—that is, consist of liquid droplets with temperatures at or below 0°C (32°F). Seeding of winter clouds that form along mountain barriers (orographic clouds) has been tried on numerous occasions. These clouds are thought to be good candidates for seeding because, under normal conditions, only a small percentage of the water that condenses in cold orographic clouds actually falls as precipitation. Since 1977 Colorado’s Vail and Beaver Creek ski areas have used this method to
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increase winter snows. An additional benefit to cloud seeding is that the increased precipitation, which melts and runs off during spring and summer months, can be collected in reservoirs for irrigation and hydroelectric power generation. In recent years the seeding of warm convective clouds with hygroscopic (water-seeking) particles has received renewed attention. The interest in this technique arose when it was discovered that a pollution-belching paper mill near Nelspruit, South Africa, seemed to be triggering precipitation. Research aircraft flying through clouds near the paper mill collected samples of the particulate matter emitted from the mill. It turned out that the mill was emitting tiny salt crystals (potassium chloride and sodium chloride), which rose into the clouds. Because these salts attract moisture, they quickly form large cloud droplets, which grow into raindrops through the collision–coalescence process. Ongoing experiments being conducted over the arid landscape of northern Mexico are attempting to duplicate this process by seeding clouds using flares mounted on airplanes that spread hygroscopic salts. Although the results of these studies are promising, conclusive evidence that such seeding can increase rainfall over economically significant areas has not yet been established. In the United States, more than 50 weather-modification projects are currently under way in 10 states. Some of the most promising results come from western Texas, where researchers estimate a 10 percent increase in rainfall from clouds seeded with silver iodide compared to those left unseeded.
Fog and Cloud Dispersal One of the most successful applications of cloud seeding involves spreading dry ice (solid carbon dioxide) into layers of supercooled fog or stratus clouds to disperse them and
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Hail Suppression
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thereby improve visibility. Airports, harbors, and foggy stretches of interstate highway are obvious candidates. Such applications trigger a transformation in cloud composition from supercooled water droplets to ice crystals. The ice crystals then settle out, leaving an opening in the cloud or fog (Figure 5-28). The U.S. Air Force has practiced this technology for many yearsat airbases, and commercial airlines have used this method at selected foggy airports in the western United States. In northern Utah, where supercooled fog can persist in mountain valleys for weeks, the state transportation department uses liquid carbon dioxide to disperse fog and improve visibility. When released, the liquid carbon dioxide evaporates quickly, thereby cooling the air and causing the supercooled droplets to freeze and precipitate as fine ice crystals. Unfortunately, most fog does not consist of supercooled water droplets. The more common “warm fogs” are more expensive to combat because seeding will not diminish them. Successful attempts at dispersing warm fogs have involved mixing drier air from above into the fog. When the layer of fog is very shallow, helicopters have been used. By flying just above the fog, the helicopter creates a strong downdraft that forces drier air toward the surface, where it mixes with the saturated foggy air. At some airports where warm fogs are common, it is becoming more common to heat the air and thus evaporate the fog. A sophisticated thermal fog-dissipation system, called Turboclair, was installed in 1970 at Orly Airport in Paris. It uses eight jet engines in underground chambers along the upwind edge of the runway. Although expensive to install, the system is capable of improving visibility for about 900 meters (3000 feet) along the approach and landing zones.
Each year hailstorms inflict an average of $500 million in property damage and crop loss in the United States (Figure 5-29). Occasionally, a single severe hailstorm can produce damages that exceed that amount. Examples include large hailstorms that occurred in Germany in 1984, Canada in 1991, and Australia in 1999; in addition, a 2001 Kansas City hailstorm inflicted an estimated $2 billion in damage. As a result, some of history’s most interesting efforts at weather modification have focused on hail suppression. Farmers who are desperate to find ways to save their crops have long believed that strong noises—explosions, cannon shots, or ringing church bells—can help reduce the amount of hail that is produced during a thunderstorm. In Europe it was common practice for village priests to ring church bells to shield nearby farms from hail. Although this practice was banned in 1780 due to bell ringers being killed by lightning, it is still employed in a few locations. In 1880, a new hypothesis of hail formation sparked renewed interest in hail suppression. This untested proposal suggested that hailstones form in cold moist clouds during calm conditions. (We now know that large hailstones form in clouds with strong vertical air currents.) In this environment the pointed ends of tiny ice needles were thought to collect around a central embryo to form a spherical ice pelFigure 5-29)BJMEBNBHFUPBTPZCFBOmFMEOPSUIPG4JPVY 'BMMT 4PVUI%BLPUB "18JEF8PSME1IPUPT
Chapter 5 Forms of Condensation and Precipitation
let. Once a layer was complete, another layer of ice crystals was added, which explains the layered structure of hailstones. It was also believed that the migration of the tiny crystals toward the embryo would not take place if there was chaos in the environment. This incorrect hypothesis of hail formation led to the development of the first “anti-hail cannons” in the 1890s. These hail cannons were vertical muzzle-loading mortars that resembled huge megaphones. The one shown in Figure 5-30 weighted 9000 kilograms (10 tons), was 9 meters (30 feet) tall, and pivoted in all directions. When fired, the cannons produced a loud whistling noise and a large smoke ring that rose to a height of about 300 meters (1000 feet). The noise from the cannons was thought to disrupt the development of large hailstones. During a two-year test of hail cannons near the town of Windisch-Feistritz, Austria, no hail was observed. At the same time, nearby provinces suffered severe hail damage. From this unscientific study, it was concluded that hail cannons were effective. Use of these devices spread to other areas
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in Europe where crops of great value frequently experienced hail losses. Soon much of Europe acquired “cannon fever,” and by 1899 more than 2000 cannons were in use in Italy alone. Over the years, however, the anti-hail cannons proved to be ineffective, and the practice was largely abandoned. In 1972 a French company began producing a modern version of the anti-hail cannon, and today several small companies still manufacture these devices. In the United States they can be found in scattered sites in Colorado, Texas, and California. A Japanese auto manufacturer has installed anti-hail cannons to protect new cars at a plant located in Mississippi. Modern attempts at hail suppression have employed various methods of cloud seeding using silver iodide crystals to disrupt the growth of hailstones. In an effort to verify the effectiveness of cloud-seeding, the U.S. government established the National Hail Research Experiment in northeastern Colorado. This experiment included several randomized cloud-seeding experiments. An analysis of the data collected after three years revealed no statistically significant difference in the occurrence of hail between the seeded and non-seeded clouds, so the planned five-year experiment was abandoned. Nevertheless, cloud seeding is still employed today in many locations, including Colorado.
Frost Prevention
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Frost, a strictly temperature-dependent phenomena, occurs when the air temperature falls to 0°C (32°F) or below. Deposits of ice crystals, called white frost, form only when the air becomes saturated. A frost or freeze hazard can be generated in two ways: when a cold air mass moves into a region or when sufficient radiation cooling occurs on a clear night. Frost associated with an invasion of cold air is characterized by low daytime temperatures and long periods of freezing conditions that inflict widespread crop damage. By contrast, frost induced by radiation cooling is strictly a nighttime phenomenon that tends to be confined to low-lying areas. Obviously, the latter phenomenon is much easier to combat. Several methods of frost prevention are being used with varying success. They either conserve heat (reduce the heat lost at night) or add heat to warm the lowermost layer of air. Heat-conservation methods include covering plants with insulating material, such as paper or cloth, and generating particles that, when suspended in air, reduce the rate of radiation cooling. Warming methods employ water sprinklers, airmixing techniques, and/or orchard heaters. Sprinklers (Figure 5-31a) add heat in two ways: first, from the warmth of the water and, more importantly, from the latent heat of fusion released when the water freezes. As long as an ice–water mixture remains on the plants, the latent heat released will keep the temperature from dropping below 0°C (32°F).
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(b)
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Air mixing works best when the air temperature 15meters (50 feet) above the ground is at least 5°C (9°F) warmer than the surface temperature. By using a wind machine, the warmer air aloft is mixed with the colder surface air (Figure5-31b). Orchard heaters probably produce the most successful results (Figure 5-32). However, because as many as 30 to 40 heaters are required per acre, fuel cost can be significant.
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Chapter 5 Forms of Condensation and Precipitation
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Give It Some Thought 1. Clouds are classified into three major height categories: low, middle, and high. Explain why low clouds (or clouds of vertical development) are much more likely to produce precipitation than middle or high clouds. 2. Which of the three basic cloud forms (cirrus, cumulus, or stratus) is illustrated by each of the accompanying images labeled a–f?
9. Describe or sketch the vertical temperature profile that would cause precipitation that began as snow to reach the ground as each of the following: a. Snow b. Rain c. Freezing rain 10. Why is it unlikely to have virga and fog occur simultaneously? 11. The accompanying images show two different types of precipitation after they have been deposited. a. Name the types of precipitation shown. b. Describe the nature (form) of each type of precipitation before it was deposited. c. In what way are these types of precipitation similar to one another? d. In what way are they different?
3. Fog can be defined as a cloud with its base at or very near the ground, yet fogs and clouds form by different processes. Describe the similarities and differences in the formation of fogs and clouds. 4. Why does radiation fog form mainly on clear nights as opposed to cloudy nights? 5. The accompanying three images (a–c) depict three types of fog. a. Identify each fog type shown. b. Describe the mechanism that generated each of thefogs.
6. Assume that it is a midwinter day in central Illinois. Mild conditions prevail because of a steady wind from the south. As the day progresses, fog forms across a broad area. Identify the likely type of fog. 7. Imagine that you are driving in a hilly area early in the morning and encounter fog as you descend into the valleys, but as you drive out of the valleys, the conditions clear. Identify the likely type of fog. 8. Cloud droplets form and grow as water vapor condenses onto hydroscopic condensation nuclei. Research has shown that the maximum radius for cloud droplets is about 0.05 millimeter. However, typical raindrops are thousands of times larger. Describe in your own words one way in which cloud droplets become raindrops.
12. What are the advantages and disadvantages of using rain gauges compared to weather radar in measuring rainfall? 13. Weather radar provides information on the intensity as well as the total amount of precipitation. Table A shows the relationship between radar reflectivity values and rainfall rates. If radar measured a reflectivity value of 47 dBZ for 2½ hours at a particularlocation, how much rain has fallen there? 14. Cloud seeding efforts have met with varying degreesof success. If humans develop effective cloud seeding techniques that reliably increase rainfall in designated areas, what, if any, drawbacks might there be?
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FORMS OF CONDENSATION AND PRECIPITATION IN REVIEW ● ●
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Condensation occurs when water vapor changes to a liquid. For condensation to take place, the air must be saturated and there must be a surface on which the vapor can condense. Clouds, visible aggregates of minute droplets of water and/or tiny crystals of ice, are one form of condensation. Clouds are classified on the basis of two criteria: form and height. The three basic cloud forms are cirrus (high, white, and thin), cumulus (globular, individual cloud masses), and stratus (sheets or layers). Cloud heights can be either high, with bases above 6000 meters (20,000 feet); middle, from 2000 to 6000 meters; or low, below 2000 meters (6500 feet). Clouds of vertical development have bases in the low height range and extend upward into the middle or high range. Based on the two criteria, 10 basic cloud types exist. Fog, generally considered an atmospheric hazard, is a cloud with its base at or very near the ground. Fogs formed by cooling include radiation fog, advection fog, and upslope fog. Fogs formed by the addition of water vapor are steam fog and frontal fog. Dew is the condensation of water vapor on objects that have radiated sufficient heat to lower their temperature below the dew point of the surrounding air. White frost forms when the dew point of the air is below freezing. For precipitation to form, millions of cloud droplets must coalesce into drops large enough to sustain themselves during
● ●
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their descent. The two mechanisms that have been proposed to explain this phenomenon are the Bergeron process, which produces precipitation from cold clouds primarily in the middle latitudes, and the warm-cloud process most associated with the tropics, called the collision–coalescence process. The two most common and familiar forms of precipitation are rain and snow. Other forms include sleet, freezing rain (glaze), hail, and rime. The most common instruments used to measure rain are the standard rain gauge, which is read directly, and the tippingbucket gauge and weighing gauge, both of which recordthe amount and intensity of rain. The two most common measurements of snow are depth and water equivalent. Although the quantity of water in a given volume of snow is not constant, a general ratio of 10 units of snow to 1 unit of water is often used when exact information is not available. Intentional weather modification is deliberate human intervention to influence atmospheric processes that constitute the weather. Weather modification falls into three categories: (1) using energy to forcefully alter the weather; (2) modifying land and water surfaces to change their natural interaction with the lower atmosphere; and (3) triggering, intensifying, or redirecting atmospheric processes.
VOCABULARY REVIEW advection fog (p. 138) Bergeron process (p. 141) cirrus (p. 131) cloud (p. 130) cloud condensation nucleus (p. 130) cloud seeding (p. 153) cloud of vertical development (p. 132) collision–coalescence process (p. 143) cumulus (p. 131) fog (p. 137) freezing nucleus (p. 142) freezing rain, or glaze (p. 147)
frontal, or precipitation, fog (p. 140) frost (p. 155) hail (p. 147) high cloud (p. 132) hydrophobic nucleus (p. 130) hygroscopic nucleus (p. 130) intentional weather modification (p. 152) low cloud (p. 132) middle cloud (p. 132) radiation fog (p. 137) rain (p. 145) rime (p. 150)
sleet (p. 146) snow (p. 146) standard rain gauge (p. 150) steam fog (p. 138) stratus (p. 131) supercooled (p. 142) tipping-bucket gauge (p. 151) trace of precipitation (p. 151) upslope fog (p. 138) weighing gauge (p. 151)
PROBLEMS 1. Suppose that the air temperature is 20°C (68°F) and the relative humidity is 50 percent at 6:00 pm and that during the evening, the air temperature drops but its water vapor content does not change. If the air temperature drops 1°C (1.8°F) every two hours, will fog occur by sunrise (6:00 am) the next morning? Explain your answer. (Hint: The data you need are found in Table 4–1, p. 105.) 2. By using the same conditions as in Problem 1, will fog occur if the air temperature drops 1°C (1.8°F) every hour? If so, when will it first appear? What name is given to fog of this type that forms because of surface cooling during the night?
3. Assuming that the air is still, how long would it take a large raindrop (5 mm) to reach the ground if it fell from a cloud base at 3000 meters? (See Table 5-3, p. 143.) How long would a typical raindrop (2 mm) take to fall to the ground from the same cloud? How long if it were a drizzle drop (0.5 mm)? 4. Assuming that the air is still, how long would it take for a typical cloud droplet (0.02 mm) to reach the ground if it fell from a cloud base at 1000 meters? (See Table 5-3.) It is very unlikely that a cloud droplet would ever reach the ground, even if the air were perfectly still. Explain why.
Chapter 5 Forms of Condensation and Precipitation
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5. The dimensions of the large cumulonimbus cloud pictured in Figure 5-33 are roughly 12 kilometers high, 8 kilometers wide, and 8 kilometers long. Assume that the droplets in every cubic meter of the cloud total 0.5 cubic centimeter of water. How much liquid water does the cloud contain? How many gallons does it contain ()? 6. The record for the largest hailstone in the United States was set on July 23, 2010, in Vivian, South Dakota. This stone was 8 inches in diameter and 18.62 inches in circumference,
and it weighed nearly 2 pounds. The maximum fall speed of a spherical hailstone of diameter d can be calculated using , where if d is in centimeters and V is in meters per second. Estimate the strength (speed) of the updrafts in this storm that were necessary to support the stone just before it began to fall. Convert your answer from meters per second to feet per second and then to miles per hour.
Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Forms of Condensation and Precipitation.
Air Pressure and Winds Of the various elements of weather and climate, changes in air pressure are the least perceptible to humans; however, they are very important in producing changes in our weather. For example, variations in air pressure generate winds that trigger changes in temperature and humidity. In addition, air pressure is a significant factor in weather forecasting and is closely tied to the other elements of weather (temperature, moisture, and wind) in cause-and-effect relationships.
Horizontal differences in air pressure created the winds that propelled these wind surfers. (Photo by Sam Pellissier/ SuperStock)
After completing this chapter, you should be able to:
t Define air pressure and explain why it changes with altitude.
t Explain the principle of the mercury barometer and how it differs from the aneroid barometer.
t Describe how horizontal pressure differences are generated and how differences in solar heating can cause global variations in air pressure.
t List and describe the three forces that act on the atmosphere to either create or alter winds.
t Explain why the winds aloft flow roughly parallel to the isobars, while surface winds travel at an angle across the isobars.
t Describe how horizontal airflow (wind) can generate vertical air motion.
t Contrast the weather associated with low-pressure centers (cyclones) and high-pressure centers (anticyclones).
t Describe how wind direction is expressed using compass directions. 161
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Wind and Air Pressure We know that air moves vertically if it is forced over a barrier or if it is warmer and thus more buoyant than surrounding air. But what causes air to move horizontally— the phenomenon we call wind (Figure 6-1)? Simply stated, wind is the result of horizontal differences in atmospheric pressure. Air flows from areas of higher pressure to areas of lower pressure. You may have experienced this condition when opening a can of something vacuum packed. The noise you hear is caused by air rushing from the area of higher pressure outside the can to the lower pressure inside. Wind is nature’s attempt to balance inequalities in air pressure. We live at the bottom of the atmosphere, and just as the creatures living at the bottom of the ocean are subjected to pressure exerted by water, humans are subjected to the pressure exerted by the weight of the atmosphere above. Although we do not generally notice the pressure exerted by the ocean of air around us (except when rapidly ascending or descending in an elevator or airplane), it is nonetheless substantial. The pressurized suits used by astronauts on space walks were designed to duplicate the atmospheric pressure experienced at Earth’s surface. Without these protective suits to keep body fluids from boiling away, astronauts would perish in minutes. Air pressure can be visualized by examining the behavior of gases. Gas molecules, unlike those of the liquid and solid phases, are not “bound” to one another but are freely moving about, filling all space available to them. When two gas molecules collide, which happens frequently under normal atmospheric conditions, they bounce off each other like elastic balls. If a gas is confined to a container, this motion is restricted by its sides, much as the walls of a handFigure 6-1 Strong winds created these waves that battered the coast of Great Britain near Tynemouth. Ocean waves are energy traveling along the interface between the ocean and atmosphere, often transferring energy from a storm far out at sea. (Photo by Geoff Love/Alamy)
Pressure exerted by the atmosphere
1 kg/cm2 or 14.7 lbs/in2
Figure 6-2 Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch.
ball court redirect the motion of a handball. The continuous bombardment of gas molecules against the sides of the container exerts an outward force called air pressure. Although the atmosphere is without walls, it is confined from below by Earth’s surface and effectively from above because the force of gravity prevents its escape. Thus, we define atmospheric pressure, or simply air pressure, as the force exerted against a surface by the continuous collision of gas molecules. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch. Specifically, a column of air 1 square inch in cross section, measured from sea level to the top of the atmosphere, would weigh about 14.7 pounds (Figure 6-2). (This is roughly the same pressure produced by a 1-square-inch column of water 10 meters [33 feet] in height.) The pressure that air exerts at Earth’s surface is much greater than most people realize. For example, the air pressure exerted on the top of a small (50-centimeter-by-100-centimeter) school desk exceeds 5000 kilograms (11,000 pounds), or about the weight of a 50-passenger school bus. Why doesn’t the desk collapse under the weight of the air above? Simply, air pressure is exerted in all directions—down, up, and sideways. Thus, the air pressure around all sides of the desk is exactly balanced. Imagine the base of a tall aquarium with the same dimensions as the desktop. When this aquarium is filled to a height of 10 meters (33 feet), the water pressure at the bottom equals 1 atmosphere (14.7 pounds per square inch).
Chapter 6 Air Pressure and Winds
Now imagine what would happen if this aquarium were placed on top of the desk so that all the force was directed downward? By contrast, if the desk is placed inside the aquarium and allowed to sink to the bottom, the desk survives because the water pressure is exerted in all directions, not just downward, as in our earlier example. The desk, like a human body, is built to withstand the pressure of 1 atmosphere.
Concept Check 6.1 1 What is wind and what is its basic cause? 2 What is standard sea-level pressure, in pounds per square inch? 3 Describe atmospheric pressure in your own words.
Measuring Air Pressure ATMOSPHERE
Air Pressure and Winds ▸ Measuring Air Pressure
When describing atmospheric pressure, meteorologists use a unit of force from physics called the newton.* Under average conditions (at sea level) the atmosphere exerts a force of 101,325 newtons per square meter. To simplify this large number, the National Weather Service adopted the millibar (mb), which equals 100 newtons per square meter. Thus, standard sea-level pressure is stated as 1013.25 millibars (Figure 6-3).** You may be acquainted with the expression “inches of mercury,” which weather reporters use to describe atmospheric pressure. This expression dates from 1643, when Torricelli, a student of the famous Italian scientist Galileo, invented the mercury barometer. Torricelli correctly described the atmosphere as a vast ocean of air that exerts pressure on us and all things about us. To measure this force, he closed one end of a glass tube and filled it with mercury. He then inverted the tube into a dish of mercury (Figure 6-4). Torricelli found that the mercury flowed out of the tube until the weight of the mercury column was balanced by the pressure exerted on the surface of the mercury by the air above. In other words, the weight of the mercury in the column equaled the weight of a similar-diameter column of air that extended from the ground to the top of the atmosphere. Torricelli noted that when air pressure increased, the mercury in the tube rose; conversely, when air pressure decreased, so did the height of the column of mercury. The length of the column of mercury, therefore, became the measure of the air pressure–“inches of mercury.” With some refinements, the mercury barometer invented by Torricelli remains the standard pressure-measuring instrument. *A newton is the force needed to accelerate a 1 kilogram mass 1 meter per second squared. **The standard unit of pressure in the SI system is the pascal, which is the name given to a newton per square meter (N/m2). In this notation a standard atmosphere has a value of 101,325 pascals, or 101.325 kilopascals. If the National Weather Service officially converts to the metric system, it will probably adopt this unit.
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Highest recorded sea level pressure: 1084 mb (32.01 in.) Agata, Siberia (December, 1968) Highest recorded sea level pressure in United States: 1064 mb (31.42 in.) Miles City, Montana (December, 1983) Strong high pressure system (anticyclone) Average sea level pressure: 1013.25 mb (29.92 in.)
Strong low pressure system (Midlatitude cyclone)
Hurricane Katrina (August, 2005): 902 mb (26.71 in.) Hurricane Wilma (October, 2005): 882 mb (26.12 in.) Lowest pressure recorded for an Atlantic hurricane Lowest recorded sea level pressure: 870 mb (25.70 in.) Typhoon Tip (October, 1979)
Figure 6-3 A comparison of atmospheric pressure in inches of mercury and millibars.
Vacuum
Mercury column
Height 76 cm (29.92 in.)
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Figure 6-4 Mercury barometer. (a) The weight of the column of mercury is balanced by the pressure exerted on the dish of mercury by the air above. If the pressure decreases, the column of mercury falls; if the pressure increases, the column rises. (b) Image of a mercury barometer. (Photo by D. Winters/Photo Researchers, Inc.)
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Standard atmospheric pressure at sea level equals 29.92 inches (760 millimeters) of mercury. In the United States the National Weather Service uses millibars on weather maps and charts, but it reports surface pressure to the public in inches of mercury. The need for a smaller and more portable instrument for measuring atmospheric pressure led to the development of the aneroid barometer (aneroid means “without liquid”). Rather than a mercury column held up by air pressure, the aneroid barometer uses a partially evacuated metal chamber (Figure 6-5). The chamber, being very sensitive to pressure variations, changes shape, compressing as pressure increases and expanding as pressure decreases. As shown in Figure 6-5, the face of an aneroid barometer intended for home use includes the words fair, change, rain, and stormy. Notice that “fair weather” corresponds with high-pressure readings, whereas “rain” is associated with low pressures. To “predict” weather in a local area, the change in air pressure over the past few hours is more important than the current pressure reading. Falling pressure is often associated with increasing cloudiness and the possibility of precipitation, whereas rising air pressure generally indicates clearing conditions. Another advantage of the aneroid barometer is that it can easily be connected to a recording mechanism. This recording instrument, called a barograph, provides a con-
Figure 6-6 An aneroid barograph makes a continuous record of pressure changes. (Photo courtesy of Qualimetrics, Inc., Sacramento, California.)
tinuous record of pressure changes over time (Figure 6-6). Another important adaptation of the aneroid barometer is its use to indicate altitude for aircraft, mountain climbers, and mapmakers (see Box 6-1).
Concept Check 6.2 1 What is average sea-level pressure when measured in millibars? In inches of mercury?
2 Describe the operating principles of the mercury barometer
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measuring barometric pressure?
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Why is mercury used in a barometer? I thought it was poisonous.
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You are correct: Mercury poisoning can be quite serious. However, the mercury in a barometer is held in a reservoir where the chance of spillage is minimal. A barometer could be constructed using any number of different liquids, including water. The problem with a waterfilled barometer is size. Because water is 13.6 times less dense than mercury, the height of a water column at standard sea-level pressure would be 13.6 times taller than that of a mercury barometer. Consequently, a water-filled barometer would need to be nearly 34 feet tall.
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Figure 6-5 Aneroid barometer. (a) Illustration of an aneroid barometer. (b) The aneroid barometer has a partially evacuated chamber that changes shape, compressing as atmospheric pressure increases and expanding as pressure decreases.
Pressure Changes with Altitude Just as scuba divers experience a decrease in water pressure as they rise toward the surface, we experience a decrease in air pressure as we ascend into the atmosphere. The relation-
Chapter 6 Air Pressure and Winds
Less than 10% of the mass of the atmosphere is found above 16 km 16
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ship between air pressure and the air’s density largely explains the drop in air pressure that occurs with altitude. Recall that at sea level a column of air weighs 14.7 pounds per square inch and therefore exerts that amount of pressure. As we ascend through the atmosphere, we find that the air becomes less dense because of the continual decrease in the amount (weight) of air above. Therefore, as would be expected, there is a corresponding decrease in pressure with an increase in altitude. The fact that density decreases with altitude is why the term “thin air” is normally associated with mountainous regions. Except for the Sherpas (indigenous peoples of Nepal), most of the climbers who reach the summit of Mount Everest use supplementary oxygen for the final leg of the journey. Even with the aid of supplementary oxygen, many of these climbers experience periods of disorientation because of an inadequate supply of oxygen to their brains. The decrease in pressure with altitude also affects the boiling temperature of water, which at sea level is 100°C (212°F). For example, in Denver, Colorado—the Mile High City—water boils at about 95°C (203°F). Although water comes to a boil faster in Denver than in San Diego, it takes longer to cook spaghetti in Denver because of its lower boiling temperature.
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Figure 6-7 Illustration of the U.S. standard atmosphere. Each layer contains about 10 percent of the atmosphere. Notice that at roughly 5 kilometers, the pressure is about one-half of its sea-level value.
Recall from Chapter 1 that the rate at which pressure decreases with altitude is not constant. The rate of decrease is much greater near Earth’s surface, where pressure is high, than aloft, where air pressure is low. A model of the U.S. standard atmosphere, shown in Figure 6-7, depicts the idealized vertical distribution of atmospheric pressure at various altitudes. Near Earth’s surface, air pressure decreases by about 10 millibars for every 100-meter increase in elevation, or about a 1-inch drop of mercury for every 1000-foot rise in elevation. Further, atmospheric pressure is reduced by approximately one-half for each 5-kilometer increase in altitude. Therefore, at 5 kilometers the pressure is 500 millibars, about one-half its sea-level value; at 10 kilometers it is onefourth, at 15 kilometers it is one-eighth, and so forth. Thus, at the altitude at which commercial jets fly (10 kilometers), the air exerts a pressure equal to only one-fourth that at sea level.
(AP Photo/ Jae C. Hong)
AZ NM (NASA)
In May and June 2011, the Wallow Fire burned more than 538 thousand acres (840 square miles) in southeastern Arizona. It was the largest wildfire in Arizona history. Question 1 Can you suggest two ways in which wind helps sustain wildfires such as this?
Concept Check 6.3 1 Explain why air pressure decreases with an increase in altitude.
2 Why do climbers use supplemental oxygen when summiting Mount Everest?
3 What is the U.S. standard atmosphere?
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Students Sometimes Ask…
Cold (dense) air
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Surface pressure (1013 mb)
Surface pressure (1013 mb)
Why do my ears sometimes feel painful when I fly? When airplanes take off and land, some people experience pain in their ears because of a change in cabin pressure. (Although most commercial airplanes are designed to keep the cabin pressure relatively constant, small changes in pressure occur.) Normally the air pressure in one’s middle ear is the same as the pressure of the surrounding atmosphere because the eustachian tube connects the ear to the throat. However, when an individual has a cold, his or her eustachian tubes may become blocked, preventing the flow of air either into or out of the middle ear. When an airplane ascends or descends, the resulting change in pressure can cause mild discomfort or, less often, excruciating pain, which subsides when the ears “pop” to equalize the pressure.
Why Does Air Pressure Vary? Why does atmospheric pressure vary daily, and why is that important? Recall that variations in air pressure cause the wind to blow, which in turn causes changes in temperature and humidity. In short, difference in air pressure create global winds that become organized into the systems that bring us our weather. Therefore, the National Weather Service closely monitors daily changes in air pressure because this information is critical to forecasting. Although vertical pressure changes are important, meteorologists are much more interested in the horizontal pressure differences that occur daily from place to place around the globe. Pressure differences from place to place are relatively small. Extreme readings are rarely more than 30 millibars (1 inch of mercury) above average sea-level pressure or 60 millibars (2 inches) below average sea-level pressure. Occasionally, the barometric pressure measured in severe storms, such as hurricanes, is lower (see Figure 6-3). As you will see, these small differences in air pressure can be sufficient to generate violent winds.
Influence of Temperature on Air Pressure How do pressure differences arise? One of the easiest ways to envision this is to picture northern Canada in midwinter. Here the snow-covered surface is continually radiating heat to space, while receiving minimal incoming solar radiation. The frigid ground cools the air above so that daily lows of –34°C (–30°F) are common. Recall that temperature is a measure of the average molecular motion (kinetic energy) of a substance. As a result, cold Canadian air is composed of comparatively slowmoving gas molecules that are packed closely together. These cold, dense air masses are associated with high surface pressures and are labeled highs (H) on weather maps. By contrast, during the summer, large areas of the American Southwest experience extremely high temperatures that are accompanied by low surface pressures labeled lows (L) on weather maps. Although a cold air column is generally
Figure 6-8 A comparison of the density of a column of cold air and a column of warm air. With an increase in altitude, air pressure drops more rapidly in the column of cold air.
associated with high surface pressure, and a warm air column is often associated with low surface pressure, other factors also influence surface pressures. These factors are examined in the next two sections. Figure 6-8 shows another important difference between cold and warm air: Air pressure drops more rapidly with altitude in a column of cold (dense) air than in a column of warm (less dense) air. For purposes of illustration only, we assume that both columns of air exert the same surface pressure, and (although greatly exaggerated) differences in the spacing of air molecules represent differences in density. Looking at the line drawing halfway up the two columns, notice that there are more air molecules above this altitude in the warm column than in the cold column (Figure 6-8). As a result, warm air aloft tends to exhibit a higher pressure than cold air at the same altitude. This important concept has implications in aviation (see Box 6-1) and will be considered in more detail when we discuss the forces that generate airflow.
Influence of Water Vapor on Air Pressure Although of less importance, factors other than temperature affect the amount of pressure a column of air will exert. For example, the amount of water vapor contained in a volume of air influences its density. Contrary to popular perception, water vapor reduces the density of air. The air may feel “heavy” on hot, humid days, but it is not. You can easily verify this fact for yourself by examining a periodic table of the elements and noting that the molecular weights of nitrogen (N2) and oxygen (O2) are greater than that of water vapor
Chapter 6 Air Pressure and Winds
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Box 6-1 Air Pressure and Aviation The cockpit of nearly every aircraft contains a pressure altimeter, an instrument that allows a pilot to determine the altitude of a plane. A pressure altimeter is essentially an aneroid barometer marked in meters instead of millibars and, as such, responds to changes in air pressure. For example, Figure 6-7 shows that the standard atmospheric pressure of about 800 millibars “normally” occurs at a height of 2000 meters. Therefore, when the pressure reaches 800 millibars, the altimeter will indicate an altitude of 2000 meters. Because of temperature variations and moving pressure systems, actual conditions are usually different from that shown by an aircraft’s altimeter. When the air is warmer than predicted by the standard atmosphere, the plane will fly higher than
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variation will result in a change in the plane’s altitude. When pressure increases (warm air column) along a flight path, the plane will climb, and when pressure decreases (cold air column), the plane will descend. There is little risk of midair collisions because nearby aircraft are assigned different flight levels in order to maintain sufficient separation. Large commercial aircraft also use radar altimeters to measure heights above the terrain. The time required for a radio signal to reach the surface and return is used to accurately determine the height of the plane above the ground. This system is not without drawbacks. Because a radar altimeter provides the elevation above the ground rather than above sea level, a knowledge of the underlying terrain is required. However, radar altimeters are useful when measuring the height above ground level during landing.
the height indicated by the altimeter. By contrast, in cold air the plane will fly lower than indicated. This could be especially dangerous if the pilot is flying a small plane through mountainous terrain with poor visibility (Figure 6-A). To avoid dangerous situations, pilots make altimeter corrections before takeoffs and landings, and in some cases they make corrections en route as well. Above about 5.5 kilometers (18,000 feet), pressure changes are more gradual, so corrections cannot be made as precisely as at lower levels. Consequently, commercial aircraft have their altimeters set at the standard atmosphere and fly paths of constant pressure, called flight levels, instead of constant altitude (Figure 6-B). In other words, when an aircraft flies at a constant altimeter Cold air setting, a pressure
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(H2O). In a mass of air the molecules of these gases are intermixed, and each takes up roughly the same amount of space. As the water content of an air mass increases, lighter water vapor molecules displace heavier nitrogen and oxygen molecules. Therefore, humid air is lighter (less dense) than dry air. Nevertheless, even very humid air is only about 2 percent less dense than dry air at the same temperature. In summary, cold, dry air produces higher surface pressures than warm, humid air. Further, a warm, dry air mass exhibits higher pressure than an equally warm, but humid, air mass. Conse-
360 mb Altimieter reads 8 km
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FIGURE 6-A Aircraft altimeters are calibrated based on the relationship between air pressure and altitude, using data provided by the standard atmosphere. However, when a plane enters a warm air column, its altitude is higher than indicated by the altimeter. By contrast, when a plane flies into a cold air mass, its altitude is lower than shown on the altimeter, which can be a problem in mountainous terrains.
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FIGURE 6-B Commercial aircraft above 5.5 kilometers (18,000 feet) generally fly paths of constant pressure instead of constant altitude.
quently, differences in temperature and to a lesser extent moisture content are largely responsible for the pressure variations observed at Earth’s surface.
Airflow and Pressure The movement of air can also cause variations in air pressure. For example, in situations where there is a net flow of air into a region, a phenomenon called convergence, air accumulates. As it converges horizontally, the air is squeezed
Concept Check 6.4 1 Explain why a cold, dry air mass produces a higher surface pressure than a warm, humid air mass.
2 If all other factors are equal, does a dry or moist air mass
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into a smaller space, which results in a more massive air column that exerts more pressure at the surface. By contrast, in regions where there is a net outflow of air, a situation referred to as divergence, the surface pressure drops. We will return to the important mechanisms of convergence and divergence, which help generate areas of high and low pressure, later in this chapter. In summary, the pressure at the surface will increase when there is a net convergence in a region and decrease when there is a net divergence.
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Strong winds
Figure 6-9 Isobars are lines connecting places of equal atmospheric pressure. The spacing of isobars indicates the amount of pressure change occurring over a given distance—called the pressure gradient force. Closely spaced isobars indicate a strong pressure gradient and high wind speeds, whereas widely spaced isobars indicate a weak pressure gradient and low wind speeds.
exert more air pressure? Explain.
3 Explain how horizontal convergence affects surface pressure.
Factors Affecting Wind ATMOSPHERE
Air Pressure and Winds ▸Factors Affecting Wind
If Earth did not rotate and if there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. However, because both factors exist, wind is controlled by a combination of forces, including: 1. Pressure gradient force 2. Coriolis force 3. Friction
Pressure Gradient Force If an object experiences an unbalanced force in one direction, it will accelerate (experience a change in velocity). The force that generates winds results from horizontal pressure differences. When air is subjected to greater pressure on one side than on another, the imbalance produces a force that is directed from the region of higher pressure toward the area of lower pressure. Thus, pressure differences cause the wind to blow, and the greater these differences, the greater the wind speed. Variations in air pressure over Earth’s surface are determined from barometric readings taken at hundreds of weather stations. These pressure measurements are shown on surface weather maps using isobars (iso = “equal,” bar = “pressure”) lines connecting places of equal air pressure (Figure 6-9). The spacing of the isobars indicates the amount of pressure change occurring over a given distance and is called the pressure gradient force. Pressure gradient is analogous to gravity acting on a ball rolling down a hill. A steep pressure gradient, like a steep hill, causes greater acceleration of a parcel of air than does a weak pressure gradient (a gentle hill). Thus, the relationship between wind speed
and the pressure gradient is straightforward: Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds. Figure 6-9 illustrates the relationship between the spacing of isobars and wind speed. Note also that the pressure gradient force is always directed at right angles to the isobars. In order to draw isobars on a weather map to show air pressure patterns, meteorologists must compensate for the elevation of each station. Otherwise, all high-elevation locations, such as Denver, Colorado, would always be mapped as having low pressure. This compensation is accomplished by converting all pressure measurements to sea-level equivalents (Figure 6-10). Doing so requires meteorologists to determine the pressure that would be exerted by an imaginary column of air equal in height to the elevation of the recording station and adding it to the station’s pressure reading. Because temperature greatly affects the density, and hence the weight of this imaginary column, temperature is also considered in the calculations. Thus, the corrected reading would give the pressure, as if it were taken at sea level under the same conditions.
Denver, CO
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Figure 6-10 To compare atmospheric pressures, meteorologists first convert all pressure measurements to sea-level values. This is done by adding the pressure that would be exerted by an imaginary column of air (shown in red) to the station’s pressure reading.
Chapter 6 Air Pressure and Winds
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Figure 6-11 Cross sectional view illustrating the formation of a sea breeze. (a) Just before sunrise; (b) after sunrise; (c) sea breeze established.
How Temperature Differences Generate Wind To illustrate how temperature differences can generate a horizontal pressure gradient and thereby create winds, consider a common example, the sea breeze. Figure 6-11a shows a vertical cross section of a coastal location before sunrise. At this time of day, we will assume that temperatures and pressures do not vary horizontally. Because there is no horizontal variation in pressure (zero horizontal pressure gradient) either aloft or at the surface, there is no wind. After sunrise, the temperature of Earth’s land surface begins to increase, while the temperature over the ocean remains nearly constant. As air over the land warms, it expands and forms a less dense air column. Although the air pressure at the surface remains essentially the same, this is
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not true at higher altitudes. As we saw earlier, in Figure 6-8, warm air aloft tends to have higher air pressure compared to a column of cooler air. Consequently, above the land a high pressure develops, and the air aloft begins to flow away from the land (Figure 6-11b). The mass transfer of air aloft creates a surface high-pressure area over the ocean, where the air is collecting, and a surface low over the land. The surface circulation that develops from this redistribution of mass is from the sea toward the land (sea breeze), as shown in Figure 6-11c. Thus, a simple thermal circulation develops, with a seaward flow aloft and a landward flow at the surface. Note that vertical movement is also required to make the circulation complete. An important relationship exists between air pressure and temperature, as you saw in the preceding discussion. Temperature variations create pressure differences and ultimately wind. Daily temperature differences caused by unequal heating as in the sea breeze example tend to be confined to a zone only a few kilometers thick. On a global scale, however, variations in the amount of solar radiation received in the polar versus the equatorial latitudes generate the much larger pressure systems that in turn produce the planetary atmospheric circulation. Therefore, the underlying cause of global pressure differences and, by extension, wind is mainly the unequal heating of Earth’s land–sea surface. Isobars on a Surface Chart Figure 6-12 is a surface weather map that shows isobars and winds. Wind direction is shown as wind arrow shafts and speed as wind bars (see the accompanying key). Isobars, used to depict pressure patterns, are rarely straight or evenly spaced on surface maps. Consequently, wind generated by the pressure gradient force typically changes speed and direction as it flows. The area of somewhat circular closed isobars represented by the red letter L is a low-pressure system. Low-pressure systems that occur in the middle latitudes are called cyclones, or midlatitude cyclones, to differentiate them from tropical cyclones. Midlatitude cyclones tend to produce stormy weather. (Tropical cyclones are also called hurricanes or typhoons, depending on their locations.) In western Canada, a high-pressure system, denoted by the blue letter H, can also be seen. High-pressure areas such as this are called anticyclones. In contrast to cyclones, anticyclones tend to be associated with clearing conditions. In summary, the horizontal pressure gradient is the driving force of wind. The magnitude of the pressure gradient force is shown by the spacing of isobars. The direction of force is always from areas of higher pressure toward areas of lower pressure and at right angles to the isobars.
Coriolis Force The weather map in Figure 6-12 shows the typical airflow associated with surface high- and low-pressure systems. As expected, the air moves out of the regions of higher pressure and into the regions of lower pressure. However, the wind rarely crosses the isobars at right angles, as the pres-
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Figure 6-12 Isobars are used to show the distribution of pressure on daily weather maps. Isobars are seldom straight but usually form broad curves. Concentric isobars indicate cells of high and low pressure. The “wind flags” indicate the expected airflow surrounding pressure cells and are plotted as “flying” with the wind (that is, the wind blows toward the station circle). Notice on this map that the isobars are more closely spaced and the wind speed is faster around the low-pressure center than around the high.
sure gradient force directs. This deviation is the result of Earth’s rotation and has been named the Coriolis force, after the French scientist Gaspard-Gustave Coriolis, who first expressed its magnitude quantitatively. It is important to note that the Coriolis force cannot generate wind; rather, it modifies airflow. The Coriolis force causes all freemoving objects, including wind, to be deflected to the right of their path of motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The reason for this deflection can be illustrated by imagining the path of a rocket launched from the North Pole toward a target on the equator (Figure 6-13). If
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Figure 6-13 The Coriolis force, illustrated using the one-hour flight of a rocket traveling from the North Pole to a location on the equator. (a) On a nonrotating Earth, the rocket would travel straight to its target. (b) However, Earth rotates 15° each hour. Thus, although the rocket travels in a straight line, when we plot the path of the rocket on Earth’s surface, it follows a curved path that veers to the right of the target.
Chapter 6 Air Pressure and Winds
the rocket travels one hour toward its target, Earth would have rotated 15° to the east during its flight. To someone watching the rocket’s path from the location of the intended target, it would look as if the rocket veered off its path and hit Earth 15° west of its target. The true path of the rocket was straight and would appear as such to someone in space looking toward Earth. Earth’s rotation under the rocket produced the apparent deflection. Note that the rocket did not hit its target and as such was deflected to the right of its path of motion because of the counterclockwise rotation of the Northern Hemisphere. Clockwise rotation produces a similar deflection in the Southern Hemisphere, but to the left of the path of motion. Although it is usually easy for people to visualize the Coriolis deflection when the motion is from north to south, as in our rocket example, it is not as easy to see how a westto-east flow would be deflected. Figure 6-14 illustrates this situation, using winds blowing eastward at four different latitudes (0°, 20°, 40°, and 60°). Notice that after a few hours the winds along the 20th, 40th, and 60th parallels appear to be veering off course. However, when viewed from space, it is apparent that these winds have maintained their original direction. It is the “change” of orientation of North America as Earth rotates on its axis that produces the deflection we observe in Figure 6-14. We can also see in Figure 6-14 that the amount of deflection is greater at 60° latitude than at 40° latitude, and likewise greater at 40° than at 20°. Furthermore, there is no deflection observed for the airflow along the equator. We conclude, therefore, that the magnitude of the Coriolis force is dependent on latitude—it is strongest at the poles and weakens equatorward, where it eventually becomes nonex-
istent. We can also see that the amount of Coriolis deflection increases with wind speed because faster winds travel farther than slower winds in the same time period. All “free-moving” objects are affected by the Coriolis force. This fact was dramatically discovered by the U.S. Navy at the beginning of World War II. During target practice, long-range guns on battleships continually missed their targets by as much as several hundred yards until ballistic corrections were made for the changing position of seemingly stationary targets. Across short distances, however, the Coriolis force is relatively small. Nevertheless, in the middle latitudes this deflecting force is great enough to potentially affect the outcome of a baseball game. A ball hit a horizontal distance of 100 meters (330 feet) in 4 seconds down the right field line will be deflected 1.5 centimeters (more than 1/2 inch) to the right by the Coriolis force. This could be enough to turn a potential home run into a foul ball! In summary, the Coriolis force acts to change the direction of a moving body to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflecting force (1) is always directed at right angles to the direction of airflow; (2) affects only wind direction, not wind speed; (3) is affected by wind speed (the stronger the wind, the greater the deflecting force); and (4) is strongest at the poles and weakens equatorward, becoming nonexistent at the equator.
Friction Earlier we stated that the pressure gradient force is the primary driving force of the wind. As an unbalanced force, it causes air to accelerate from regions of higher pressure
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Figure 6-14 Coriolis deflection of winds blowing eastward at different latitudes. After a few hours the winds along the 20th, 40th, and 60th parallels appear to veer off course. This deflection (which does not occur at the equator) is caused by Earth’s rotation, which changes the orientation of the surface over which the winds are moving.
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Figure 6-15 Friction as a factor in wind speed. (a) Wind increases in strength with an increase in altitude because it is less affected by friction from objects near Earth’s surface. (b) This snow-covered tree shows the effects of strong winds in a high mountain setting. (Photo by E. J. Tarbuck)
toward regions of lower pressure. Thus, we might expect wind speeds to continually increase (accelerate) as long as this imbalance existed. But we know from personal experience that winds do not become faster indefinitely. Rather, friction, which acts to slow moving objects, comes into play. Although friction significantly influences airflow near Earth’s surface, its effect is negligible above a height of a few kilometers (Figure 6-15). Therefore, we will examine the flow aloft first (where the effect of friction is small) and then analyze surface winds (where friction significantly influences airflow).
Students Sometimes Ask… I’ve been told that water goes down a sink in one direction in the Northern Hemisphere and in the opposite direction in the Southern Hemisphere. Is that true? Not necessarily! The origin of this myth comes from applying a scientific principle to a situation where it does not fit. Recall that the Coriolis deflection causes cyclonic systems to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.It was inevitable that someone would suggest (without checking) that a sink should drain in a similar manner. However, a cyclone is more than 1000 kilometers in diameter and may exist for several days. By contrast, a typical sink is less than 1 meter in diameter and drains in a matter of seconds. On this scale, the Coriolis force is minuscule. Therefore, the shape of the sink and how level it is has more to do with the direction of waterflow than the Coriolis force.
Concept Check 6.5 1 List three forces that combine to direct horizontal airflow (wind).
2 What force is responsible for generating wind? 3 Write a generalization relating the spacing of isobars to wind speed.
4 Briefly describe how the Coriolis force modifies air movement. 5 Which two factors influence the magnitude of the Coriolis force?
Winds Aloft In this section we address airflow that occurs at least a few kilometers above Earth’s surface, where the effects of friction are small enough to disregard.
Geostrophic Flow Aloft, the Coriolis force is responsible for balancing the pressure gradient force and thereby directing airflow. Figure6-
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16 shows how a balance is achieved between these opposing forces. For illustration only, we assume a nonmoving parcel of air at the starting point in Figure 6-16. (Remember that air is rarely stationary in the upper atmosphere.) Because our parcel of air has no motion, the Coriolis force exerts no influence. Under the influence of the pressure gradient force, the parcel begins to accelerate directly toward the area of low pressure. As soon as the flow begins, the Coriolis force commences and causes a deflection to the right for winds in the Northern Hemisphere. As the parcel accelerates, the Coriolis force intensifies. (Recall that the magnitude of the Coriolis force is proportional to wind speed.) Thus, the increased speed results in further deflection. Eventually the wind turns so that it is flowing parallel to the isobars. When this occurs, the pressure gradient force is balanced by the opposing Coriolis force, as shown in Figure6-16. As long as these forces remain balanced, the resulting wind will continue to flow parallel to the isobars at a constant speed. Stated another way, the wind can be considered to be coasting (not accelerating or decelerating) along a pathway defined by the isobars. Under these idealized conditions, when the Coriolis force is exactly equal in strength but acting in the opposite direction of the pressure gradient force, the airflow is said to be in geostrophic balance. The winds generated by this balance are called geostrophic (“turned by Earth”) winds. Geostrophic winds flow in relatively straight paths, parallel to the isobars, with velocities proportional to the pressure gradient force. A steep pressure gradient creates strong winds, and a weak pressure gradient produces light winds. It is important to note that the geostrophic wind is an idealized model that only approximates the actual behavior
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Figure 6-16 The geostrophic wind. The only force acting on a stationary parcel of air is the pressure gradient force. Once the air begins to accelerate, the Coriolis force deflects it to the right in the Northern Hemisphere. Greater wind speeds result in a stronger Coriolis force (deflection) until the flow is parallel to the isobars. At this point the pressure gradient force and Coriolis force are in balance, and the flow is called a geostrophic wind. It is important to note that in the “real” atmosphere, airflow is continually adjusting for variations in the pressure field. As a result, the adjustment to geostrophic equilibrium is much more irregular than shown.
of airflow aloft. Under natural conditions in the atmosphere, winds are never purely geostrophic. Nonetheless, the geostrophic model offers a useful estimation of the actual winds aloft. By measuring the pressure field (orientation and spacing of isobars) that Surface exists aloft, meteorologists can determine both wind direction and speed (Figure 6-17). As we have seen, wind direction is directly linked to the prevailing pressure pattern. Therefore, if we know the wind direction, we can also establish a rough approximation of the pressure distribution. This rather straightforward relationship between wind direction and pressure distribution was first formulated by the Dutch meteorologist Buys Ballot in 1857. Essentially, Buys Ballot’s law states that in the Northern Hemisphere, if you stand with your back to the wind, low pressure will be found to your left and high pressure to your right. In the Southern Hemisphere, the situation is reversed. Although Buys Ballot’s law holds for airflow aloft, it must be used with caution when applied to surface winds. At the surface, friction and topography interfere with the idealized circulation. At the surface, if you stand with your back to the wind and then turn clockwise about 30°, low pressure will be to your left and high pressure to your right. In summary, winds above a few kilometers can be considered geostrophic—that is, they flow in a straight path parallel to the isobars at speeds that can be calculated from the pressure gradient. The major discrepancy from true geostrophic winds involves the flow along highly curved paths, a topic considered next.
Curved Flow and the Gradient Wind A casual glance at an upper-level weather map shows that the isobars are not generally straight; instead, they make broad, sweeping curves (see Figure 6-17). Occasionally the isobars connect to form roughly circular cells of either high or low pressure. Thus, unlike geostrophic winds that flow along relatively straight paths, winds around cells of high or low pressure follow curved paths in order to parallel
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Figure 6-17 Upper-air weather chart. This simplified weather chart shows the direction and speed of the upper-air winds. Note from the flags that the airflow is almost parallel to the contours. Like most other upper-air charts, this one shows variations in the height (in meters) at which a selected pressure (500 millibars) is found instead of showing variations in pressure at a fixed height, like surface maps. Do not let this confuse you because there is a simple relationship between height contours and pressure. Places experiencing 500-millibar pressure at higher altitudes (toward the south on this map) are experiencing higher pressures than places where the height contours indicate lower altitudes. Thus, higher-elevation contours indicate higher pressures, and lower-elevation contours indicate lower pressures.
the isobars. Winds of this nature, which blow at a constant speed parallel to curved isobars, are called gradient winds. Let us examine how the pressure gradient force and Coriolis force combine to produce gradient winds. Figure6-18a shows the gradient flow around a center of low pressure. As soon as the flow begins, the Coriolis force causes the air to be deflected. In the Northern Hemisphere, where the Coriolis force deflects the flow to the right, the resulting wind blows counterclockwise about a low. Conversely, around a high-pressure cell, the outward-directed pressure gradient force is opposed by the inward-directed Coriolis force, and a clockwise flow results. Figure 6-18b illustrates this idea. Because the Coriolis force deflects the winds to the left in the Southern Hemisphere, the flow is reversed—clockwise around low-pressure centers and counterclockwise around high-pressure centers. Recall that meteorologists call centers of low pressure cyclones and the flow around them cyclonic. There are several types and scales of cyclones. Large low pressure systems that are major weather makers in the United States are called midlatitude cyclones. Other examples include tropical cyclones (hurricanes), which are generally smaller than midlatitude cyclones, and tornadoes, which are tiny and extremely in-
tense cyclonic storms. Cyclonic flow has the same direction of rotation as Earth: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Centers of high pressure are called anticyclones and exhibit anticyclonic flow (opposite that of Earth’s rotation). Whenever isobars curve to form elongated regions of low and high pressure, these areas are called troughs and ridges, respectively (see Figure 6-17). The flow around a trough is cyclonic; conversely, the flow around a ridge is anticyclonic. Now let us consider the forces that produce the gradient flow associated with cyclonic and anticyclonic circulations. Wherever the flow is curved, a force has deflected the air (changed its direction), even when no change in speed results. This is a consequence of Newton’s first law of motion, which states that a moving object will continue to move in a straight line unless acted upon by an unbalanced force. You have undoubtedly experienced the effect of Newton’s law when the automobile in which you were riding made a sharp turn and your body tried to continue moving straight ahead (see Appendix E). Referring to Figure 6-18a, we see that in a low-pressure center, the inward-directed pressure gradient force is opposed by the outward-directed Coriolis force. But to keep
Chapter 6 Air Pressure and Winds
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Do Baseballs Really Fly Farther at Denver’s Coors Field? Adair, Sterling Professor Emeritus of Physics at Yale University, a 400-foot blast in Atlanta could carry perhaps 425 feet in Denver, although Adair admits that calculating the actual difference is tricky for reasons related to fluid dynamics. Recently a group of researchers at the University of Colorado at Denver tested the assumption that batted balls travel farther in the “thin air” at Coors Field than in ballparks near sea level. They concluded that the assumed elevation enhancement of fly ball distance has been greatly overestimated.
Since Denver’s Coors Field was built in 1995, it has become known as the “homerun hitter’s ballpark” (Figure 6-C). This notoriety is warranted because Coors Field led all Major League ballparks in both total home runs and home runs per at-bat during seven of its first eight seasons. In theory, a well-struck baseball should travel roughly 10 percent farther in Denver (elevation 5280 feet) than it would in a ballpark at sea level. This so-called elevation enhancement results from lower air density at mile-high Coors Field. According to Robert
Instead, they suggest that the hitter-friendly conditions should be attributed to the prevailing weather conditions of the nearby Front Range of the Rocky Mountains and the effects of low air density on the act of pitching a baseball. For example, wind can make or break a home run. According to Professor Adair, if there’s a 10-mile-an-hour breeze behind a batter, it will add an extra 30 feet to a 400-foot home run. Conversely, if the wind is blowing in toward home plate at 10 miles per hour, the flight of the ball will be reduced by about 30 feet. During the summer months winds most frequently blow from the south and southwest in the Denver area. Because of the orientation of Coors Field, these winds blow toward the outfield, thus aiding the hitters rather than the pitchers. The act of pitching a baseball is also greatly affected by air density. In particular, one factor that determines how much a curveball will curve is air density. At higher elevations, thinner air causes a ball to break less, which makes it easier for a batter to hit a pitch.
FIGURE 6-C Denver’s Coors Field, home of the Colorado Rockies, is nearly 1 mile above sea-level. It is known as the “homerun hitter’s ballpark.” (Photo by Russell Lansford/Icon SMI/Newscom)
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the path curved (parallel to the isobars), the inward pull of the pressure gradient force must be strong enough to balance the Coriolis force as well as to turn (accelerate) the air inward. The inward turning of the air is called centripetal acceleration. Stated another way, the pressure gradient force must exceed the Coriolis force to overcome the air’s tendency to continue moving in a straight line.* The opposite situation exists in anticyclonic flows, where the inward-directed Coriolis force must balance the pressure gradient force as well as provide the inward acceleration needed to turn the air. Notice in Figure 6-18 that the pressure gradient force and Coriolis force are not balanced (the arrow lengths are different), as they are in geostrophic flow. This imbalance provides a change in direction (centripetal acceleration) that generates curved flow.
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Concept Check 6.6
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1 Explain the formation of geostrophic wind. 2 Describe how geostrophic winds blow relative to isobars. 3 Describe the direction of cyclonic and anticyclonic flow in both the Northern and Southern Hemispheres.
Despite the importance of centripetal acceleration in establishing curved flow aloft, near the surface friction comes into play and greatly overshadows this much weaker force. Consequently, except with rapidly rotating storms such as tornadoes and hurricanes, the effect of centripetal acceleration is negligible and therefore is not considered in the discussion of surface circulation.
Surface Winds Friction as a factor affecting wind is important only within the first few kilometers of Earth’s surface. We know that friction acts to slow the movement of air (Figure 6-19). *The tendency of a particle to move in a straight line when rotated creates an imaginary outward force called centrifugal force.
Figure 6-19 A snow fence slows the wind, thereby decreasing the wind’s ability to transport snow. As a result, snow accumulates on the downwind side of the fence. (Photo by Scott T. Slattery/Shutterstock))
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Figure 6-20 Comparison between upper-level winds and surface winds, showing the effects of friction on airflow. Friction slows surface wind speed, which weakens the Coriolis force, causing the winds to cross the isobars.
By slowing air movement, friction also reduces the Coriolis force, which is proportional to wind speed. Because the pressure gradient force is not affected by wind speed, it wins the tug of war against the Coriolis force (Figure 620). The result is the movement of air at an angle across the isobars, toward the area of lower pressure. The roughness of the surface determines the angle at which the air will flow across the isobars and influences the speed at which it will move. Over relatively smooth surfaces, where friction is low, air moves at an angle of 10° to 20° to the isobars and at speeds roughly two-thirds of geostrophic flow (Figure 6-20b). Over rugged terrain, where friction is high, the angle can be as great as 45° from the isobars, with wind speeds reduced by as much as 50 percent (Figure 6-20c). We have learned that above the friction layer in the Northern Hemisphere, winds blow counterclockwise around
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a cyclone and clockwise around an anticyclone, with winds nearly parallel to the isobars. Combined with the effect of friction, we find that the airflow crosses the isobars at varying angles, depending on the terrain, but always from higher to lower pressure. In a cyclone, in which pressure decreases inward, friction causes a net flow toward its center (Figure 621). In an anticyclone, the opposite is true: The pressure decreases outward, and friction causes a net flow away from the center. Therefore, the resultant winds blow into and counterclockwise about a cyclone and outward and clockwise about an anticyclone (Figure 6-21). Of course, in the Southern Hemisphere the Coriolis force deflects the winds to the left and reverses the direction of flow. Regardless of hemisphere, however, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.
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Concept Check 6.7 1 Unlike winds aloft, which blow nearly parallel to the isobars, surface winds generally cross the isobars. Explain what causes this difference.
2 Prepare a diagram with isobars and wind arrows that shows the winds associated with surface cyclones and anticyclones in both the Northern and Southern Hemispheres.
How Winds Generate Vertical Air Motion ATMOSPHERE
Air Pressure and Winds ▸ Highs and Lows
So far we have discussed wind without regard to how airflow in one region might affect airflow elsewhere. As one researcher put it, a butterfly flapping its wings in South America can generate a tornado in the United States. Although this is an exaggeration, it suggests that airflow in one region might cause a change in weather at some later time at a different location.
Cold winds blowing from Greenland encountered moist air over the Greenland Sea, and their convergence generated parallel rows of clouds called cloud streets in the skies around Jan Mayen Island. The island acts as an obstacle that causes the wind to form spiraling eddies (called van Karman vortices) on the south (leeward) side of the island. (NASA) Question 1 Based on the orientation of the cloud streets, what direction is the wind blowing over the Greenland Sea? Question 2 Can you think of an analogy for the spiraling eddies that formed downwind of Jan Mayen Island? Question 3 Eddies are also generated by bicycles, cars, and airplanes. Participants in bicycle and car races use these eddies, which reduce wind resistance, by traveling directly behind another competitor. What term is used to describe this competitive advantage?
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Of particular importance is the question of how horizontal airflow (wind) relates to vertical flow. Although the rate at which air ascends or descends (except in violent storms) is slow compared to horizontal flow, it is very important as a weather maker. Rising air is associated with cloudy conditions and often precipitation, whereas subsidence produces adiabatic heating and clearing conditions. In this section we will examine how the movement of air can create pressure change and hence generate winds. In doing so, we will examine the interrelationship between horizontal and vertical flow and its effect on the weather.
Vertical Airflow Associated with Cyclones and Anticyclones Let us first consider the situation of a surface low-pressure system (cyclone) in which the air is spiraling inward (see Figure 6-21). The net inward transport of air causes a shrinking of the area it occupies, a process called horizontal convergence (Figure 6-22). Whenever air converges horizontally, it piles up—that is, it increases in mass to allow for the decreased area it occupies. This process generates a “denser” column. We seem to have encountered a paradox: Low-pressure centers cause a net accumulation of air, which increases their pressure. Consequently, a surface cyclone should quickly eradicate itself in a manner not unlike what happens to the vacuum in a coffee can when it is opened. You can see that for a surface low to exist for a substantial duration, compensation must occur aloft. For example, surface convergence could be maintained if divergence (spreading out) aloft occurred at a rate equal to the inflow below. Figure 6-22 illustrates the relationship between surface convergence (inflow) and the divergence aloft (outflow) needed to maintain a low-pressure center.
Convergence aloft
Divergence aloft may even exceed surface convergence, thereby accelerating vertical motion and intensifying surface inflow. Because rising air often results in cloud formation and precipitation, the passage of a low-pressure center is generally accompanied by “bad weather.” Like their cyclonic counterparts, anticyclones must also be maintained from above. Outflow near the surface is accompanied by convergence aloft and general subsidence of the air column (Figure 6-22). Because descending air is compressed and warmed, cloud formation and precipitation are less likely in an anticyclone. Thus, fair weather can usually be expected with the approach of a high-pressure system. For these reasons, it is common to see “stormy” at the low end of household barometers and “fair” at the high end (Figure 6-23). By noting the pressure trend—rising, falling, or steady—we have a good indication of forthcoming weather. Such a determination, called the pressure tendency, or barometric tendency, is useful in short-range weather prediction. The generalizations relating cyclones and anticyclones to weather conditions are stated nicely in this verse (where “glass” refers to the barometer): When the glass falls low, Prepare for a blow; When it rises high, Let all your kites fly.
In conclusion, it should be obvious why local television weather broadcasters emphasize the positions and projected paths of cyclones and anticyclones. The “villain” on these weather programs is always the lowpressure system, which produces “foul” weather in any season. Lows move in roughly a west-to-east direction across the United States and require from a few days to more than a week for the journey. Because their paths can be erratic, accurate prediction of their migration is difficult, yet it is es-
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Figure 6-22 Airflow associated with surface cyclones (L) and anticyclones (H). A low, or cyclone, has converging surface winds and rising air, resulting in cloudy conditions. A high, or anticyclone, has diverging surface winds and descending air, which leads to clear skies and fair weather.
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to land, for instance, the increased friction causes an abrupt drop in wind speed. This reduction of wind speed downstream results in a pileup of air upstream. Thus, converging winds and ascending air accompany flow from the ocean to the land. This effect contributes to the cloudy conditions over the humid coastal regions of Florida. Conversely, when air moves from the land to the ocean, general divergence and subsidence accompany the seaward flow of air because of lower friction and increasing wind speed over the water. The result is often subsidence and clearing conditions. Mountains also hinder the flow of air and cause divergence and convergence. As air passes over a mountain range, it is compressed vertically, which produces horizontal spreading (divergence) aloft. On reaching the leeward side of the mountain, the air experiences vertical expansion, which causes horizontal convergence. This effect greatly influences the weather in the United States east of the Rocky Mountains, as we shall examine later. As a result of the connections between surface conditions and those aloft, significant research has been aimed at understanding atmospheric circulation, especially in the midlatitudes. Once we examine global atmospheric circulation in the next chapter, we will again consider the relationships between horizontal airflow (wind) and vertical motions (rising and descending air currents).
Concept Check 6.8 1 For surface low pressure to exist for an extended period, what condition must exist aloft?
2 What general weather conditions are to be expected when the pressure tendency is rising? When the pressure tendency is falling?
3 Converging winds and ascending air are often associated (b)
Figure 6-23 These two photographs illustrate the basic weather generalizations associated with pressure centers. (a) A rainy day in London. Low pressure systems are frequently associated with cloudy conditions and precipitation. (Photo by Lourens Smak/Alamy) (b) By contrast, clear skies and “fair” weather may be expected when an area is under the influence of high pressure. (Photo by Prisma Bildagentur/Alamy)
sential for short-range forecasting. Meteorologists must also determine whether the flow aloft will intensify an embryo storm or act to suppress its development.
Factors That Promote Vertical Airflow Because of the close tie between vertical motion in the atmosphere and our daily weather, we will consider some other factors that contribute to surface convergence and surface divergence. Friction can cause both convergence and divergence. When air moves from the relatively smooth ocean surface
with the flow of air from the oceans onto land. Conversely, divergence and subsidence often accompany the flow of air from land to sea. What causes this convergence over land and divergence over the ocean?
Students Sometimes Ask . . . What causes “mountain sickness”? When visitors drive up to a mountain pass above 3000 meters (10,000 feet) and take a walk, they typically notice shortness of breath and possibly fatigue. These symptoms are caused by breathing air with roughly 30 percent less oxygen than at sea level. At these altitudes our bodies try to compensate for the oxygen deficiency of the air by breathing more deeply and increasing the heart rate, thereby pumping more blood to the body’s tissues. The additional blood is thought to cause brain tissues to swell, resulting in headaches, insomnia, and nausea—the main symptoms of acute mountain sickness. Mountain sickness is generally not life threatening and usually can be alleviated with a night’s rest at a lower altitude. Occasionally people become victims of high-altitude pulmonary edema. This life-threatening condition involves a buildup of fluid in the lungs and requires prompt medical attention.
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These satellite images show four different tropical cyclones (Hurricanes) that occurred on different dates in different parts of the world. (NASA)
Question 1 For each storm, examine the cloud pattern and determine whether the flow is clockwise or counterclockwise.
Wind Measurement Two basic wind measurements—direction and speed—are important to weather observers. Winds are always labeled by the direction from which they blow. A north wind blows from the north toward the south; an east wind blows from
Question 2 In which hemisphere is each storm located, Northern or Southern?
the east toward the west. One instrument commonly used to determine wind direction, the wind vane, is often seen on the tops of buildings (Figure 6-24a). Sometimes the wind direction is shown on a dial that is connected to the wind vane. The dial indicates the direction of the wind either by points of the compass—that is, N, NE, E, SE, and so on—or
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Figure 6-24 Wind measurement. (a) Wind vane (right) and cup anemometer (left). The wind vane shows wind direction, and the anemometer measures wind speed. (Photo by Belfort Instrument Company) (b) A wind sock is a device for determining wind direction and estimating wind speed. They are common sights at small airports and landing strips. (Photo by Lourens Smak/Almay Images)
by a scale of 0° to 360°. On the latter scale, 0° (or 360°) is north, 90° is east, 180° is south, and 270° is west. When the wind consistently blows more often from one direction than from any other, it is called a prevailing wind. You may be familiar with the prevailing westerlies that dominate the circulation in the midlatitudes. In the United States, for example, these winds consistently move the “weather” from west to east across the continent. Embed-
ded within this general eastward flow are cells of high and low pressure, with their characteristic clockwise and counterclockwise flow. As a result, the winds associated with the westerlies, as measured at the surface, often vary considerably from day to day and from place to place. A wind rose provides a method of representing prevailing winds by indicating the percentage of time the wind blows from various directions (Figure 6-25). The length of the lines on
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Figure 6-25 The percentage of directional windflow is charted on a wind rose and may represent daily, weekly, monthly, seasonal, or annual totals. (a) Wind frequency for the winter in the eastern United States. (b) Wind frequency for the winter in northern Australia. Note the reliability of the southeast trades in Australia as compared to the westerlies in the eastern United States. (Data from G. T. Trewartha)
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Box 6-3
Wind Energy: An Alternative with Potential
Air has mass, and when it moves (that is, when the wind blows), it contains the energy of that motion—kinetic energy. A portion of that energy can be converted into other forms—mechanical force or electricity—that we can use to perform work (Figure 6-D). Mechanical energy from wind is commonly used for pumping water in rural or remote places. The “farm windmill,” still a familiar sight in many rural areas, is an example. Mechanical energy converted from wind can also be used for other purposes, such as sawing logs, grinding grain, and propelling sailboats. By contrast, windpowered electric turbines generate electricity for homes, businesses, and for sale to utilities.
Today, modern wind turbines are being installed at breakneck speed. In fact, worldwide, in 2010 the installed wind power capacity exceeded 203,000 megawatts, an increase of 28 percent over 2009.* This is equivalent to the total electrical demand of Italy—2 percent of global electricity production. Worldwide, wind energy installations have doubled every three years since 2000. The United States is the world’s leading producer (22.3 percent), followed by China (16.3 percent), Germany (16.2 percent), and Spain (11.5 percent). Within the next decade, China *One megawatt is enough electricity to supply 250–300 average American households.
is expected to produce the most windgenerated electricity. Wind speed is a crucial element in determining whether a place is a suitable site for installing a wind-energy facility. Generally, a minimum average wind speed of 21 kilometers (13 miles) per hour is necessary for a large-scale wind-power plant to be profitable. A small difference in wind speed results in a large difference in energy production, and therefore a large difference in the cost of the electricity generated. For example, a turbine operating on a site with an average wind speed of 12 miles per hour would generate about 33 percent more electricity than one operating at 11 miles per hour. Also, there
FIGURE 6-D Farm windmills such as the one on the left remain familiar sights in some areas. Mechanical energy from wind is commonly used to pump water. (Photo by Mehmet Dilsiz/Shutterstock) The wind turbines on the right are operating near Palm Springs, California. Although California is the state where significant wind-power development began, it has been surpassed by Texas and Iowa. (Photo by John Mead/Science Photo Library/Photo Researchers, Inc.)
the wind rose indicates the percentage of time the wind blew from that direction. As seen in Figure 6-25b, the direction of airflow associated with the belt of trade winds is much more consistent than the westerlies. Knowledge of the wind patterns for a particular area can be useful. For example, during the construction of an airport, the runways are aligned with the prevailing wind
to assist in takeoffs and landings. Furthermore, prevailing winds greatly affect the weather and climate of a region. Mountain ranges that trend north–south, such as the Cascade Range of the Pacific Northwest, cause the ascent of the prevailing westerlies. Thus, the windward (west) slopes of these ranges are rainy, whereas the leeward (east) sides are dry.
Chapter 6 Air Pressure and Winds
is little energy to be harvested at low wind speeds: 6-mile-per-hour winds contain less than one-eighth the energy of 12-mile-perhour winds. The United States has tremendous wind energy resources (Figure 6-E). In 2010, 36 states had commercial facilities that produced electricity from wind power. Texas was number one in installed capacity, followed by Iowa, California, and Minnesota. Despite the fact that the modern U.S. wind industry began in California, 16 states have greater wind potential. As shown in Table 6-A, the top five states for wind energy potential are North Dakota, Texas, Kansas, South Dakota, and Montana. Although only a small fraction of U.S. electrical generation currently comes from wind energy, it has been estimated that wind energy potential equals more than twice the total electricity currently consumed.
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TABLE 6-A The Leading States for Wind Energy Potential Rank
State
Potential*
Rank
State
Potential*
1
North Dakota
1210
11
Colorado
481
2
Texas
1190
12
New Mexico
435
3
Kansas
1070
13
Idaho
73
4
South Dakota
1030
14
Michigan
65
5
Montana
1020
15
New York
62
6
Nebraska
868
16
Illinois
61
7
Wyoming
747
17
California
59
8
Oklahoma
725
18
Wisconsin
58
9
Minnesota
657
19
Maine
56
Iowa
551
20
Missouri
52
10
* The total amount of electricity that could potentially be generated each year, measured in billions of kilowatt hours. A typical American home would use several hundred kilowatt hours per month.
The U.S. Department of Energy has announced a goal of obtaining 5 percent of U.S. electricity from wind by the year 2020— a goal that seems consistent with the current
Wind energy potential Excellent Good Moderate
Wind speed is often measured with a cup anemometer, which has a dial much like the speedometer of an automobile (Figure 6-24a). Sometimes an aerovane is used instead of a wind vane and cup anemometer. As illustrated in Figure 6-26, this instrument resembles a wind vane with a propeller at one end. The fin keeps the propeller facing into the wind, allowing the blades to rotate at a rate proportional
growth rate of wind energy nationwide. Thus wind-generated electricity appears to be shifting from being an “alternative” to a “mainstream” energy source.
FIGURE 6-E The wind energy potential for the United States. Large wind systems require average wind speeds of about 6 meters per second (13 miles per hour). In the key, “moderate” refers to regions that experience wind speeds of 6.4 to 6.9 meters per second (m/s), “good” means 7 to 7.4 m/s, and “excellent” means 7.5 m/s and higher.
to the wind speed. This instrument is commonly attached to a recorder that produces a continuous record of wind speed and direction. This information is valuable for determining locations where winds are steady and speeds are relatively high—potential sites for tapping wind energy (Box 6-3). At small airstrips wind socks are frequently used (Figure 6-24b). A wind sock consists of a cone-shaped bag
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that is open at both ends and free to change position with shifts in wind direction. The degree to which the sock is inflated indicates the strength of the wind. Recall that 70 percent of Earth’s surface is covered by water, making conventional methods of measuring wind speed difficult. Weather buoys and ships at sea provide limited coverage, but the availability of satellite-derived wind data has dramatically improved weather forecasts. For example, wind speed and direction can be established using satellite images to track cloud movements. This is currently being accomplished by comparing a sequence of satellite images separated by intervals of 5 to 30 minutes—an innovation especially useful in predicting the timing and location of a hurricane making landfall (Figure 6-27).
Concept Check 6.9 1 A southwest wind blows from the _____ (direction) toward the _____ (direction).
2 When the wind direction is 315°, from what compass direction is it blowing?
3 What is the name of the prevailing winds that affect the contiguous United States?
Figure 6-26 An aerovane. (Photo by imagebroker/Alamy)
Key 100–250-mb 251–350-mb 351–500-mb
Figure 6-27 Upper-level winds obtained from the GOES meteorological satellite. (Image by NOAA)
Chapter 6 Air Pressure and Winds
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Give It Some Thought 1. The accompanying satellite images show the cloud
patterns associated with two low pressure systems. a. Determine the airflow around each of these pressure cells (clockwise or counterclockwise). b. Which one of these two areas of low pressure is located in the Northern Hemisphere? c. Can you identify which one of these storm systems is a tropical cyclone (hurricane)? (Hint: Compare these images to Figures 11-1 and 11-4.)
(a)
(NASA)
(b)
(NASA)
2. Temperature variations create pressure differences, which in turn produce winds. On a small scale, the sea breeze is a good illustration of this principle. Prepare and label a sketch that illustrates the formation of a sea breeze. 3. Why is the Coriolis force considered an apparent force and not a “real” force? 4. Given the following descriptions, identify the direction (for example, east, west, northwest) in which the Coriolis force is acting on the moving object. a. A commercial jet flying from New York to Chicago b. A baseball thrown from south to north in South Dakota c. A blimp floating from St. Louis northeast toward Detroit d. A boomerang thrown from west to east in Australia e. A football thrown along the equator 5. Geostrophic winds are maintained by a balance between the pressure gradient force and the Coriolis force. Explain why gradient winds cannot achieve a similar balance between these two forces. 6. If a weather system with strong winds approached Lake Michigan from the west, how might the speed of the winds change as the system traversed the lake? Explain your answer. 7. The accompanying map is a simplified surface weather map for April 2, 2011, on which the centers of three pressure cells are numbered. a. Identify which of the pressure cells are anticyclones (highs) and which are midlatitude cyclones (lows).
b. Which pressure system has the steepest pressure gradient and hence exhibits the strongest winds? c. Refer to Figure 6-3 to determine whether pressure system 3 should be considered strong or weak.
8. If you live in the Northern Hemisphere and are directly west of the center of a midlatitude cyclone, what is the probable wind direction? 9. Sketch a cross section of a low-pressure cell that shows surface flow (inward or outward), vertical flow, and flow aloft. 10. Given the following time line of barometric pressure readings, interpret the likely weather conditions, with regard to cloudiness and precipitation, for each of the following days: Day 1: Pressure steady at 1025 millibars Day 2: Pressure at 1010 millibars and falling Day 3: Pressure reaches four-day minimum of 992 millibars Day 4: Pressure at 1008 millibars and rising 11. If you wanted to erect wind turbines to generate electricity, would you search for a location that typically experiences a strong pressure gradient or a weak pressure gradient? Explain. 12. When designing an airport, it is important to have the runways positioned so that planes take off into the wind. Refer to the accompanying wind rose and discuss the orientation of the runway and the direction the planes would travel when they take off. 13. You and a friend are watching TV on a rainy day
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when the weather reporter states, “The barometric pressure is 28.8 inches and rising.” Hearing this, you say, “It looks like fair weather is on its way.” How would you respond if your friend asked the following questions?
a. “I thought air pressure had something to do with the weight of air. How does ‘inches’ relate to weight?” b. “Why do you think the weather is going to improve?”
AIR PRESSURE AND WINDS IN REVIEW r Wind, the horizontal movement of air, is the result of r r
r r
r
horizontal differences in air pressure. Air pressure is the force exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter, or 14.7 pounds per square inch, or 1013.25 millibars. Two instruments used to measure atmospheric pressure are the mercury barometer, where the height of a mercury column provides a measure of air pressure (standard atmospheric pressure at sea level equals 29.92 inches, or 760 millimeters), and the aneroid barometer, which uses a partially evacuated metal chamber that changes shape as air pressure changes. The pressure at any given altitude is equal to the weight of the air above that point. The rate at which pressure decreases with an increase in altitude is much greater near Earth’s surface than aloft. Two factors that largely determine the amount of air pressure exerted by an air mass are temperature and humidity. A cold, dry air mass will produce higher surface pressures than a warm, humid air mass. Wind is controlled by a combination of (1) the pressure gradient force, (2) the Coriolis force, and (3) friction. The pressure gradient force is the primary driving force of wind that results from pressure differences that are depicted by the spacing of isobars on a map. Closely spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate a weak pressure gradient and light winds. The Coriolis force produces a deviation in the path of wind due to Earth’s rotation (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Friction, which significantly influences airflow near Earth’s surface, is negligible above a height of a few kilometers.
r Above a height of a few kilometers, the Coriolis force is equal
r
r
r
r
to and opposite the pressure gradient force, which results in geostrophic winds. Geostrophic winds flow in a nearly straight path, parallel to the isobars, with velocities proportional to the pressure gradient force. Winds that blow at a constant speed parallel to curved isobars are termed gradient winds. In low-pressure systems, such as midlatitude cyclones, the circulation of air, referred to as cyclonic flow, is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Centers of high pressure, called anticyclones, exhibit anticyclonic flow, which is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Near the surface, friction plays a major role in determining the direction of airflow. The result is a movement of air at an angle across the isobars, toward the area of lower pressure. Therefore, the resultant winds blow into and counterclockwise about a Northern Hemisphere surface cyclone. In a Northern Hemisphere surface anticyclone, winds blow outward and clockwise. A surface low-pressure system with its associated horizontal convergence is maintained or intensified by divergence (spreading out) aloft. Surface convergence in a cyclone accompanied by divergence aloft causes a net upward movement of air. Therefore, the passage of a low-pressure center is often associated with stormy weather. By contrast, fair weather can usually be expected with the approach of a high-pressure system. Two basic wind measurements—direction and speed—are important to a weather observer.
VOCABULARY REVIEW aerovane (p. 183) aneroid barometer (p. 164) anticyclone (p. 169) anticyclonic flow (p. 174) atmospheric (air) pressure (p. 162) barograph (p. 164) barometric tendency (p. 178) Buys Ballot’s law (p. 173) convergence (p. 167) Coriolis force (p. 169)
cup anemometer (p. 183) cyclone (p. 169) cyclonic flow (p. 174) divergence (p. 167) geostrophic wind (p. 173) gradient wind (p. 173) isobar (p. 168) mercury barometer (p. 163) midlatitude cyclone (p. 169) millibar (p. 163)
newton (p. 163) pressure gradient force (p. 168) pressure tendency (p. 178) prevailing wind (p. 181) ridge (p. 174) trough (p. 174) U.S. standard atmosphere (p. 165) wind (p. 162) wind vane (p. 180)
Chapter 6 Air Pressure and Winds
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PROBLEMS 1. Figure 6-4, illustrates a simple mercury barometer. When a glass tube is completely evacuated of air and placed into the dish of mercury, the mercury rises to a height such that the force of the air pushing on the open dish matches the gravity pulling the mercury back down the tube. The density of mercury is 13,534 kilograms per cubic meter, which is very dense compared to water (1000 kilograms per cubic meter).
In the mercury barometer, the mercury will rise to 29.92 inches under standard sea-level pressure. How tall would the barometer need to be if you used water instead of mercury? 2. Average air pressure at sea level is about 14.7 pounds per square inch. Using this information, how much does the entire atmosphere weigh? (Hint: The radius of Earth is 3963 miles.)
Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Air Pressure and Winds
Circulation of the Atmosphere Winds are generated by pressure differences that arise because of unequal heating of Earth’s surface. Earth’s winds blow in an unending attempt to balance these surface temperature differences. Because the zone of maximum solar heating migrates with the seasons—moving northward during the Northern Hemisphere summer and southward as winter approaches—the wind patterns that make up the general circulation also migrate latitudinally. This chapter focuses on global circulation and local wind systems, and it concludes with a discussion of global precipitation patterns.
Wind turbines, Tehachapi, California. (Photo T.J. Florian/Rainbow/Robert Harding)
After completing this chapter, you should be able to:
t Distinguish between macroscale, mesoscale, and microscale winds and give an example of each.
t List four types of local winds and describe their formation.
t Describe or sketch the three-cell model of global circulation.
t Label and describe the basic characteristics of Earth’s idealized zonal pressure belts on an appropriate map.
t Describe the seasonal change in Earth’s global circulation that produces the Asian monsoon.
t Explain why the airflow aloft in the middle latitudes has a strong west-to-east component.
t Explain the origin of the polar jet stream and its relationship to midlatitude cyclonic storms.
t Sketch and label the major ocean currents on a world map.
t Describe the Southern Oscillation and its relationship to El Niño and La Niña.
t List the climate impacts to North America that are associated with El Niño and La Niña.
t Describe the relationships between global pressure systems and the global distribution of precipitation. 189
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Scales of Atmospheric Motion Earth’s highly integrated wind system can be thought of as a series of deep rivers of air that encircle the planet. Embedded in the main currents are vortices of various sizes, including hurricanes, tornadoes, and midlatitude cyclones (Figure 7-1). Like eddies in a stream, these rotating wind systems develop and die out with somewhat predictable regularity. In general, the smallest eddies, such as dust devils, last only a few minutes, whereas larger and more complex systems, such as midlatitude cyclones and hurricanes, may survive for several days. Residents of the United States and Canada are familiar with the term westerlies, which describes winds that predominantly blow across the midlatitudes from west to east. However, over short time periods, the winds may blow from any direction. You may recall being in a storm when shifts in wind direction and speed came in rapid succession. With such variations, how can we describe our winds as westerly? The answer lies in our attempt to simplify descriptions of the atmospheric circulation by sorting out events according to size. On the scale of a weather map, for instance, where observing stations are spaced about 150 kilometers (nearly 100 miles) apart, small whirl-
Figure 7–1 Gale force winds. (Photo by Emma Lee/Life Files/Photolibrary)
winds that carry dust skyward are far too small to be identified. Instead, weather maps reveal larger-scale wind patterns, such as those associated with traveling cyclones and anticyclones. In addition, equal consideration is given to the time frame in which the wind systems occur. In general, large weather patterns persist longer than their smaller counterparts. For example, dust devils usually last a few minutes. By contrast, midlatitude cyclones typically take a few days to cross the United States and sometimes dominate the weather for a week or longer.
Small- and Large-Scale Circulation The wind systems shown in Figure 7-2 illustrate the three major categories of atmospheric circulation: microscale, mesoscale, and macroscale. Microscale Winds The smallest scale of air motion is referred to as microscale circulation. These small, often chaotic winds normally last for seconds or at most minutes. Examples include simple gusts, which hurl debris into the air (Figure 7-2a), downdrafts, and small, well-developed vortices such as dust devils (see Box 7-1).
Chapter 7 Circulation of the Atmosphere
191
L I F E S P A N (a)
(b)
(c)
Waves in the westerlies, Trade winds
Days to weeks
Midlatitude cyclones, Anticyclones, Hurricanes Local winds: Land/sea breeze, Santa Ana, etc.
Hours to days Minutes to hours
Tornadoes, Thunderstorms
Seconds to minutes
Wind gusts, Microbursts, Dust devils
Microscale
SCALE
Mesoscale
Synoptic scale Weather-map scale
Planetary scale
Macroscale 0
1 km
10 km
1000 km
5000 km
40,000 km
Figure 7–2 Three scales of atmospheric motion. (a) Gusts illustrate microscale winds. (b) Tornadoes exemplify mesoscale wind systems. (c) Satellite image of a hurricane, an example of macroscale circulation. (Photos by (a) E. J. Tarbuck, (b) Erin Nguyen/Photo Researchers Inc., (c) NASA/Science Photo Library/Photo Researchers, Inc.)
Mesoscale Winds Mesoscale winds generally last for several minutes and may exist for hours. These middle-size phenomena are usually less than 100 kilometers (62 miles) across. Further, some mesoscale winds—for example, thunderstorms and tornadoes—also have a strong vertical component (Figure 7-2b). It is important to remember that thunderstorms and tornadoes are often embedded within a larger cyclonic storm and thus move with it. Further, much of the vertical air movement within a midlatitude cyclone, or hurricane, is provided by thunderstorms, where updrafts in excess of 100 kilometers (62 miles) per hour have been measured. Land and sea breezes, as well as mountain and valley winds, also fall into this category and will be discussed in the next section, with other local winds. Macroscale Winds The largest wind patterns, called macroscale winds, are exemplified by the westerlies and trade winds that carried sailing vessels back and forth across
the Atlantic during the opening of the New World. These planetary-scale flow patterns extend around the entire globe and can remain essentially unchanged for weeks at a time. A somewhat smaller macroscale circulation is called synoptic scale—also referred to as weather-map scale. Synopticscale wind systems are about 1000 kilometers in diameter and are easily identified on weather maps. Two wellknown synoptic-scale systems are the individual traveling midlatitude cyclones and anticyclones that appear on weather maps as areas of low and high pressure, respectively. These weather producers are found in the middle latitudes and are sometimes hundreds to thousands of kilometers across. Somewhat smaller macroscale systems are tropical storms and hurricanes that develop in late summer and early fall over warm tropical oceans (Figure 7-2c). Airflow in these systems is inward and upward, as in the larger midlatitude cyclones. However, the rate of horizontal flow associated
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Box 7–1 Dust Devils near Earth’s surface becomes unstable. the afternoon, when surface temperatures A common phenomenon in arid regions of In this situation, warm surface air begins are highest. the world are the whirling vortices called to rise, causing air near the ground to be Recall that when the air near the surface dust devils (Figure 7–A). Although they drawn into the developing whirlwind. The is considerably warmer than the air a few resemble tornadoes, dust devils are generrotating winds that are associated with dust dozen meters overhead, the layer of air ally much smaller and less intense than their devils are produced by the destructive same phenomenon that causes cousins. Most dust devils are ice skaters to spin faster as only a few meters in diameter they pull their arms closer to and reach heights no greater their body. As the inwardly spithan about 100 meters (300 raling air rises, it carries sand, feet). Further, these whirldust, and other loose debris winds are usually short-lived dozens of meters into the air— phenomena that die out within making a dust devil visible. minutes. Most dust devils are small Unlike tornadoes, which and short-lived; consequently, are associated with convecthey are not generally destructive clouds, dust devils form tive. Occasionally, however, on days when clear skies these whirlwinds grow to be dominate. Further, these 100 meters or more in diamwhirlwinds develop from the eter and over a kilometer high. ground upward, which is opWith wind speeds that may posite of tornado formation. reach 100 kilometers (60 miles) Because surface heating is FIGURE 7–A Dust devil. Although these whirling vortices resemble tornadoes, they critical to their formation, dust have a different origin and are much smaller and less intense. (Courtesy of St. Meyers/ per hour, large dust devils can do considerable damage. devils occur most frequently in Okapia/Photo Researchers, Inc.)
with hurricanes is usually more rapid than that of their more poleward cousins.
Structure of Wind Patterns Although it is common practice to divide atmospheric motions according to size, remember that global winds are a composite of motion on all scales—much like a meandering river that contains large eddies composed of smaller eddies containing still smaller eddies. As an example, we will examine winds associated with hurricanes that form over the North Atlantic. When we view one of these tropical cyclones on a satellite image, the storm appears as a large whirling cloud migrating slowly across the ocean (Figure 7-2c). From this perspective, which is at the weather-map (synoptic) scale, the general counterclockwise rotation of the storm is easily seen. In addition to their rotating motions, hurricanes often move from east to west or northwest. (Once hurricanes move into the belt of the westerlies, they tend to change course and move in a northeasterly direction.) This motion demonstrates that these large eddies are embedded in a still larger flow (planetary scale) that is moving westward across the tropical portion of the North Atlantic. When we examine a hurricane more closely by flying an airplane through it, some of the small-scale aspects of the storm become noticeable. As the plane approaches
the outer edge of the system, it is evident that the large rotating cloud that we see in the satellite image consists of many individual cumulonimbus towers (thunderstorms). Each of these mesoscale phenomena lasts for a few hours and must be continually replaced by new ones for the hurricane to persist. During the flight, we also realize that the individual thunderstorms are made up of even smallerscale turbulences. The small thermals of rising and descending air in these clouds make for a rough flight. In summary, a typical hurricane exhibits several scales of motion, including many mesoscale thunderstorms, which consist of numerous microscale turbulences. Furthermore, the counterclockwise circulation of the hurricane (weather-map scale) is embedded in the global winds (planetary scale) that flow from east to west in the tropical North Atlantic.
Concept Check 7.1 1 List the three major categories of atmospheric circulation and give at least one example of each.
2 Describe how the size of a wind system is related to its duration (life span).
3 What scale of atmospheric circulation includes midlatitude cyclones, anticyclones, and tropical cyclones (hurricanes)?
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Local Winds Local winds are examples of mesoscale winds (time frame of minutes to hours and size of 1 to 100 kilometers). Remember that most winds have the same cause: pressure differences that arise because of temperature differences caused by unequal heating of Earth’s surface. Most local winds are linked to temperature and pressure differences that result from variations in topography or in local surface conditions.
Students Sometimes Ask… What is the highest wind speed ever recorded in the United States? The highest wind speed recorded at a surface station is 372 kilometers (231 miles) per hour, measured April 12, 1934, at Mount Washington, New Hampshire. Located at an elevation of 1879 meters (6262 feet), the observatory atop Mount Washington has an average wind speed of 56 kilometers (35 miles) per hour. Faster wind speeds have undoubtedly occurred on mountain peaks, but no instruments were in place to record them.
Recall that winds are named for the direction from which they blow. This holds true for local winds. Thus, a sea breeze originates over water and blows toward the land, whereas a valley breeze blows upslope, away from its source.
Land and Sea Breezes The daily temperature differences that develop between the sea and adjacent land areas, and the resulting pressure pattern that creates a sea breeze, were discussed in Chapter 6 (see Figure 6–11). Recall that land is heated more intensely during daylight hours than is an adjacent body of water. As a result, the air above the land heats and expands, creating an area of high pressure aloft. This, in turn, causes the air above the land to move seaward. This mass transfer of air aloft creates a
surface low over the land. A sea breeze develops as cooler air over the the water moves landward toward the area of lower pressure (Figure 7-3a). At night, the reverse may take place; the land cools more rapidly than the sea, and a land breeze develops, with airflow off the land (Figure 7-3b). A sea breeze has a significant moderating influence in coastal areas. Shortly after a sea breeze begins, air temperatures over the land may drop by as much as 5° to 10°C. However, the cooling effect of these breezes is generally noticeable for only 100 kilometers (60 miles) inland in the tropics and often less than half that distance in the middle latitudes. These cool sea breezes generally begin shortly before noon and reach their greatest intensity—about 10 to 20 kilometers per hour—in the midafternoon. Smaller-scale sea breezes can also develop along the shores of large lakes. Cities near the Great Lakes, such as Chicago, benefit from the “lake effect” during the summer, when residents typically enjoy cooler temperatures near the lake compared to warmer inland areas. In many places sea breezes also affect the amount of cloud cover and rainfall. The peninsula of Florida, for example, experiences increased summer precipitation partly because of convergence associated with sea breezes from both the Atlantic and Gulf coasts. The intensity and extent of land and sea breezes vary by location and season. Tropical areas where intense solar heating is continuous throughout the year experience more frequent and stronger sea breezes than midlatitude locations. The most intense sea breezes develop along tropical coastlines adjacent to cool ocean currents. In the middle latitudes, sea breezes are best developed during the warmest months, but land breezes are often missing because at night the land does not always cool below the temperature of the ocean surface.
Mountain and Valley Breezes A daily wind similar to land and sea breezes occurs in mountainous regions. During the day, air along mountain slopes is heated more intensely than air at the same elevation over
Pressure (mb) 988
Pressure (mb)
L
H
988
L
H
992
992
996
996
1000
1000
1004
1004 1008
1008 1016
H Cool Water
(a) Sea breeze
Hot
L Land
Cool
L
Warm
Water (b) Land breeze
Figure 7–3 Illustration of a sea breeze and a land breeze. (a) During the daylight hours, cooler and denser air over the water moves onto the land, generating a sea breeze. (b) At night the land cools more rapidly than the sea, generating an offshore flow called a land breeze.
H
1016
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The Atmosphere: An Introduction to Meteorology
During the day air above mountain slopes is heated and rises creating a valley breeze
Warm air rising
At night air above the mountain slopes cools rapidly and drains into the warm valley below creating a mountain breeze
Cool air sinking
Warm air rising ez e
y lle Va
br ee ze
y lle Va
e br
Cold air sinking M ou nt ain bre eze
Cool air
Cold air sinking e
194
ee br n i ta un Mo
z
Warm air rising
(a) Valley breeze
(b) Mountain breeze
Figure 7–4 Valley and mountain breezes. (a) Heating during the daylight hours warms the air along the mountain slopes. This warm air rises, generating a valley breeze. (b) After sunset, cooling of the air near the mountain slopes can result in cool air draining into the valley, producing a mountain breeze.
the valley floor (Figure 7-4a). This warmer air glides up the mountain slope and generates a valley breeze. Valley breezes can often be identified by the cumulus clouds that develop over adjacent mountain peaks and may account for late afternoon thundershowers that occur on warm summer days (Figure 7-5). After sunset the pattern is reversed. Rapid heat loss along the mountain slopes cools the air, which drains into the valley and causes a mountain breeze (Figure 7-4b). Similar cool air drainage can occur in hilly regions that have modest slopes. The result is that the coldest pockets of air are usually found in the lowest spots. Like many other winds, mountain and valley breezes have seasonal preferences. Valley breezes are most common during warm seasons, when solar heating is most intense,
Figure 7–5 The occurrence of a daytime upslope (valley) breeze is identified by cloud development on mountain peaks, sometimes becoming a midafternoon thunderstorm. (Photo by Herber Koeppel/Alamy)
whereas mountain breezes tend to be more frequent during cold seasons.
Chinook (Foehn) Winds Warm, dry winds called chinooks sometimes move down the slopes of mountains in the United States. Similar winds in the Alps are called foehns. Such winds are usually created when a strong pressure gradient develops in a mountainous region. As the air descends the leeward slopes of the mountains, it is heated adiabatically (by compression). Because condensation may have occurred as the air ascended the windward side, releasing latent heat, the air descending the leeward side will be warmer and drier than at a similar elevation on the windward side. Chinooks commonly flow down the east slopes of the Colorado Rockies in the winter and spring, when the affected area may be experiencing subfreezing temperatures. Thus, these dry, warm winds often bring drastic change. Within minutes of the arrival of a chinook, the temperature may climb 20°C (36°F). When the ground has a snow cover, these winds melt it rapidly, which explains why the Native American word chinook, meaning “snow-eater,” has been applied to these winds. Chinook winds have been known to melt more than a foot of snow in a single day. A chinook that moved through Granville, North Dakota, on February 21, 1918, caused the temperature to rise from –33°F to 50°F—an increase of 83°F! Chinooks are sometimes viewed as beneficial to ranchers east of the Rockies because they keep the grasslands clear of snow during much of the winter. However, this benefit is offset by the loss of moisture that the snow would bequeath to the land if it remained until the spring melt. Another chinook-like wind that occurs in the United States is the Santa Ana. Found in southern California, the hot, dry Santa Ana winds greatly increase the threat of fire
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in this already dry area (see Severe and Hazardous Weather: Santa Ana Winds and Wildfires on page 196).
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from average worldwide pressure distribution. We will then modify this idealized model by adding more recently discovered aspects of the atmosphere’s complex motions.
Katabatic (Fall) Winds In the winter, areas adjacent to highlands may experience a local wind called a katabatic or fall wind. These winds originate when cold dense air, situated over a highland area such as the ice sheets of Greenland and Antarctica, begins to move. Under the influence of gravity, the cold air cascades over the rim of a highland like a waterfall. Although the air is heated adiabatically, the initial temperatures are so low that the wind arrives in the lowlands still colder and more dense than the air it displaces. As this frigid air descends, it occasionally is channeled into narrow valleys, where it acquires velocities capable of great destruction. A few of the better-known katabatic winds have local names. Most famous is the mistral, which blows from the French Alps toward the Mediterranean Sea. Another is the bora, which originates in the mountains of the Balkan Peninsula and blows to the Adriatic Sea.
Country Breezes One mesoscale wind, called a country breeze, is associated with large urban areas. As the name implies, this circulation pattern is characterized by a light wind blowing into the city from the surrounding countryside. In cities, the massive buildings composed of rocklike materials tend to retain the heat accumulated during the day more than the open landscape of outlying areas (see Box 3–3 on the urban heat island). The result is that the warm, less dense air over cities rises, which in turn initiates the country-to-city flow. A country breeze is most likely to develop on a relatively clear, calm night. One of the unfortunate consequences of the country breeze is that pollutants emitted near the urban perimeter tend to drift in and concentrate near the city’s center.
Students Sometimes Ask… What is a haboob? A haboob is a type of local wind that occurs in arid regions. The name was originally applied to strong dust storms in the African Sudan, where one city experiences an average of 24 haboobs per year. (The name comes from the Arabic word habb, meaning “wind.”) Haboobs generally occur when downdrafts from large thunderstorms reach the surface and swiftly spread out across the desert. Tons of silt, sand, and dust are lifted, forming a whirling wall of debris hundreds of meters high. These dense, dark “clouds” can completely engulf desert towns and deposit enormous quantities of sediment. The deserts of the southwestern United States occasionally experience dust storms produced in this manner.
Single-Cell Circulation Model One of the first contributions to the classical model of global circulation came from George Hadley in 1735. Well aware that solar energy drives the winds, Hadley proposed that the large temperature contrast between the poles and the equator creates a large convection cell in both the Northern and Southern Hemispheres (Figure 7-6). In Hadley’s model, warm equatorial air rises until it reaches the tropopause, where it spreads toward the poles. Eventually, this upper-level flow reaches the poles, where cooling causes it to sink and spread out at the surface as equatorward-moving winds. As this cold polar air approaches the equator, it is reheated and rises again. Thus, the
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Figure 7–6 Global circulation on a nonrotating Earth. A simple convection system is produced by unequal heating of the atmosphere on a nonrotating Earth.
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Santa Ana Winds and Wildfires
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Although Santa Ana winds occur every anta Ana winds are chinook-like year, they were particularly hazardous in the winds that characteristically sweep fall of 2003, and to a lesser extent in 2007, through southern California and northwhen hundreds of thousands of acres were western Mexico in the fall and winter. scorched. In late October 2003, Santa Anas These hot, dry winds are infamous for fanbegan blowing toward the coast of southern ning regional wildfires. Santa Ana winds are California, with speeds that driven by strong high-presThese hot, dry winds sometimes exceeded 100 sure systems with subsiding air that tend to develop in are infamous for fanning kilometers (60 miles) per hour. Much of this area is covered the fall over the Great Basin. regional wildfires. by brush known as chaparral The clockwise flow from the and related shrubs. It didn’t take much—a anticyclone directs desert air from Arizona careless camper or motorist, a lightning strike, and Nevada westward toward the Pacific or an arsonist—to ignite fires. Soon a number (Figure 7–B inset). The wind gains speed as it of small fires occurred in portions of Los Angeis funneled through the canyons of the Coast les, San Bernardino, Riverside, and San DiRanges, in particular the Santa Ana Canyon, ego counties (Figure 7–B). Several developed from which the winds derive their name. quickly into wildfires that moved almost as fast Adiabatic heating of this already warm, dry air as it descends mountain slopes further accentuates the already parched conditions. Vegetation, seared by the summer heat, is dried even further by these hot, dry winds.
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FIGURE 7–B Ten large wildfires rage across southern California in this image taken on October 27, 2003, by NASA’s Aqua satellite. Inset shows an idealized high-pressure area composed of cool dry air that drives Santa Ana winds. Adiabatic heating causes the air temperature to increase and the relative humidity to decrease. (Photo courtesy of NASA)
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as the ferocious Santa Ana winds sweeping through the canyons. Within a few days more than 13,000 firefighters were on firelines that extended from north of Los Angeles to the Mexican border. Nearly two months later, when all the fires were officially extinguished, more than 742,000 acres had been scorched, more than 3000 homes destroyed, and 26 people killed (Figure 7–C). The Federal Emergency Management Agency put the dollar losses at over $2.5 billion. The 2003 southern California wildfires became the worst fire disaster in the state’s history. Strong Santa Ana winds, coupled with dry summers, have produced wildfires in southern California for millennia. These fires are nature’s way of burning out chaparral thickets and sage scrub to prepare the land for new growth. When people began building homes and crowding into the fire-prone area between Santa Barbara and San Diego, the otherwise natural problems were compounded. Landscaped properties consisting of highly flammable eucalyptus and pine trees have further increased the hazard risk. In addition, fire prevention efforts have resulted in the accumulation of large quantities of flammable material over time, ultimately producing fewer but larger and more destructive fires. Clearly, wildfires will remain a major threat in these areas into the foreseeable future.
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FIGURE 7–C Flames from a wildfire fanned by Santa Ana winds move toward a home south of Valley Center, California, on October 27, 2003. (Photo by Denis Poroy/Associated Press)
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another way, the Coriolis force causes a general pileup of air (convergence) aloft. As a result, general subsidence occurs in the zones between 20° and 35° latitude. The subsiding air between 20° and 35° latitude is relatively dry because it has released its moisture near the equator. In addition, the effect of adiabatic heating during descent further reduces the relative humidity of the air. ConseThree-Cell Circulation Model quently, this zone of subsidence is the site of the world’s subtropical deserts. The Sahara Desert of North Africa and the In the 1920s a three-cell circulation model was proposed. Great Australian Desert are examples of subtropical deserts Although this model has been modified to fit upper-air oblocated in these regions of subsiding air. Further, because surservations, it remains a useful tool for examining global cirface winds are sometimes weak in the zones between 20° and culation. Figure 7-7 illustrates the idealized three-cell model 35° latitude, this belt was named the horse latitudes (Figure and the surface winds that result. 7-7). The name stems from the fact that Spanish sailing ships In the zones between the equator and roughly 30° laticrossing the Atlantic were sometimes becalmed and stalled tude north and south, the circulation closely resembles the for long periods of time in these waters. If food and water model used by Hadley and is known as the Hadley cell. supplies for the horses on board became depleted, the SpanNear the equator, the warm rising air that releases latent ish sailors were forced to throw the horses overboard. heat during the formation of cumulus towers is believed From the center of the horse latitudes, the surface flow to provide the energy that drives the Hadley cells. As the splits into two branches—one flowing poleward and one flow aloft moves poleward, the air begins to subside in a flowing toward the equator. The equatorward flow is dezone between 20° and 35° latitude. Two factors contribute flected by the Coriolis force to form the reliable trade winds, to this general subsidence: (1) As upper-level flow moves so called because they enabled early sailing ships to move away from the stormy equatorial region, radiation cooling goods between Europe and North America. In the Northern becomes the dominant process. As a result, the air cools, beHemisphere, the trades blow from the northeast, while in comes more dense, and sinks. (2) In addition, the Coriolis the Southern Hemisphere, the trades are from the southeast. force becomes stronger with increasing distance from the The trade winds from both hemispheres meet near the equaequator, causing the poleward-moving upper air to be detor, in a region that has a weak pressure gradient. This zone flected into a nearly west-to-east flow by the time it reaches is called the doldrums. Here light winds and humid condi30° latitude. This restricts the poleward flow of air. Stated tions provide the monotonous weather that is the basis for the expression “the doldrums.” Polar cell In the three-cell model, the circulation between 30° and Polar high Subpolar low 60° latitude (north and south), called the Ferrel cell, was Polar cell proposed by William Ferrel to account for the westerly surFerrel Polar easterlies face winds in the middle latitudes (see Figure 7-7). These cell 60° prevailing westerlies were known to Benjamin FrankFerrel lin, perhaps the first American weather forecaster, who cell noted that storms migrated from west to east across the Polar front colonies. Franklin also observed that the westerlies Hadley 30° were much more sporadic and, therefore less cell Westerlies reliable than the trade winds for sail power. S ub We now know that it is the migration of tropic al high (Horse titudes) la cyclones and anticyclones across the midHadley cell latitudes that disrupts the general westerly NE trade flow at the surface. The Ferrel cell, howwinds 0° ever, is not a good model for the flow aloft because it predicts a flow from east to west, just opposite of what is observed. In fact, the Equato Hadley cell rial low (Doldrums) westerlies become stronger with an increase in Hadley altitude and reach their maximum speeds 10-12 cell kilometers above Earth’s surface in zones called SE trade jet streams. Because of the importance of the midwinds latitude circulation in producing our daily weather, we Ferrel will consider the westerlies in more detail later. Ferrel cell cell The circulation in a polar cell is driven by subsidence near the poles that produces a surface flow that moves equatorward; this is called the polar easterlies in both Figure 7–7 Idealized global circulation for the three-cell circulation model on a rotating Earth. hemispheres. As these cold polar winds move equatorward, circulation proposed by Hadley has upper-level air flowing poleward and surface air moving equatorward. Although correct in principle, Hadley’s model does not take into account Earth’s rotation. (Hadley’s model would better approximate the circulation of a nonrotating planet.)
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they eventually encounter the warmer westerly flow of the midlatitudes. The region where the flow of cold air clashes with warm air has been named the polar front. The significance of this region will be considered later.
Concept Check 7.3 1 Briefly describe the idealized global circulation proposed by George Hadley. What are the shortcomings of the Hadley model?
2 Which two factors cause air to subside between 20° and 35° latitude?
3 Referring to the idealized three-cell model of atmospheric circulation, most of the United States is situated in which belt of prevailing winds?
4 What wind belts are found between the equator and 30° latitude?
Pressure Zones Drive the Wind Earth’s global wind patterns are derived from a distinct distribution of surface air pressure. To simplify this discussion, we will first examine the idealized pressure distribution that would be expected if Earth’s surface were uniform—that is, composed entirely of water or smooth land.
Idealized Zonal Pressure Belts If Earth had a uniform surface, each hemisphere would have two east-west oriented belts of high pressure and two of low pressure (Figure 7-8a). Near the equator, the warm rising branch of the Hadley cells is associated with the low-pressure zone known as the equatorial low. This region of ascending moist, hot air is marked by abundant precipitation. Because this region of low pressure is where the trade winds
Sally Benson: Climate and Energy Scientist Sally Benson has spent her career researching solutions to some of the most pressing environmental problems of our time. In the mid1970s, as a young scientist with Lawrence Berkeley National Laboratory, she tackled the first oil shortage by investigating ways to harness the power of geothermal energy.
“I get to do important work . . . and have fun while I do it.” Ten years later, she was elbow-deep in the mud of Kesterson Reservoir in California’s Central Valley. Irrigation runoff had caused selenium from local soil to accumulate in the water, causing local birds to hatch chicks with horrifying deformities. Benson’s studies made her realize microbes could be environmentally friendly tools to clean up other sites with toxic metal contamination. “The experience was so positive because I was doing this cutting-edge science at the same time regulators had to make a decision about how to clean up the site. I got an idea of the impact that science can make, and how research could get results,” Benson says. By the mid-1990s, Benson was directing all Earth science research at the laboratory. “I talked with many people, read a lot, and came to the conclusion that climate change was the most significant issue facing the world.” Benson organized research programs that developed regional models of climate change to help residents plan for droughts and temperature shifts, and studied the carbon cycle of the oceans, among other projects.
In 2007, Benson was appointed director of the Global Climate and Energy Project at Stanford University, which seeks to develop energy sources that release fewer greenhouse gases. “Energy efficiency in lighting, heating and cooling systems, and autos makes all the sense in the world. But at the end of the day, we need to do a lot more than that. If our current understanding is correct, we need to cut overall emissions by 80 percent of today’s levels,” Benson states. Benson sees many promising ways to achieve that goal. One is renewable energy. “Today’s biofuels don’t provide much advantage in terms of carbon dioxide emissions. But alternatives such as cellulosic ethanol (producing ethanol with plant fibers), where there are low emissions in the process of growing and making them, are incredibly important,” Benson opines. Another is to continue using some fossil fuels to run power plants and ships, but to capture the greenhouse gases they emit. “More than half of the electricity worldwide is produced by burning coal, a plentiful resource in many places. We need to find a way to make it carbon neutral,” Benson says. One way to remove such emissions from the atmosphere altogether would be to inject them in aquifers deep within Earth. Benson herself has studied how to use technology developed by the oil and gas industry to select sites where the rock offers a good seal, inject the gas, and monitor the area for leaks. “I get to do important work solving critical problems, get to be outdoors, do experiments
at scale, and have fun while I do it. Interacting with the huge number of people impacted by these issues makes the Earth sciences very rewarding.” —Kathleen Wong
PROFESSOR SALLY BENSON is Executive Director of Stanford University’s Global Climate and Energy Project, a $225 million effort to develop energy resources that are not harmful to the environment, especially energy sources that release fewer greenhouse gases. Dr. Benson has a multidisciplinary background with degrees in geology, materials science, and minerals engineering, and with applied research in hydrology and reservoir engineering. (Courtesy Dr. Sally Benson)
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converge, it is also referred to as the intertropical convergence zone (ITCZ). In Figure 7-9, the ITCZ is visible as a band of clouds near the equator. In the belts about 20° to 35° on either side of the equator, where the westerlies and trade winds originate and go their separate ways, are the high-pressure zones known as the subtropical highs. In these zones a subsiding air column produces weather that is normally warm and dry. Another low-pressure region is situated at about 50° to 60° latitude, in a position corresponding to the polar front. Here the polar easterlies and the westerlies clash in the lowpressure convergence zone known as the subpolar low. As you will see later, this zone is responsible for much of the stormy weather in the middle latitudes, particularly in the winter. Finally, near Earth’s poles are the polar highs, from which the polar easterlies originate (see Figure 7-8a). The polar highs exhibit high surface pressure mainly because of surface cooling. Because air near the poles is cold and dense, it exerts higher-than-average pressure.
Semipermanent Pressure Systems: The Real World Up to this point, we have considered the global pressure systems as if they were continuous belts around Earth. How-
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Figure 7–9 The intertropical convergence zone (ITCZ) is seen as the band of clouds that extends east–west just north of the equator. (Courtesy of NOAA)
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ever, because Earth’s surface is not uniform, the only true zonal distribution of pressure exists along the subpolar low in the Southern Hemisphere, where the ocean is continuous. To a lesser extent, the equatorial low is also continuous. At other latitudes, particularly in the Northern Hemisphere, where there is a higher proportion of land compared to ocean, the zonal pattern is replaced by semipermanent cells of high and low pressure. The idealized pattern of pressure and winds for the “real” Earth is illustrated in Figure 7-8b. The pattern shown is always in a state of flux because of seasonal temperature
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changes, which serve to either strengthen or weaken these pressure cells. In addition, the position of these pressure systems moves either poleward or equatorward with the seasonal migration of the zone of maximum solar heating. As a consequence of these factors, Earth’s pressure patterns vary in strength and location during the course of the year. Views of average global pressure patterns and resulting winds for the months of January and July are shown in Figure 7-10. Notice on these maps that the observed pressure patterns are circular (or elongated) instead of zonal (east-west bands). The most prominent features on both
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maps are the subtropical highs. These systems are centered between 20° and 35° latitude over the subtropical oceans. When we compare Figures 7-10a (January) and 7-10b (July), we see that some pressure cells are year-round features—the subtropical highs, for example. Others, however, are seasonal. For example, the low-pressure cell over northern Mexico and the southwestern United States is a summer phenomenon and appears only on the July map. The main cause of these variations is the greater seasonal temperature fluctuations experienced over landmasses, especially in the middle and higher latitudes. January Pressure and Wind Patterns The Siberian high, a very strong high-pressure center positioned over the frozen landscape of northern Asia, is the most prominent feature on the January pressure map (Figure 7-10a). A weaker polar high is located over the chilled North American continent. These cold anticyclones consist of very dense air that accounts for the significant weight of these air columns. In fact, the highest sea-level pressure ever measured, 1084 millibars (32.01 inches of mercury), was recorded in December 1968 at Agata, Siberia. Subsidence within these air columns results in clear skies and divergent surface flow. As the Arctic highs strengthen over the continents, the subtropical anticyclones situated over the oceans become weaker. Further, the average position of the subtropical highs tends to be closer to the eastern shore of the oceans in January than in July. For example, notice in Figure 7-10a that the center of the subtropical high is located in the eastern part of the North Atlantic (sometimes called the Azores high). Also shown on the January map but absent in July are two intense semipermanent low-pressure centers. Named the Aleutian low and the Icelandic low, these cyclonic cells are situated over the North Pacific and North Atlantic, respectively. They are not stationary cells but rather composites of numerous cyclonic storms that traverse these regions. In other words, so many midlatitude cyclones occur during the winter that these regions are almost always experiencing low pressure, hence the term semipermanent. As a result, the areas affected by the Aleutian and Icelandic lows are frequently cloudy and receive abundant winter precipitation. Because a large number of cyclonic storms form over the North Pacific and travel eastward, the coastal areas of southern Alaska receive abundant precipitation. This fact is exemplified by the climate data for Sitka, Alaska, a coastal town that receives 215 centimeters (85 inches) of precipitation each year, more than five times that received in Churchill, Manitoba, Canada. Although both towns are situated at roughly the same latitude, Churchill is located in the continental interior, far removed from the influence of the cyclonic storms associated with the Aleutian low. July Pressure and Wind Patterns The pressure pattern over the Northern Hemisphere changes dramatically in summer (see Figure 7-10b). High surface temperatures over the continents generate lows that replace wintertime highs. These thermal lows consist of warm ascending air that induces
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inward-directed surface flow. The strongest of these lowpressure centers develops over southern Asia, while a weaker thermal low is found in the southwestern United States. Notice in Figure 7-10 that during the summer months, the subtropical highs in the Northern Hemisphere migrate westward and become stronger than during the winter months. These strong high-pressure centers dominate the summer circulation over the oceans and pump warm moist air onto the continents that lie to the west of these highs. This results in an increase in precipitation over parts of eastern North America and Southeast Asia. During the peak of the summer season, the subtropical high found in the North Atlantic is positioned near the island of Bermuda, hence the name Bermuda high. In the Northern Hemisphere winter, the Bermuda high is located near Africa and is called the Azores high (see Figure 7-10).
Concept Check 7.4 1 What is the intertropical convergence zone (ITCZ)? 2 If Earth had a uniform surface, east–west belts of high and low pressures would exist. Name these zones and the approximate latitude in which each would be found.
3 During what season is the Siberian high strongest? 4 During what season is the Bermuda high strongest?
Monsoons Large seasonal changes in Earth’s global circulation are called monsoons. Contrary to popular belief, monsoon does not mean “rainy season”; rather, it refers to a particular wind system that reverses its direction twice each year. In general, winter is associated with winds that blow predominantly off the continents, called the winter monsoon. In contrast, in summer, warm moisture-laden air blows from the sea toward the land. Thus, the summer monsoon is usually associated with abundant precipitation over affected land areas and is the source of the misconception.
The Asian Monsoon The best-known and best-developed monsoon circulation occurs in southern and southeastern Asia—affecting India and the surrounding areas as well as parts of China, Korea, and Japan. As with most other winds, the Asian monsoon is driven by pressure differences that are generated by unequal heating of Earth’s surface. As winter approaches, long nights and low sun angles result in the accumulation of frigid air over the vast landscape of northern Russia. This generates the cold Siberian high, which ultimately dominates Asia’s winter circulation. The subsiding dry air of the Siberian high produces surface flow that moves across southern Asia, producing predominantly offshore winds (Figure 7-11a). By the time this flow reaches India, it has warmed considerably but remains extremely dry. For example, Kolkata, India, receives less than
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by these dust plumes help to replenish nutrients in rainforest soils that are continually being washed out of the soils by heavy tropical rains. Question 1 Notice that the dust plume is following a curved path. Does the atmospheric circulation
carrying this dust plume exhibit a clockwise or counterclockwise rotation? Question 2 What global pressure system was responsible for transporting this dust from Africa to South America?
Chapter 7 Circulation of the Atmosphere
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2 percent of its annual precipitation in the cooler six months. The remainder comes in the warmer six months, with the vast majority falling from June through September. In contrast, summertime temperatures in the interior of southern Asia often exceed 40°C (104°F). This intense solar heating generates a low-pressure area over the region similar to that associated with a sea breeze, but on a much larger scale. This in turn generates outflow aloft that encourages inward flow at the surface. With the development of the low-pressure Cm Tucson, Arizona center over southeastern Asia, 8 moisture-laden air from the In7 dian and Pacific Oceans flows 6 landward, thereby generating a 5 pattern of precipitation typical 4 of the summer monsoon. 3 One of the world’s rainiest 2 regions is found on the slopes 1 0 of the Himalayas, where oroJ F M A M J J A S graphic lifting of moist air from the Indian Ocean produces copious precipitation. Cherrapunji, India, once recorded an annual rainfall of 25 meters (82.5 feet), most of which fell during the four months of the summer monsoon (Figure 711b). The Asian monsoon is complex and strongly influenced by the seasonal change in solar heating received by the vast Asian continent. However, another factor, related to the annual migration of the Sun’s vertical rays, also contributes to the monsoon circulation of southern Asia. As shown in Figure 7-11, the Asian monsoon is associated with a largerthan-average seasonal migration of the ITCZ. With the onset of summer, the ITCZ moves northward over the continent and is accompanied by peak rainfall. The opposite occurs in the Asian winter, as the ITCZ moves south of the equator. Nearly half the world’s population inhabits regions affected by the Asian monsoons. Many of these people depend on subsistence agriculture for their survival. The timely arrival of monsoon rains often means the difference between adequate nutrition and widespread malnutrition.
The North American Monsoon Other regions experience seasonal wind shifts like those associated with the Asian monsoon. For example, a seasonal wind shift influences a portion of North America. Sometimes called the North American monsoon, this circulation pattern produces a dry spring followed by a relatively rainy summer that impacts large areas of the southwestern United States and northwestern Mexico.* This is illustrated by precipitation patterns observed in Tucson, Arizona,
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which typically receives nearly 10 times more precipitation in August than in May. As shown in Figure 7-12, summer rains typically last into September before drier conditions are reestablished. Summer daytime temperatures in the American Southwest, particularly in the low deserts, can be extremely high. This intense surface heating generates a low-pressure center over Arizona. The resulting circulation pattern brings warm, moist air from the Gulf of California and to a lesser extent the Gulf of Mexico (Figure 7-13). The supply of atmospheric moisture from nearby marine sources, coupled with the convergence and upward flow of the thermal low, is responsible for generating the precipitation this region experiences during the hottest months. Although often associated with the state of Arizona, this monsoon is actually strongest in northwestern Mexico and is also quite pronounced in New Mexico.
Concept Check 7.5 1 Define monsoon. 2 Explain the cause of the Asian monsoon. Which season (summer or winter) is the rainy season?
3 What areas of North America experience a pronounced *This event is also called the Arizona monsoon and the Southwest monsoon because it has been extensively studied in this part of the United States.
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monsoon circulation?
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important discoveries was that airflow aloft in the middle latitudes has a strong west-to-east component, thus the name westerlies.
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Figure 7–13 High summer temperatures over the southwestern United States create a thermal low that draws moist air from the Gulf of California and the Gulf of Mexico. This summer monsoon produces an increase in precipitation, which often comes in the form of thunderstorms, over the southwestern United States and northwestern Mexico.
The Westerlies Prior to World War II, upper-air observations were scarce. Since then, aircraft and radiosondes have provided a great deal of data about the upper troposphere. Among the most
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Let us consider the reason for the predominance of westerly flow aloft. In the case of the westerlies, the temperature contrast between the poles and equator drives these winds. Figure 7-14 illustrates the pressure distribution with height over the cold polar region as compared to the much warmer tropics. Recall that because cold air is more dense (compact) than warm air, air pressure decreases more rapidly in a column of cold air than in a column of warm air. The pressure surfaces (planes) in Figure 7-14 represent a simplified view of the pressure distribution we would expect to observe from pole to equator. Over the equator, where temperatures are higher, air pressure decreases more gradually than over the cold polar regions. Consequently, at the same altitude above Earth’s surface, higher pressure exists over the tropics and lower pressure is the norm above the poles. Thus, the pressure gradient aloft is directed from the equator (area of higher pressure) toward the poles (area of lower pressure). Once the air from the tropics begins to advance poleward in response to this pressure-gradient force (red arrow in Figure 7-14), the Coriolis force causes a change in the direction of airflow. Recall that in the Northern Hemisphere, the Coriolis force causes winds to be deflected to the right. Eventually, a balance is reached between the polewarddirected pressure-gradient force and the equatorwarddirected Coriolis force to generate a wind with a strong west-to-east component (Figure 7-14). Recall that such winds are called geostrophic winds. Because the equator-topole temperature gradient shown in Figure 7-14 is typical over the globe, a westerly flow aloft should be expected, and it does prevail on most occasions. The pressure gradient also increases with altitude; as a result, so do wind speeds. This increase in wind speed occurs as we ascend into the tropopause but begins to decrease upward into the stratosphere. These zones of fast wind speeds are called jet streams and will be considered in the next section.
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Figure 7–14 Idealized pressure gradient that develops aloft because of density differences between cold polar air and warm tropical air. Notice that the poleward-directed pressure-gradient force is balanced by an equatorward-directed Coriolis force. The result is a prevailing flow from west to east called the westerlies.
Chapter 7 Circulation of the Atmosphere
(Figure 7-16a). These fast streams of air, once considered analogous to jets of water, were named jet streams. Jet streams occur near the top of the troposphere and have widths that vary from less than 100 kilometers (60 miles) to over 500 kilometers (300 miles). Wind speeds often exceed 100 kilometers (60 miles) per hour and occasionally approach 400 kilometers (240 miles) per hour. Although jet streams had been detected earlier, their existence was first dramatically illustrated during World War II. American bombers heading westward toward Japaneseoccupied islands sometimes encountered unusually strong headwinds. On abandoning their missions, the planes experienced strong westerly tailwinds on their return flights.
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Figure 7–15 Idealized airflow of the westerlies at the 500-millibar level. The five long-wavelength undulations, called Rossby waves, compose this flow. The jet stream is the fast core of this wavy flow.
Altitude 7–12 km Polar jet stream
Waves in the Westerlies Studies of upper-level wind charts show that the westerlies follow wavy paths that have long wavelengths. Much of our knowledge of these large-scale motions is attributed to C. G. Rossby, who first explained the nature of these waves. The longest wave patterns (called Rossby waves), shown in Figure 7-15, usually consist of four to six meanders that encircle the globe. Although the air flows eastward along this wavy path, these long waves tend to remain stationary or drift slowly from west to east. Rossby waves can have a tremendous impact on our daily weather, especially when they meander widely from north to south. We will consider this role in the following section and again in Chapter 9.
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Concept Check 7.6 1 Why is the flow aloft in the midlatitudes predominantly westerly?
2 What name is given to the long-wavelength flow that is apparent on upper air charts? Equatorial low
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Figure 7–16 Jet streams. (a) Approximate positions of the polar and subtropical jet streams. (b) A cross-sectional view of the polar and subtropical jets in relation to the three-cell model of global circulation.
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Modern commercial aircraft pilots use the strong flow within jet streams to increase their speed when making eastward flights around the globe. On westward flights, of course, they avoid these fast currents of air when possible.
The Polar Jet Stream What is the origin of the distinctive energetic winds that exist within the somewhat slower general westerly flow? The key is that large temperature differences at the surface produce steep pressure gradients aloft and hence faster upper-air winds. In winter and early spring, it is not unusual to have a warm balmy day in southern Florida and near-freezing temperatures in Georgia, only a few hundred kilometers to the north. Such large wintertime temperature contrasts lead us to expect faster westerly flow at that time of year. In general, the fastest upper-air winds are located above regions of the globe having large temperature contrasts across very narrow zones. These large temperature contrasts occur along linear zones called fronts. The most prevalent jet stream occurs along a major frontal zone called the polar front and is appropriately named the polar jet stream, or simply the polar jet (Figure 7-16b). Because this jet stream is often found in the middle latitudes, particularly in the winter, it is also known as the midlatitude jet stream (Figure 7-17). Recall that the polar front is situated between the cool winds of the polar easterlies and the relatively warm westerlies. Instead of flowing nearly straight west to east, the polar jet stream usually has a meandering path. Occasionally, it flows almost due north–south. Sometimes it splits into two jets that may, or may not, rejoin. Like the polar front, this zone of high-velocity airflow is not continuous around the globe. On average, the polar jet travels at 125 kilometers (75 miles) per hour in the winter and roughly half that
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speed in the summer (Figure 7-18). This seasonal difference is due to the much stronger temperature gradient that exists in the middle latitudes during the winter. Because the location of the polar jet roughly coincides with that of the polar front, its latitudinal position migrates with the seasons. Thus, like the zone of maximum solar heating, the jet moves northward during summer and southward in winter. During the cold winter months, the polar jet stream may extend as far south as central Florida (Figure 7-18). With the coming of spring, the zone of maximum solar heating, and therefore the jet, begins a gradual northward migration. By midsummer, its average position is near the Canadian border, but it can be located much farther poleward. The polar jet stream plays a very important role in the weather of the midlatitudes. In addition to supplying energy to help drive the rotational motion of surface storms, it also directs the paths of these storms. Consequently, determining changes in the location and flow pattern of the polar jet is an important part of modern weather forecasting. As the polar jet shifts northward, there is a corresponding change in the region where outbreaks of severe thunderstorms and tornadoes occur. For example, in February most thunderstorms and tornadoes occur in the states bordering the Gulf of Mexico. By midsummer the center of this activity shifts to the northern plains and Great Lakes states. In addition, the location of the polar jet stream affects other surface conditions, particularly temperature and humidity. When it is situated substantially equatorward of your location, the weather will be colder and drier than normal. Conversely, when the polar jet moves poleward of your location, warmer and more humid conditions will prevail. Thus, depending on the position of the jet stream, the weather could be hotter, colder, drier, or wetter than normal.
Chapter 7 Circulation of the Atmosphere
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temperatures prevail to the north. Then, with minimal warning, the flow aloft may begin to meander and produce large-amplitude waves and a general north-to-south flow, as shown in Figure 7-20b and Figure 7-20c. Such a change causes cold air to advance equatorward and warm air to flow poleward. In addition, a cold air mass may become detached and produce an outbreak of cold air, as shown in Figure 7-20d. This redistribution of energy eventually results in a weakened temperature gradient and a return to a flatter flow aloft (Figure 7-20d). Therefore, the wavy flow of the westerlies centered on the polar jet plays an important role in Earth’s heat budget.
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Figure 7–19 Infrared image of the subtropical jet stream shown as a band of cloudiness extending from Mexico to Florida. (Photo courtesy of NOAA)
Concept Check 7.7
Subtropical Jet Stream
1 How are jet streams generated? 2 At what time of year should we expect the fastest polar jet streams? Explain.
A semipermanent jet over the subtropics is called the subtropical jet stream (Figure 7-16b). The subtropical jet is mainly a wintertime phenomenon. Somewhat slower than the polar jet, this west-to-east current is centered at 25° latitude (in both hemispheres) at an altitude of about 13 kilometers (8 miles). In the Northern Hemisphere, when the subtropical jet sweeps northward in the winter, it may bring warm humid conditions to the Gulf states, particularly southern Florida (Figure 7-19).
Jet Streams and Earth’s Heat Budget Let us return to the wind’s function of maintaining Earth’s heat budget by transporting heat from the equator toward the poles. Although the flow in the tropics (Hadley cell) is somewhat meridional (north to south), at most other latitudes the flow is zonal (west to east). The reason for the zonal flow, as we have seen, is the Coriolis force. The question we now consider is: How can wind with a west-to-east flow transfer heat from south to north? The important function of heat transfer is accomplished by the wavy flow (Rossby waves) of the westerlies centered on the polar jet stream. There may be periods of a week or more when the flow in the polar jet is essentially west to east, as shown in Figure 7-20a. When this condition prevails, relatively mild temperatures occur south of the jet stream, and cooler Figure 7–20 Cyclic changes that occur in the upper-level airflow of the westerlies. The flow, which has the jet stream as its axis, starts out nearly straight and then develops meanders and cyclonic activity that dominate the weather.
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3 Why is the polar jet sometimes referred to as the midlatitude jet stream?
4 Describe the expected winter temperatures in the north-central states when the polar jet stream is located over central Florida.
5 Explain how the wavy flow centered on the jet stream helps balance Earth’s heat budget.
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(Image courtesy of NASA) In mid-April 2010, Iceland’s Eyjafjallajökull volcano produced an ash plume that rose nearly 50,000 feet into the atmosphere as it moved eastward across the North Atlantic. This event prompted authorities in the United Kingdom, Ireland, France, and Scandinavia to close airspace over their countries for fear that the volcanic ash, if sucked into an airplane’s turbines, could cause engine failure.
Students Sometimes Ask… Why do pilots of commercial aircraft always remind passengers to keep their seat belts fastened, even in ideal flying conditions? The reason for this request is a phenomenon known as clear air turbulence. Clear air turbulence occurs when airflow in two adjacent layers is moving at different velocities. This can happen when the air at one level is traveling in a different direction than the air above or below it. More often, however, it occurs when air at one level is traveling faster than air in an adjacent layer. Such movements create eddies (turbulence) that can cause the plane to move suddenly up or down.
Global Winds and Ocean Currents Energy is passed from moving air to the surface of the ocean through friction. As a consequence, winds blowing steadily across the ocean drag the water along with them. Because
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United Kingdom Question 1 Based on the fact that Iceland is located in the zone of the polar easterlies (winds that blow from the northeast toward the southwest), explain why this ash plume moved primarily from west to east toward Northern Europe.
winds are the primary driving force of surface ocean currents, a relationship exists between atmospheric circulation and oceanic circulation. A comparison of Figures 7-21 and 7-10 illustrates this. As shown in Figure 7-21, north and south of the equator are two westward-moving currents, the North and South Equatorial Currents. These currents derive their energy principally from the trade winds that blow from the northeast and southeast, respectively, toward the equator. Because of the Coriolis force, surface currents are deflected poleward to form clockwise spirals in the Northern Hemisphere and counterclockwise spirals in the Southern Hemisphere. These nearly circular ocean currents, called gyres, are found in each of the major ocean basins centered around the subtropical high-pressure systems (Figure 7-21). In the North Atlantic, the equatorial current is deflected northward through the Caribbean, where it becomes the Gulf Stream. As the Gulf Stream moves along the eastern coast of the United States, it is strengthened by the prevailing westerly winds. As it continues northeastward beyond the Grand Banks, it gradually widens and slows
Chapter 7 Circulation of the Atmosphere
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until it becomes a vast, slowly moving current known as the North Atlantic Drift. The North Atlantic Drift splits as it approaches Western Europe. Part moves northward past Great Britain toward Norway, while the other portion is deflected southward as the cool Canary Current. As the Canary Current moves south, it eventually merges with the North Equatorial Current.
The Importance of Ocean Currents Currents have an important effect on climate. The moderating effect of poleward-moving warm ocean currents is well known. The North Atlantic Drift, an extension of the warm Gulf Stream, keeps Great Britain and much of northwestern Europe warmer than might be expected considering their latitudes. In addition to influencing temperatures of adjacent land areas, cold currents have other climatic effects. For example, in tropical deserts that exist along the west coasts of continents, such as the Atacama in Peru and Chile, and the Namib in southern Africa, cold ocean currents have a dramatic impact. The aridity along these coasts is intensified because air flowing over the cold currents is chilled from below. This causes the air to become very stable and resist the upward movements necessary to create precipitationproducing clouds. Cold currents may also cause the lowest layer of air to chill below its dew-point temperature. As a result, these desert areas are characterized by high relative humidities and abundant fog. Thus, not all tropical deserts are hot with low humidities and clear skies. Rather, the presence of cold
currents transforms some tropical deserts into relatively cool, damp places that are often shrouded in fog. Ocean currents also play a major role in maintaining Earth’s heat balance. They accomplish this task by transferring heat from the tropics, where there is an excess of heat, to the polar regions, where a deficit exists. Ocean currents account for about one-quarter of this total heat transport, and winds make up the remainder.
Ocean Currents and Upwelling Upwelling, the rising of cold water from deeper layers to replace warmer surface water, is a common wind-induced vertical movement. It is most characteristic along the eastern shores of the global oceans, most notably along the coasts of California, Peru, and West Africa. Upwelling occurs in areas where winds blow parallel to the coast toward the equator. Because of the Coriolis force, the surface water is directed (deflected) away from the shore. As the surface layer moves away from the coast, it is replaced by water that “upwells” from below the surface. This slow upward flow from depths of 50 to 300 meters (165 to 1000 feet) brings water that is cooler than the water it replaces and creates a characteristic zone of relatively cool water adjacent to the shore. Swimmers accustomed to the waters along the midAtlantic states of the United States might find a dip in the Pacific on central California’s coast a chilling surprise. In August, when water temperatures along the Atlantic shore usually exceed 21°C (70°F), the surf along the coast of California is only about 15°C (60°F).
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Concept Check 7.8 1 What drives Earth’s surface ocean currents? 2 Ocean currents tend to form spirals. a In what direction (clockwise or counterclockwise) do these ocean currents travel in each hemisphere?
b What general name is given to these spirals? Name the major examples in each hemisphere.
3 Give an example of how ocean currents affect the temperature of adjacent landmasses.
4 Describe the role that ocean currents play in maintaining Earth’s heat balance.
El Niño and La Niña and the Southern Oscillation El Niño was first recognized by fishermen from Ecuador and Peru, who noted a gradual warming of waters in the eastern Pacific in December or January. Because the warm-
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ing usually occurred near the Christmas season, the event was named El Niño—“little boy,” or “Christ child,” in Spanish. These periods of abnormal warming happen at irregular intervals of two to seven years and usually persist for spans of nine months to two years. La Niña, which means “little girl,” is the opposite of El Niño and refers to colder-thannormal sea-surface temperatures along the coastline of Ecuador and Peru. As Figure 7-22a illustrates, the atmospheric circulation in the central Pacific during a La Niña event is dominated by strong trade winds. These wind systems, in turn, generate a strong equatorial current that flows westward from South America toward Australia and Indonesia. In addition, a cold ocean current is observed flowing equatorward along the coast of Ecuador and Peru. The latter flow, called the Peru Current, encourages upwelling of cold, nutrient-filled waters that serve as the primary food source for millions of small feeder fish, particularly anchovies. Therefore, fishing is particularly good during the periods of strong upwelling. Every few years, however, the circulation associated with La Niña is replaced by an El Niño event (Figure 7-22b).
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Figure 7–22 The relationship between El Niño, La Niña, and the Southern Oscillation is illustrated by these diagrams. (a) During a La Niña event, strong trade winds drive the equatorial currents toward the west. At the same time, the strong Peru Current causes upwelling of cold water along the west coast of South America. (b) When the Southern Oscillation occurs, the pressure over the eastern and western Pacific flip-flops. This causes the trade winds to diminish, leading to an eastward movement of warm water along the equator and the beginning of an El Niño. As a result, the surface waters of the central and eastern Pacific warm, with far-reaching consequences for weather patterns.
Chapter 7 Circulation of the Atmosphere
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an El Niño, strong equatorial countercurrents amass large quantities of warm water that block the upwelling of colder, El Niño is noted for its potentially catastrophic impact on nutrient-filled water along the west coast of South America. the weather and economies of Peru, Chile, and Australia, As a result, the anchovies, which support the population of among other countries. As shown in Figure 7-22b, during game fish, starve, devastating the fishing industry. At the same time, some inland areas of Peru and Chile that are normally arid receive above-average rainfall, which Warm can cause major flooding. These cliWarm Warm matic fluctuations have been known Warm Cool Wet for years, but they were considered and wet Dry local phenomena. Warm Wet Dry Scientists now recognize that and dry Warm Warm and wet and wet El Niño is part of the global atmoDry Warm spheric circulation pattern that afWarm and dry Warm fects the weather at great distances Wet from Peru. One of the most severe El Niño events on record occurred in (a) El Niño: December to February 1997–1998 and was responsible for a variety of weather extremes in many parts of the world. During the 1997– 1998 El Niño episode, ferocious Wet winter storms struck the California Warm coast, causing unprecedented beach and dry Dry Dry erosion, landslides, and floods. In the southern United States, heavy Wet rains also brought floods to Texas Warm and the Gulf states. Warm Cool and dry Wet Although the effects of El Niño are somewhat variable, some locales appear to be affected more consis(b) El Niño: June to August tently. In particular, during the winter, warmer-than-normal conditions prevail in the north-central United Cool States and parts of Canada (Figure 7Wet Wet Cool 23a). In addition, significantly wetter Warm and dry winters are experienced in the southWet western United States and northCool Dry Cool Wet Cool western Mexico, while the southeastand dry and dry ern United States experiences wetter Cool and wet and cooler conditions. In the western Cool Pacific, drought conditions are observed in parts of Indonesia, Aus(c) La Niña: December to February tralia, and the Philippines (Figure 723a). One major benefit of El Niño is
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Figure 7–23 Climatic impacts of El Niño and La Niña in various locations during December through February and June through August. El Niño has the most significant impact on the climate of North America during the winter. In addition, El Niño affects the areas around the tropical Pacific in both winter and summer. Likewise, La Niña has its most significant impact on North America in the winter but affects other areas during all seasons.
The Atmosphere: An Introduction to Meteorology
a lower-than-average number of Atlantic hurricanes. El Niño is credited with suppressing hurricanes during the 2009 hurricane season, the least active in 12 years.
Impact of La Niña La Niña was once thought to be the normal conditions that occur between two El Niño events, but meteorologists now consider La Niña an important atmospheric phenomenon in its own right. Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered and exhibits a distinctive set of weather patterns (Figure 7-22a). Typical La Niña winter weather includes cooler and wetter conditions over the northwestern United States and especially cold winter temperatures in the Northern Plains states (Figure 7-23c). In addition, unusually warm conditions occur in the Southwest and Southeast. In the western Pacific, La Niña events are associated with wetter-thannormal conditions. The 2010–2011 La Niña contributed to a deluge in Australia, which resulted in one of the country’s worst natural disasters: Large portions of the state of Queensland were extensively flooded (Figure 7-24). Another La Niña impact is more frequent hurricane activity in the Atlantic. A recent study concluded that the cost of hurricane damages in the United States is 20 times greater in La Niña years than in El Niño years.
Southern Oscillation Major El Niño and La Niña events are intimately related to the large-scale atmospheric circulation. Each time an El Niño occurs, the barometric pressure drops over large por-
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Figure 7–25 This graph illustrates the Southern Oscillation. Negative values (blue) represent the cold La Niña phase, whereas positive values (red) represent the warm El Niño phase. The graph was created by analyzing six variables, including sea-surface temperatures and sea-level pressures.
tions of the eastern Pacific and rises in the western Pacific (see Figure 7-22b). Then, as a major El Niño event comes to an end, the pressure difference between these two regions swings back in the opposite direction, triggering a La Niña event (Figure 7-22a). This seesaw pattern of atmospheric pressure between the eastern and western Pacific is called the Southern Oscillation (Figure 7-25). Winds are the link between the pressure change associated with the Southern Oscillation and the ocean warming and cooling associated with El Niño and La Niña. The start of an El Niño event begins with a rise in surface pressure over Australia and Indonesia and a decrease in pressure
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Figure 7–24 Flooding of large portions of Rockhampton, Queensland, Australia, January 2011. The Queensland floods of 2010–2011 have been attributed to one of the strongest La Niña events as far back as records have been kept. Unusually warm sea-surface temperatures around Australia contributed to the heavy rains. In other parts of Australia, this strong La Niña event brought relief from a decade-long drought. (Courtesy of NASA)
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Global Distribution of Precipitation
over the eastern Pacific (Figure 7-22b). During a strong El Niño, this pressure change causes the trade winds to weaken and countercurrents to develop and move warm water eastward. The resulting change in atmospheric circulation takes the rain with it, causing drought in the western Pacific and increased rainfall in the normally dry regions of Peru and Chile. The opposite circulation develops during La Niña events, as shown in Figure 7-22a. When a strong La Niña develops, the trade winds strengthen, causing dryer-than-normal conditions in the eastern Pacific, while extreme flooding may occur in Indonesia and northeastern Australia.
Figure 7-26 shows the distribution pattern for average annual precipitation over the globe. Although this map may appear complicated, the general features of the pattern can be explained using knowledge of global winds and pressure systems. In general, regions influenced by high pressure, with its associated subsidence and divergent winds, experience dry conditions. Conversely, regions under the influence of low pressure, with its converging winds and ascending air, receive ample precipitation. However, if the wind-pressure regimes were the only control of precipitation, the pattern shown in Figure 7-26 would be much simpler. Air temperature is also important in determining precipitation potential. Because cold air has a low capacity for moisture compared with warm air, we would expect a latitudinal variation in precipitation, with low latitudes (warm regions) receiving the greatest amounts of precipitation and high latitudes (cold regions) receiving the least. In addition to latitudinal variations in precipitation, the distribution of land and water complicates the precipitation pattern. Large landmasses in the middle latitudes commonly experience decreased precipitation toward their interiors. For example, North Platte, Nebraska, receives less than half the precipitation that falls on the coastal community of Bridgeport, Connecticut, despite being located at the same
Concept Check 7.9 1 Describe how a major El Niño event tends to affect the weather in Peru and Chile as compared to Indonesia and Australia.
2 Describe the sea-surface temperatures on both sides of the tropical Pacific during a La Niña event.
3 How does a major La Niña event influence the hurricane season in the Atlantic Ocean?
4 Briefly describe the Southern Oscillation and how it is related to El Niño and La Niña.
5 Describe how an El Niño event might affect the climate in North America during the winter. Describe the same for a La Niña event.
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of land and water. Recall from our earlier discussion that on a uniform Earth, four major pressure zones emerge in each hemisphere (see Figure 7-8a, page 199). These zones are the equatorial low (ITCZ), the subtropical high, the subpolar low, and the polar high. Also, remember that these pressure belts show a marked seasonal shift toward the summer hemisphere. The idealized precipitation regimes expected from these pressure systems are shown in Figure 7-27. Near the equator the trade winds converge (ITCZ), resulting in heavy precipitation in all seasons. Poleward of the equatorial low in each hemisphere lie the belts of subtropical high pressure. In these regions, subsidence contributes to dry conditions throughout the year. Between the wet equatorial regime and the dry subtropical regime lies a zone that is influenced by both high- and low-pressure systems. Because the pressure systems migrate seasonally with the Sun, these transitional regions receive most of their precipitation in the summer, when they are under the influence of the ITCZ. They experience a dry season in the winter, when the subtropical high moves equatorward. The midlatitudes receive most of their precipitation from traveling cyclonic storms (Figure 7-28). This region is the site of the polar front, the convergence zone between
Phytoplankton bloom
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South America
(Image courtesy of NASA) This true-color image, taken in the winter of 2010–2011, shows swirls of green and blue color off the coast of South America that are caused by a bloom of phytoplankton—tiny marine organisms that contain chlororophyll and other pigments. These phytoplankton blooms usually occur when nutrientrich cold water upwells toward the surface. Upwelling is most pronounced along the west coast of South America when the Peru Current is strong. Question 1 Based on the fact that the Peru Current was stronger than normal and caused a significant phytoplankton bloom, does this image represent an El Niño event or a La Niña event? Question 2 How do these periods of strong upwelling affect commercial fishing along the west coast of South America?
Polar High Sparse precipitation in all seasons
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Subpolar low Ample precipitation in all seasons
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Winter wet, Summer dry 30° Subtropical High Dry in all seasons Summer wet, Winter dry
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latitude. Furthermore, mountain barriers alter precipitation patterns. Windward mountain slopes receive abundant precipitation, whereas leeward slopes and adjacent lowlands are usually deficient in moisture.
Subpolar low Ample precipitation in all seasons Polar High Sparse precipitation in all seasons
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Zonal Distribution of Precipitation We will first examine the zonal distribution of precipitation that we would expect on a uniform Earth covered entirely in water and then add the variations caused by the distribution
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Figure 7–27 Zonal precipitation patterns.
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Figure 7–28 Satellite image of a well-developed midlatitude cyclone over the British Isles. These traveling storms produce most of the precipitation in the middle latitudes. (Courtesy of European Space Agency/Science Photo Library/Photo Researchers, Inc.)
cold polar air and the warmer westerlies. Because the position of the polar front migrates freely between approximately 30° and 70° latitude, most midlatitude areas receive ample precipitation. The polar regions are dominated by high pressure and cold air that holds little moisture. Throughout the year, these regions experience only meager precipitation.
many of the world’s great deserts but also regions of abundant rainfall (Figure 7-26). This pattern results because the subtropical high-pressure centers that dominate the circulation in these latitudes have different characteristics on their eastern and western sides (Figure 7-29). Subsidence is most
H
Distribution of Precipitation over the Continents The zonal pattern outlined in the previous section roughly approximates general global precipitation. Abundant precipitation occurs in the equatorial and midlatitude regions, whereas substantial portions of the subtropical and polar realms are relatively dry. Numerous exceptions to this idealized zonal pattern are obvious in Figure 7-26. For example, several arid areas are found in the midlatitudes. The desert region of southern South America, known as Patagonia, is one example. Midlatitude deserts such as Patagonia exist mostly on the leeward (rain shadow) side of a mountain barrier or in the interior of a continent, cut off from a source of moisture. The most notable anomaly in the zonal distribution of precipitation occurs in the subtropics. Here we find not only
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Figure 7–29 Characteristics of subtropical high-pressure systems. Subsidence on the east side of these systems produces stable conditions and aridity. Convergence and uplifting on the western flank promote uplifting and instability.
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Box 7–2 Precipitation Regimes on a Hypothetical Continent When we consider the influence of land and water on the distribution of precipitation, the pattern illustrated in Figure 7–D emerges. This figure is a highly idealized precipitation scheme for a hypothetical continent located in the Northern Hemisphere. The “continent” is shaped as it is to approximate the percentage of land found at various latitudes. Notice that this hypothetical landmass has been divided into seven zones. Each zone represents a different precipitation regime. Stated another way, all locations within the same zone (precipitation regime) experience roughly the same precipitation pattern. As you will see, the odd shapes and sizes of these zones reflect the way precipitation is normally distributed across landmasses located in the Northern Hemisphere. The cities shown are representative of the precipitation pattern of each location. With this in mind, we will examine each precipitation regime (numbered 1 through 7) to look at the influences of land and water on the distribution of precipitation. First, compare the precipitation patterns for the nine cities. Notice that the precipitation regimes for west coast locations correspond to the zonal pressure and precipitation pattern shown in Figure 7–27: zone 6 equates to the equatorial low (wet all year), zone 4 matches the subtropical high (dry all year), and so on.
For illustration, Aklavik, Canada, which is strongly influenced by the polar high, has scant moisture, whereas Singapore, near the equator, has plentiful rain every month. Also note that precipitation varies seasonally with both latitude and each city’s location with respect to the ocean. For example, precipitation graphs for San Francisco and Mazatlan, Mexico, illustrate the marked seasonal variations found in zones 3 and 5. Recall that it is the seasonal migration of the pressure systems that causes these fluctuations. Notice that zone 2 shows a precipitation maximum in the fall and early winter, as illustrated by data for Juneau, Alaska. This pattern occurs because cyclonic storms over the North Pacific are more prevalent in the coolest half of the year. Further, the average path of these storms moves equatorward during the winter, impacting locations as far south as southern California in midwinter. The eastern half of the continent does not experience the zonal precipitation pattern that we observe on the western side. Only the dry polar regime (zone 1) is similar in size and position on both the eastern and western seaboards. The most noticeable departure from the zonal pattern is the absence of an arid subtropical region along the east coast. This is caused by the behavior of subtropical anticyclones located over the oceans. Recall that southern Florida,
pronounced on the eastern side, which results in stable atmospheric conditions. Because these anticyclones tend to crowd the eastern side of an ocean, particularly in the winter, we find that the western portions of the continents adjacent to subtropical highs are arid (Figure 7-29). Centered at approximately 25° north or south latitude on the west side of their respective continents, we find the Sahara Desert of North Africa, the Namib of southwest Africa, the Atacama of South America, the deserts of northwestern Mexico, and Australia. On the western flanks of these highs, however, subsidence is less pronounced. In addition, the surface air that flows out of these highs often traverses large expanses of warm water. As a result, this air acquires moisture through evaporation that acts to enhance its instability. Consequently, landmasses located west of a subtropical high generally receive ample precipitation throughout the year. Southern Florida is a good example (Figure 7-29).
which is located on the east coast of the North American continent, receives ample precipitation year-round, whereas the Baja Peninsula on the west coast is arid. Moving inland from the east coast, observe how precipitation decreases (by comparing total precipitation westward for New York, Chicago, and Omaha). This decrease, however, does not hold true for mountainous regions; even relatively small ranges, like the Appalachians, are able to extract a greater share of precipitation. Another variation is seen with latitude. As we move poleward along the eastern half of our hypothetical continent, note the decrease in precipitation (by comparing Cancun, New York, and Aklavik). This is the decrease we would expect with cooler air temperatures and corresponding lower moisture capacities. Consider next the continental interior, which shows a somewhat different precipitation pattern, especially in the middle latitudes. The precipitation regimes of the continental interior are affected to a degree by a type of monsoon circulation. Cold winter temperatures and flow off the land make winter the drier season (for example, Omaha and Chicago). In summer, the inflow of warm, moist air from the ocean is aided by a thermal low that develops over the land and causes a general increase in precipitation over the midcontinent.
Concept Check 7.10 1 Are regions that are dry throughout the year dominated by high or low pressure?
2 Describe the precipitation pattern for locations near the equator and near the poles.
3 Earth’s major subtropical deserts are located between about 20° and 35° latitude. a Name five subtropical deserts. b On what side (western or eastern) of the continent are they located? c What is the general name of the pressure systems responsible for subtropical deserts?
4 List two reasons that explain why polar regions experience meager precipitation.
5 What factors, in addition to global wind and pressure systems, influence the global distribution of precipitation?
Chapter 7 Circulation of the Atmosphere
Aklavik, NWT 68°N Annual precipitation 23 cm (9 in)
Omaha, NE 41°N Annual precipitation 65 cm (25 in) 250
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Zone 7. Juneau, AK 58°N Annual precipitation 211 cm (82 in)
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Chicago, IL 41°N Annual precipitation 84 cm (33 in) 250
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0 Phoenix, AZ 33°N Annual precipitation 19 cm (7 in) 250
Mazatlan, Mexico 23°N Annual precipitation 73 cm (28 in)
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Cancun, Mexico 21°N Annual precipitation 142 cm (56 in)
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FIGURE 7–D Idealized precipitation regimes for a hypothetical landmass in the Northern Hemisphere.
Give It Some Thought 1. It is a warm summer day, and you are shopping in
downtown Chicago, just a few blocks from Lake Michigan. All morning the winds have been calm, suggesting that no major weather systems are nearby. By midafternoon, should you expect a cool breeze from Lake Michigan or a warm breeze originating from the rural areas outside the city? 2. Boulder, Colorado, in the eastern foothills of the Rocky Mountains, is experiencing a warm, dry January day with strong westerly winds. What type of local winds are likely responsible for these weather conditions? 3. Which of the local winds described in this chapter is not heavily dependent on differences in the rates at which various ground surfaces are heated?
4. The accompanying sketch shows the three-cell
circulation model in the Northern Hemisphere. Match the appropriate number on the sketch to each of the following features: a. Hadley cell b. Equatorial low c. Polar front d. Ferrel cell e. Subtropical high f. Polar cell g. Polar high
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5. If Earth did not rotate on its axis and if its surface
6.
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were completely covered with water, what direction would a boat drift if it started its journey in the middle latitudes of the Northern Hemisphere? (Hint: What would the global circulation pattern be like for a nonrotating Earth?) Briefly explain each of the following statements that relate to the global distribution of surface pressure: a. The only true zonal distribution of pressure exists in the region of the subpolar low in the Southern Hemisphere. b. The subtropical highs are stronger in the North Atlantic in July than in January. c. The subpolar lows in the Northern Hemisphere are the result of individual cyclonic storms that are more common in the winter. d. A strong high-pressure cell develops in the winter over northern Asia. On the accompanying image of Jupiter, notice the many zones of clouds that are produced by large convective cells similar to the Hadley cells on Earth. Given your understanding of the role of the Coriolis force in producing Earth’s wind belts, do you think Jupiter rotates faster or slower on its axis than Earth? Explain. Refer to Figure 7–26, p. 213 and Figure 7-10, p. 200, to determine what aspect of global circulation (polar highs, equatorial low, etc.) is responsible for each of the following: a. Dry conditions over North Africa. b. The wet summer monsoon over southeast Asia. c. Dry conditions over west-central Australia. d. Wet conditions over northeastern South America. Explain why the west coasts of continents generally experience cold ocean currents. How do these cold currents contribute to desert conditions in some
coastal areas, such as the Atacama region of Peru and Chile? 10. The accompanying map shows sea-surface temperature anomalies (difference from normal) over the equatorial Pacific Ocean. Based on this map, answer the following questions: a. In what phase was the Southern Oscillation (El Niño or La Niña) when this image was made? b. Would the trade winds be strong or weak at this time? c. If you lived in Australia during this event, what weather conditions would you expect? d. If you were attending college in the southeastern United States during winter months, what type of weather conditions would you expect? (Hint: See Figure 7–23, p. 211.)
11. Refer to Figure 7–D on page 217 to determine in
which of the seven precipitation regimes you reside. What elements of the global atmospheric circulation interact to create the precipitation regime of your location? 12. The accompanying maps of Africa show the distribution of precipitation for July and January. Which map represents July and which represents January? How did you determine your answer?
CIRCULATION OF THE ATMOSPHERE IN REVIEW ●
The largest planetary-scale wind patterns, called macroscale winds, include the westerlies and trade winds. Mesoscale winds, such as thunderstorms, tornadoes, and land-sea breezes,
influence smaller areas and often exhibit intense vertical flow. The smallest scale of air motion is the microscale, which includes gusts and dust devils.
Chapter 7 Circulation of the Atmosphere ●
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Most winds have the same cause: pressure differences that arise because of temperature differences that are caused by unequal heating of Earth’s surface. In addition to land and sea breezes brought about by the daily temperature contrast between land and water, other mesoscale winds include mountain breezes and valley breezes, chinook (foehn) winds, katabatic (fall) winds, and country breezes. The three-cell circulation model for each hemisphere provides a simplified view of global circulation. According to this model, atmospheric circulation cells are located between the equator and 30° latitude, 30° and 60° latitude, and 60° latitude and the pole. The areas of general subsidence in the zone between 20° and 35° are called the horse latitudes. In each hemisphere, the equatorward flow from the horse latitudes forms the reliable trade winds. The circulation between 30° and 60° latitude (north and south) results in the prevailing westerlies. Air that moves equatorward from the poles produces the polar easterlies in both hemispheres. If Earth’s surface were uniform, four latitudinally oriented belts of pressure would exist in each hemisphere—two high and two low. Beginning at the equator, the four belts would be the (1) equatorial low, also referred to as the intertropical convergence zone (ITCZ), (2) subtropical high, at about 20° to 35° on either side of the equator, (3) subpolar low, situated at about 50° to 60° latitude, and (4) polar high, near Earth’s poles. In reality, the only true zonal pattern of pressure exists along the subpolar low in the Southern Hemisphere. At other latitudes, particularly in the Northern Hemisphere, where there is a higher proportion of land compared to ocean, the zonal pattern is replaced by semipermanent cells of high and low pressure. The greatest seasonal change in Earth’s global circulation is the development of monsoons, wind systems that exhibit a pronounced seasonal reversal in direction. The best-known and most pronounced monsoonal circulation is the Asian monsoon. The North American monsoon, a relatively small
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seasonal wind shift, produces a dry spring followed by a comparatively rainy summer that impacts large areas of the southwestern United States and northwestern Mexico. The temperature contrast between the poles and the equator drives the westerlies in the middle latitudes. Embedded within the flow aloft are narrow ribbons of high-speed winds, called jet streams, that meander for thousands of kilometers. The key to the origin of polar jet streams is found in great temperature contrasts at the surface. Because winds are the primary driving force of ocean currents, a strong relationship exists between the oceanic circulation and the general atmospheric circulation. Because of the circulation associated with the subtropical highs, ocean currents form clockwise spirals in the Northern Hemisphere and counterclockwise spirals in the Southern Hemisphere. El Niño refers to episodes of ocean warming along the coasts of Ecuador and Peru. When surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered. These events are part of the global circulation and are related to a seesaw pattern of atmospheric pressure between the eastern and western Pacific called the Southern Oscillation. El Niño and La Niña events influence weather on both sides of the tropical Pacific Ocean as well as the United States. The general features of the global distribution of precipitation can be explained by global winds and pressure systems. In general, regions influenced by high pressure, with its associated subsidence and divergent winds, experience dry conditions. On the other hand, regions under the influence of low pressure and its converging winds and ascending air receive ample precipitation. On a uniform Earth throughout most of the year, heavy precipitation would occur in the equatorial region, the midlatitudes would receive most of their precipitation from traveling cyclonic storms, and polar regions would be dominated by cold air that holds little moisture.
VOCABULARY REVIEW Aleutian low (p. 201) Azores high (p. 201) Bermuda high (p. 201) bora (p. 195) chinook (p. 194) country breeze (p. 195) doldrums (p. 197) El Niño (p. 210) equatorial low (p. 198) Ferrel cell (p. 197) foehn (p. 194) Hadley cell (p. 197) horse latitudes (p. 197) Icelandic low (p. 201)
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intertropical convergence zone (ITCZ) (p. 199) jet stream (p. 205) katabatic or fall wind (p. 195) land breeze (p. 193) La Niña (p. 210) macroscale wind (p. 191) mesoscale wind (p. 191) microscale circulation (p. 190) mistral (p. 195) monsoon (p. 201) mountain breeze (p. 194) polar easterlies (p. 197) polar front (p. 198)
polar high (p. 199) polar jet stream (p. 206) prevailing westerlies (p. 197) Santa Ana (p. 194) sea breeze (p. 193) Siberian high (p. 201) Southern Oscillation (p. 212) subpolar low (p. 199) subtropical high (p. 199) subtropical jet stream (p. 207) trade winds (p. 197) upwelling (p. 209) valley breeze (p. 194)
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Air Masses Most people living in the middle latitudes have experienced hot, “sticky” summer heat waves and frigid winter cold waves. In the case of a heat wave, after several days of sultry weather, the spell may come to an abrupt end that is marked by thundershowers, followed by a few days of relatively cool relief. In the case of a cold wave, thick stratus clouds and snow may replace the clear skies that had prevailed, and temperatures may climb to values that seem mild compared with what preceded them. In both examples, what was experienced was a period of generally uniform weather conditions followed by a relatively short period of change and the subsequent reestablishment of a new set of weather conditions that remained for perhaps several days before changing again.
Satellite image illustrating the process that leads to lake-effect snow storms, a phenomenon associated with cold, dry air masses. (NASA)
After completing this chapter you should be able to:
t Define air mass and air-mass weather. t List the basic criteria for an air-mass source region and explain why regions of high pressure are favored sites.
t On a map, locate and label the source regions that influence North America.
t Classify air masses. t Describe the processes by which traveling air masses are modified and discuss two examples.
t Summarize the weather conditions associated with each of the air masses that influence North America in summer and winter. 221
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What Is an Air Mass? ATMOSPHERE
Basic Weather Patterns ▸ Air Masses
The weather patterns just described result from the movements of large bodies of air, called air masses. An air mass, as the term implies, is an immense body of air, usually 1600 kilometers (1000 miles) or more across and perhaps several kilometers thick, which is characterized by homogeneous physical properties (in particular, temperature and moisture content) at any given altitude. When this air moves out of its region of origin, it will carry these temperatures and moisture conditions elsewhere, eventually affecting a large portion of a continent (Figure 8-1). An excellent example of the influence of an air mass is illustrated in Figure 8-2. Here a cold, dry mass from northern Canada moves southward. With a beginning temperature of −46°C (−51°F), the air mass warms 13°C (24°F), to −33°C (−27°F), by the time it reaches Winnipeg. It continues to warm as it moves southward through the Great Plains and into Mexico. Throughout its southward journey, the air mass becomes warmer, but it also brings some of the coldest weather of the winter to the places in its path. Thus, the air mass is modified, but it also modifies the weather in the areas over which it moves. The horizontal uniformity of an air mass is not complete because it may extend through 20° or more of latitude and cover hundreds of thousands to millions of square kilometers. Consequently, small differences in temperature and humidity from one point to another at the same level are to be expected. Still, the differences observed within an air mass are small in comparison to the rapid rates of change experienced across air-mass boundaries. Because it may take several days for an air mass to traverse an area, the region under its influence will probably experience generally constant weather conditions, a situation
Cold, dry air mass –46°C
–33°C Winnipeg
–29°C Sioux Falls –23°C Omaha –18°C Wichita –15°C Oklahoma City –9°C Dallas –4°C
10°C
Houston
Tampico
Figure 8-2 As this frigid Canadian air mass moved southward, it brought some of the coldest weather of the winter to the areas in its path. As it advanced out of Canada, the air mass slowly got warmer. Thus, the air mass was gradually modified at the same time that it modified the weather in the areas over which it moved. (From PHYSICAL GEOGRAPHY: A LANDSCAPE APPRECIATION, 9th Edition, by Tom L. McKnight and Darrell Hess, © 2008. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)
called air-mass weather. Certainly, some day-to-day variations may exist, but the events will be very unlike those in an adjacent air mass. The air-mass concept is an important one because it is closely related to the study of atmospheric disturbances. Many significant middle-latitude disturbances originate along the boundary zones that separate different air masses.
Concept Check 8.1 1 Define air mass. 2 What is air-mass weather?
Figure 8-1 Air masses that form over this high-latitude location in the South Atlantic will be cold and humid. (Photo by Michael Collier)
Chapter 8 Air Masses
Source Regions ATMOSPHERE
cA Continental arctic Very cold, very dry
Basic Weather Patterns ▸ Air Masses
Where do air masses form? What factors determine the nature and degree of uniformity of an air mass? These two basic questions are closely related because the site where an air mass forms vitally affects the properties that characterize it. Areas in which air masses originate are called source regions. Because the atmosphere is heated chiefly from below and gains its moisture by evaporation from Earth’s surface, the nature of the source region largely determines the initial characteristics of an air mass. An ideal source region must meet two essential criteria. First, it must be an extensive and physically uniform area. A region having highly irregular topography or one that has a surface consisting of both water and land is not satisfactory. The second criterion is that the area be characterized by a general stagnation of atmospheric circulation so that air will stay over the region long enough to come to some measure of equilibrium with the surface. In general, it means regions dominated by stationary or slow-moving anticyclones, with their extensive areas of calm or light winds. Regions under the influence of cyclones are not likely to produce air masses because such systems are characterized by converging surface winds. The winds in lows are constantly bringing air with unlike temperature and humidity properties into the area. Because the time involved is not long enough to eliminate these differences, steep temperature gradients result, and air-mass formation cannot take place. Figure 8-3 shows the source regions that produce the air masses that most often influence North America. The waters of the Gulf of Mexico and Caribbean Sea and similar regions in the Pacific west of Mexico yield warm air masses, as does the land area that encompasses the southwestern United States and northern Mexico. In contrast, the North Pacific, the North Atlantic, and the snow- and ice-covered areas comprising northern North America and the adjacent Arctic Ocean are major source regions for cold air masses. It is also clear that the size of the source regions and the intensity of temperatures change seasonally. Notice in Figure 8-3 that major source regions are not found in the middle latitudes but instead are confined to subtropical and subpolar locations. The fact that the middle latitudes are the site where cold and warm air masses clash, often because the converging winds of a traveling cyclone draw them together, means that this zone lacks the conditions necessary to be a source region. Instead, this latitude belt is one of the stormiest on the planet.
Concept Check 8.2 1 What two criteria must be met for an area to be an air-mass source region?
2 Why are regions that have a cyclonic circulation generally not conducive to air-mass formation?
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mP
cP
Maritime polar Cool, humid
Continental polar Cold, dry
mP Maritime polar Cool, humid
mT mT
Maritime tropical Warm, humid
Maritime tropical Warm, humid (a) Mid-winter pattern
cP Continental polar Cool, dry
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Maritime polar Cool, humid
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Maritime tropical Warm, humid
cT Continental tropical Hot, dry (b) Mid-summer pattern
Figure 8-3 Air-mass source regions for North America. Source regions are largely confined to subtropical and subpolar locations. The fact that the middle latitudes are the site where cold and warm air masses clash, often because the converging winds of a traveling cyclone draw them together, means that this zone lacks the conditions necessary to be a source region. The differences between polar and arctic are relatively small and serve to indicate the degree of coldness of the respective air masses. By comparing the summer and winter maps, it is clear that the extent and temperature characteristics of source regions fluctuate.
Classifying Air Masses ATMOSPHERE
Basic Weather Patterns ▸ Air Masses
The classification of an air mass depends on the latitude of the source region and the nature of the surface in the area of origin—ocean or continent. The latitude of the source region indicates the temperature conditions within the air mass, and the nature of the surface below strongly influences the moisture content of the air. Air masses are identified by two-letter codes. With reference to latitude (temperature), air masses are placed into
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one of three categories: polar (P), arctic (A), or tropical (T). The differences between polar and arctic are usually small and simply serve to indicate the degree of coldness of the respective air masses. The lowercase letter m (for maritime) or the lowercase letter c (for continental) is used to designate the nature of the surface in the source region and hence the humidity characteristics of the air mass. Because maritime air masses form over oceans, they have a relatively high water-vapor content compared to continental air masses that originate over landmasses. When this classification scheme is applied, the following air masses can be identified: cA
continental
arctic
cP
continental
polar
cT
continental
tropical
mT
maritime
tropical
mP
maritime
polar
Notice that the list does not include mA (maritime arctic). These air masses are not listed because they seldom, if ever, form. Although arctic air masses form over the Arctic Ocean, this water body is largely ice covered throughout the year. Consequently, the air masses that originate here consistently have the moisture characteristics associated with a continental source region.
Concept Check 8.3 1 On what basis are air masses classified? 2 Compare the temperature and moisture characteristics of the following air masses: cA, cP, mP, mT, and cT.
3 Why is mA left out of the air-mass classification scheme?
LA TX Gulf of Mexico
Air-Mass Modification After an air mass forms, it normally migrates from the area where it acquired its distinctive properties to a region with different surface characteristics. Once the air mass moves from its source region, it not only modifies the weather of the area it is traversing, but it is also gradually modified by the surface over which it is moving. This idea was clearly shown in Figure 8-2. Warming or cooling from below, the addition or loss of moisture, and vertical movements all act to bring about changes in an air mass. The amount of modification can be relatively small or, as the following example illustrates, the changes can be profound enough to alter completely the original identity of the air mass. When cA or cP air moves over the ocean in winter, it undergoes considerable change (Figure 8-4). Evaporation from the water surface rapidly transfers large quantities of moisture to the once-dry continental air. Furthermore, because the underlying water is warmer than the air above, the air is also heated from below. This factor leads to instability and vertically ascending currents that rapidly transport heat and moisture to higher levels. In a relatively short span, cold, dry, and stable continental air is transformed into an unstable mP air mass. When an air mass is colder than the surface over which it is passing, as in the preceding example, the lowercase letter k may be added after the air-mass symbol. If, however, an air mass is warmer than the underlying surface, the lowercase letter w is added. It should be remembered that the k or w suffix does not mean that the air mass itself is cold or warm. It means only that the air is relatively cold or warm in comparison with the underlying surface over which it is traveling. For example, an mT air mass from the Gulf of Mexico is usually classified as mTk as it moves over the southeastern states in summer. Although the air mass is warm, it is still cooler than the highly heated landmass over which it is passing. The k or w designation gives an indication of the stability of an air mass and hence the weather that might be FL expected. An air mass that is colder than the surface is going to be warmed in its lower layers. This fact causes greater instability that favors the ascent of the heated lower air and creates the
Figure 8-4 This satellite image from December 16, 2007, shows the modification of cold, dry, and cloud-free cP air as it moved over the Gulf of Mexico. The addition of heat and water vapor from the relatively warm water quickly modified the air mass, as evidenced by the development of clouds. (NASA)
Chapter 8 Air Masses
possibility of cloud formation and precipitation. Indeed, a k air mass is often characterized by cumulus clouds, and if precipitation occurs, it will be of the shower or thunderstorm variety. Also, visibility is generally good (except in rain) because of the stirring and overturning of the air. Conversely, when an air mass is warmer than the surface over which it is moving, its lower layers are chilled. A surface inversion that increases the stability of the air mass often develops. This condition does not favor the ascent of air, and so it opposes cloud formation and precipitation. Any clouds that do form will be stratus clouds, and precipitation, if any, will be light to moderate. Moreover, because of the lack of vertical movements, smoke and dust often become concentrated in the lower layers of the air mass and cause poor visibility. During certain times of the year, fogs, especially the advection type, may also be common in some regions. In addition to modifications resulting from temperature differences between an air mass and the surface below, upward and downward movements induced by cyclones and anticyclones or topography can also affect the stability of an air mass. Such modifications are often called mechanical or dynamic and are usually independent of the changes caused by surface cooling or heating. For example, significant modification can result when an air mass is drawn into a low. Here convergence and lifting dominate and the air mass is rendered more unstable. Conversely, the subsidence associated with anticyclones acts to stabilize an air mass. Similar alterations in stability occur when an air mass is lifted over highlands or descends the leeward side of a mountain barrier. In the first case, the air’s stability is reduced; in the second case, the air becomes more stable.
Concept Check 8.4 1 What do the lowercase letters k and w indicate about an air mass?
2 List the general weather conditions associated with k and w air masses.
Continental Polar (cP) and Continental Arctic (cA) Air Masses Continental polar and continental arctic air masses are, as their classification implies, cold and dry. Continental polar air originates over the often snow-covered interior regions of Canada and Alaska, poleward of the 50th parallel. Continental arctic air forms farther north, over the Arctic basin and the Greenland ice cap (see Figure 8-3). Continental arctic air is distinguished from cP air by its generally lower temperatures, although at times the differences may be slight. In fact, some meteorologists do not differentiate between cP and cA. Winter During the winter, both cP and cA air masses are bitterly coldand very dry. Winter nights are long, and the daytime Sun is short-lived and low in the sky. Consequently, as winter advances, Earth’s surface and atmosphere lose heat that, for the most part, is not replenished by incoming solar energy. Therefore, the surface reaches very low temperatures, and the air near the ground is gradually chilled to heights of 1 kilometer (0.6 mile) or more. The result is a strong and persistent temperature inversion, with the coldest temperatures near the ground. Marked stability is, therefore, the rule. Because the air is very cold and the surface below is frozen, the mixing ratio of these air masses is necessarily low, ranging from perhaps 0.1 gram per kilogram in cA up to 1.5 grams per kilogram in some cP air. As wintertime cP or cA air moves outward from its source region, it carries its cold, dry weather to the United States, normally entering between the Great Lakes and the Rockies. Because there are no major barriers between the high-latitude source regions and the Gulf of Mexico, cP and cA air masses can sweep rapidly and with relative ease far southward into the United States. The winter cold waves experienced in much of the central and eastern United States are closely associated with such polar outbreaks. One such cold wave is described in “Severe and Hazardous Weather,” page 227. Usually, the last freeze in spring and the first freeze in autumn can be correlated with outbreaks of polar or arctic air.
3 How might vertical movements induced by a pressure system or topography act to modify an air mass?
Students Sometimes Ask... When a cold air mass moves south from Canada into the United States, how rapidly can temperatures change?
Properties of North American Air Masses Air masses frequently pass over us, which means that theday-to-day weather we experience often depends on the temperature, stability, and moisture content of these large bodies of air. In this section, we briefly examine the properties of the principal North American air masses. In addition, Table 8-1 serves as a summary.
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When a fast-moving frigid air mass advances into the northern Great Plains, temperatures have been known to plunge 20° to 30°C (40° to 50°F) in a matter of just a few hours. One notable example is a drop of 55.5°C (100°F), from 6.7°C to −48.8°C (44° to −56°F), in 24 hours at Browning, Montana, on January 23−24, 1916. Another remarkable example occurred on Christmas Eve, 1924, when the temperature at Fairfield, Montana, dropped from 17°C (63°F) at noon to −29°C (−21°F) at midnight—an amazing 46°C (83°F) change in just 12 hours.
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TABLE 8-1
Weather Characteristics of North American Air Masses Stability in Source Region
Associated Weather
Bitterly cold and very dry in winter
Stable
Cold waves in winter
Interior Canada and Alaska
Very cold and dry in winter
Stable entire year
a. Cold waves in winter b. Modified to cPk in winter over Great Lakes, bringing lake-effect snow to leeward shores
North Pacific
Mild (cool) and humid entire year
Unstable in winter
a. Low clouds and showers in winter b. Heavy orographic precipitation on windward side of western mountains in winter c. Low stratus and fog along coast in summer; modified to cP inland
Cold and humid in winter
Unstable in winter
Air Mass
Source Region
cA
Arctic basin and Greenland ice cap (winter only)
cP
mP
mP
Temperature and Moisture Characteristics in Source Region
Northwestern Atlantic
Stable in summer
Stable in summer
Cool and humid in summer
a. Occasional nor’easter in winter b. Occasional periods of clear, cool weather in summer
cT
Northern interior Mexico and southwestern U.S. (summer only)
Hot and dry
Unstable
a. Hot, dry, and cloudless, rarely influencing areas outside source region b. Occasional drought to southern Great Plains
mT
Gulf of Mexico, Caribbean Sea, western Atlantic
Warm and humid entire year
Unstable entire year
a. In winter it usually becomes mTw, moving northward and bringing occasional widespread precipitation or advection fog b. In summer hot and humid conditions, frequent cumulus development, and showers or thunderstorms
mT
Eastern subtropical Pacific
Warm and humid entire year
Stable entire year
a. In winter it brings fog, drizzle, and occasional moderate precipitation to northwestern Mexico and the southwestern United States b. In summer occasionally reaches the western United States and is a source of moisture for infrequent convectional thunderstorms
Summer Because cA air is present principally in the winter, only cP air has any influence on our summer weather, and this effect is considerably reduced when compared with winter. During summer months the properties of the source region for cP air are very different from those during winter. Instead of being chilled by the ground, the air is warmed from below as the long days and higher Sun angle warm the snow-free land surface. Although summer cP air is warmer and has a higher moisture content than its wintertime counterpart, the air is still cool and relatively dry compared with air in areas farther south. Summer heat waves in the northern portions of the eastern and central
United States are often ended by the southward advance of cP air, which for a day or two brings cooling relief and bright, pleasant weather.
Lake-Effect Snow: Cold Air Over WarmWater A glance at the chapter-opening image (pages 220–221) provides a perspective from space of the process discussed in this section. The skies over Lake Superior and Lake Michigan exhibit long rows of dense, white, snowproducing clouds. They formed as cold, dry cP air moved
Chapter 8 Air Masses
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The Siberian Express
T
he surface weather map for December 22, 1989, shows a very large high-pressure center covering the eastern two-thirds of the United States and a substantial portion of Canada (Figure8-A). As is usually the case in winter, a large anticyclone such as this is associated with a huge mass of dense and bitterly cold arctic air. After such an air mass forms over the frozen expanses near the Arctic Circle, the winds aloft sometimes direct it toward the south and east. When an outbreak takes place, it is popularly called the “Siberian Express” by the news media, even though the air mass did not originate in Siberia. November 1989 was unusually mild for late autumn. In fact, across the United States, more than 200 daily high-temperature records were set. December, however, was different. East of the Rockies, the month’s weather was dominated by two arctic outbreaks. The second brought record-breaking cold.
More than 370record low temperatures were reported. Between December 21 and 25, as the frigid dome of high pressure advanced southward and eastward, more than 370record low temperatures were reported. On December 21, Havre, Montana, had an overnight low of −42.2°C (−44°F), breaking a record set in 1884. Meanwhile, Topeka’s −32.2°C (−26°F) was that city’s lowest temperature tor any date since record keeping had started 102 years earlier. The three days that followed saw the arctic air migrate toward the south and east. By December 24, Tallahassee, Florida, had a low temperature of −10°C (14°F). In the center of the state the daily minimum at Orlando was −5.6°C (22°F). It was actually warmer in North Dakota on Christmas Eve than in central and northern Florida!
As we would expect, utility companies in many states reported record demand. When the arctic air advanced into Texas and Florida, agriculture was especially hard hit. Some Florida citrus growers lost 40 percent of their crop, and many vegetable crops were wiped out completely (Figure 8-B).
It was actually warmer in North Dakota on Christmas Eve than in central and northern Florida! After Christmas the circulation pattern that brought this record-breaking Siberian Express from the arctic deep into the United States changed. As a result, temperatures for much of the country during January and February 1990 were well above normal. In fact, January 1990 was the second warmest January in 96 years. Thus, despite frigid December temperatures, the winter of 1989–1990 “averaged out” to be a relatively warm one.
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FIGURE 8-A A surface weather map for 7 AM EST, December 22, 1989. This simplified National Weather Service (NWS) map shows an intense winter cold wave caused by an outbreak of frigid continental arctic air. This event brought subfreezing temperatures as far south as the Gulf of Mexico. Temperatures on NWS maps are in degrees Fahrenheit.
FIGURE 8-B When an arctic air mass invades the citrus regions of Florida and Texas, even modern freeze controls may not be able to prevent significant losses. (Photo courtesy of Florida Department of Citrus)
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from the land surface across the open water. Continental polar air masses are not, as a rule, associated with dense clouds and heavy precipitation. Yet during late autumn and winter a unique and interesting weather phenomenon takes place along the downwind shores of the Great Lakes.* Periodically, brief, heavy snow showers issue from dark clouds that move onshore from the lakes (see “Severe and Hazardous Weather,” p. 230). Seldom do these storms move more than about 80 kilometers (50 miles) inland from the shore before the snows come to an end. These highly localized storms, occurring along the leeward shores of the Great Lakes, create what are known as lake-effect snows. Lake-effect storms account for a high percentage of the snowfall in many areas adjacent to the lakes. The strips of land that are most frequently affected, called snowbelts, are shown in Figure 8-5. A comparison of average snowfall totals at Thunder Bay, Ontario, on the north shore of Lake Superior, and Marquette, Michigan, along the southern shore, provides another excellent example. Because Marquette is situated on the leeward shore of the lake, it receives substantial lake-effect snow and therefore has a much higher snowfall total than does Thunder Bay (Table 8-2). What causes lake-effect snow? The answer is closely linked to the differential heating of water and land (Chapter 3) and to the concept of atmospheric instability (Chapter 4). During the summer months, bodies of water, including the Great Lakes, absorb huge quantities of energy from the Sun and from the warm air that passes over them. Although these water bodies do not reach particularly high temperatures, they nevertheless represent huge reservoirs of heat. The surrounding land, in contrast, cannot store heat nearly as effectively. Consequently, during autumn and winter, the temperature of the land drops quickly, whereas water bodies lose their heat more gradually and cool slowly. *Actually, the Great Lakes are just the best-known example. Other large lakes can also experience this phenomenon.
0 0
300 300
Monthly Snowfall at Thunder Bay, Ontario, and Marquette, Michigan
TABLE 8-2
Thunder Bay, Ontario October
November
December
January
3.0 cm (1.2 in.)
14.9 cm (5.8 in.)
19.0 cm (7.4 in.)
22.6 cm (8.8 in.)
Marquette, Michigan October
November
December
January
5.3 cm (2.1 in.)
37.6 cm (14.7 in.)
56.4 cm (22.0 in.)
53.1 cm (20.7 in.)
From late November through late January the contrasts in average temperatures between water and land range from about 8°C in the southern Great Lakes to 17°C farther north. However, the temperature differences can be much greater (perhaps 25°C) when a very cold cP or cA air mass pushes southward across the lakes. When such a dramatic temperature contrast exists, the lakes interact with the air to produce major lake-effect storms. Figure 8-6 depicts the movement of a cP air mass across one of the Great Lakes. During its journey, the air acquires large quantities of heat and moisture from the relatively warm lake surface. By the time it reaches the opposite shore, this cPk air is humid and unstable, and heavy snow showers are likely.
Maritime Polar (mP) Air Masses Maritime polar air masses form over oceans at high latitudes. As the classification indicates, mP air is cool to cold and humid, but compared with cP and cA air masses in winter, mP air is relatively mild because of the higher temperatures of the ocean surface as contrasted to the colder continents. Figure 8-5 Average annual snowfall. The snowbelts of the Great Lakes region are easy to pick out on this snowfall map. (Data from NOAA)
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Incredible. Significant structural deformation of mid- and highrise buildings.
*The original Fujita scale was developed by T. Theodore Fujita in 1971 and put into use in 1973. The Enhanced Fujita Scale is a revision that was put into use in February 2007. Winds speeds are estimates (not measurements) based on damage and represent three-second gusts at the point of damage. More information about the criteria used to evaluate tornado intensity can be found at http://www.spc.noaa.gov/efscale/.
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ample, an outbreak of 147 tornadoes brought death and destruction to a 13-state region east of the Mississippi River. More than 300 people died, and nearly 5500 people were injured (Figure 10–26).
100
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The proportion of tornadoes that result in the loss of life is small. In 2010, there were 1543 tornadoes reported in the United States. Of this total, just 22 were “killer tornadoes,” a number that is about average. Put another way, in most years slightly less than 2 percent of all reported tornadoes in the United States are “killers.” Although the percentage of tornadoes that result in death is small, every tornado is potentially lethal. If you examine Figure 10–27, which compares tornado fatalities with storm intensities, the results are quite interesting. It is clear from this graph that themajority (63 percent) of all tornadoes are weak and that the number of storms decreases as tornado intensity increases. The distribution of tornado fatalities, however, is just the opposite. Although only 2 percent of tornadoes are classified as violent, they account for nearly 70 percent of the deaths. If there is some question about the causes of tornadoes, there certainly is none about the destructive effects of these
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Because severe thunderstorms and tornadoes are small and short-lived phenomena, they are among the most difficult weather features to forecast precisely. Nevertheless, the prediction, detection, and monitoring of such storms are among the most important services provided by professional meteorologists. The timely issuance and dissemination of watches and warnings are both critical to the protection of life and property. The Storm Prediction Center (SPC) located in Norman, Oklahoma, is part of the National Weather Service (NWS) and the National Centers for Environmental Prediction (NCEP). Its mission is to provide timely and accu-
Chapter 10 Thunderstorms and Tornadoes
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Warren Faidley: Storm Chaser Warren Faidley is a storm chaser. As an extreme weather photojournalist, he has survived more blizzards, tornadoes, hailstorms, and flying debris than he would like to remember. His images of town-dwarfing tornadoes and hurricane destruction have been used in movies and magazines, news programs, and textbooks. As a frequent witness to many violent weather events, he is often interviewed for news programs and storm documentaries. Faidley’s calendar revolves around storms: Spring is tornado season, summer is lightning season, and late summer to fall is hurricane season. All the while, he analyzes weather charts, second-guesses forecasts, and consults Doppler radar data the way most people consult city maps. He spends his days zigzagging across the farm roads and lonely highways of Oklahoma, Kansas, and other states to approach storms in progress.
*UTPOFQBSUTDJFODFBOE NFUFPSPMPHZ BOEBOPUIFS QBSUBSUJTUSZ Getting near a storm is only half the job. “At the same time, I need to make images that convey what it looks like when a 2 × 4 goes through the side of a car.” One trick is finding spots of color like a red barn and green fields against gray storm clouds and a gray sky. “It’s one part science and meteorology, and another part artistry,” Faidley says. Faidley’s life has been entwined with extreme weather since childhood. He nearly drowned after being swept away in a flash flood at age 12, and he steered his bicycle into dust devils as a teen. By the mid-1980s, after earning a degree in journalism, he decided to become the country’s first full-time weather photojournalist. His first break was a bolt from the blue. Faidley snapped a photo of a white-hot arc of lightning striking a light pole, suffusing the Arizona night in an eeric purple glow. Another fork hit perilously near Faidley, almost killing him. But the episode ended on a happy note. Life magazine published the lightning photo in 1989, launching Faidley’s freelance career.
In 1992, Faidley followed up the lightning photo by obtaining some of the few existing shots of Hurricane Andrew in progress. Faidley hid under a shed in south Florida while the category 5 storm howled past.
5IFSFBSFNPNFOUTPGUFSSPSCVU BMTPNPNFOUTPGBCTPMVUFCFBVUZ Storm chasing offers Faidley a heady mix of adrenaline and grace. “It’s very awe-inspiring. There are moments of terror but also moments of absolute beauty. Capturing a picture of an orange sky cut in half by the emerald green of a coming storm is fantastic.” Faidley is no mere thrill-seeker. He does a great deal of advance planning and takes the precautions required to come back alive. In the late 1990s, he designed and built the first tornado-resistant chase vehicle, installing impact-resistant glass, a NASCAR-type roll cage, and other safety features on his SUV. “The purpose isn’t to enable us to do something stupid, like penetrate a tornado.
Rather, it’s to offer us safety in case something unexpected happens such as a sign blowing off a motel and careening down the road.” He is always aware of escape roads whenever he’s in storm country, and he speaks regularly to the public about the importance of staying informed and knowing how to respond during a violent weather episode. When Faidley first began storm chasing in the 1980s, moment-by-moment weather infomation was hard to come by. Live weather radar on the Internet did not exist back then. Instead, he got to know National Weather Service forecasters and learned storm meteorology from them on the fly. Today, Faidley watches forecasts weeks ahead to ensure that he’s within driving range when the looming clouds appear. “I live a barnstorming, gypsy life driven by visual instinct,” Faidley says. “The canvas keeps changing but the canvas wants to kill you. It’s a juggling act.” Kathleen M. Wong
8"33&/'"*%-&: is a well-known storm chaser and weather photographer. He is the author of The Ultimate Storm Survival Handbook and the autobiographical book Storm Chaser. (Photo courtesy of Warren Faidley)
rate forecasts and watches for severe thunderstorms and tornadoes. Severe thunderstorm outlooks are issued several times daily. Day 1 outlooks identify those areas likely to be affected
by severe thunderstorms during the next 6 to 30hours, and day 2 outlooks extend the forecast through the following day. Both outlooks describe the type, coverage, and intensity of the severe weather expected. Many local NWS field of-
The Atmosphere: An Introduction to Meteorology
Number of tornado deaths
296
1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0
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fices also issue severe weather outlooks that provide a more local description of the severe weather potential for the next 12 to 24 hours.
Tornado Watches and Warnings Tornado watches alert the public to the possibility of tornadoes over a specified area for a particular time interval. Watches serve to fine-tune forecast areas already identified in severe weather outlooks. A typical watch covers an area of about 65,000 square kilometers (25,000 square miles) for a four- to six-hour period. A tornado watch is an important part of the tornado alert system because it sets in motion the procedures necessary to deal adequately with detection, tracking, warning, and response. Watches are generally reserved for organized severe weather events where the tornado threat will affect at least 26,000 square kilometers (10,000 square miles) and/or persist for at least three hours. Watches typically are not issued when the threat is thought to be isolated and/or short lived. Whereas a tornado watch is designed to alert people to the possibility of tornadoes, a tornado warning is issued by local offices of the National Weather Service when a tornado has actually been sighted in an area or is indicated by weather radar. It warns of a high probability of imminent danger. Warnings are issued for much smaller areas than watches, usually covering portions of a county or counties. In addition, they are in effect for much shorter periods, typi-
1960s
1970s
1980s
1990s
2000s
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cally 30 to 60 minutes. Because a tornado warning may be based on an actual sighting, warnings are occasionally issued after a tornado has already developed. However, most warnings are issued prior to tornado formation, sometimes by several tens of minutes, based on Doppler radar data and/or spotter reports of funnel clouds or cloud-base rotation. If the direction and the approximate speed of the storm are known, an estimate of its most probable path can be made. Because tornadoes often move erratically, the warning area is fan shaped downwind from the point where the tornado has been spotted. Improved forecasts and advances in technology have contributed to a significant decline in tornado deaths over the past 50years. Figure 10–28 illustrates this trend. During a span when the U.S. population grew rapidly, tornado deaths trended downward. As noted earlier, the probability of one place being struck by a tornado, even in the area of greatest frequency, is slight. Nevertheless, although the probabilities may be small, tornadoes have provided many mathematical exceptions. For example, the small town of Codell, Kansas, was hit three years in a row—1916, 1917, and 1918—and each time on the same date, May 20! Needless to say, tornado watches and warnings should never be taken lightly.
Doppler Radar The installation of Doppler radar across the United States significantly improved our ability to track thunderstorms and issue warnings based on their potential to produce tornadoes (Figure 10–29). Conventional weather radar works by transmitting short pulses of electromagnetic energy. A small fraction of the waves that are sent out is scattered by a storm and returned to the radar. The strength of the returning signal indicates rainfall intensity, and the time difference between the transmission and return of the signal indicates the distance to the storm. However, to identify tornadoes and severe thunderstorms, we must be able to detect the characteristic circulation
Chapter 10 Thunderstorms and Tornadoes
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patterns associated with them. Conventional radar cannot do so except occasionally, when spiral rain bands occur in association with a tornado and give rise to a hook-shaped echo. Doppler radar not only performs the same tasks as conventional radar but also has the ability to detect motion directly (Figure 10–30). The principle involved is known as the Doppler effect (Figure 10–31). Air movement in clouds is determined by comparing the frequency of the reflected signal to that of the original pulse. The movement of precipitation toward the radar increases the frequency of reflected pulses, whereas moReflectivity Storm-relative velocity tion away from the radar decreases the frequency. These frequency changes are then Figure 10–305IJTJTBEVBM%PQQMFSSBEBSJNBHFPGBO&'UPSOBEPOFBS.PPSF interpreted in terms of speed toward or away 0LMBIPNB PO.BZ 5IFMFGUJNBHF SFnFDUJWJUZ TIPXTQSFDJQJUBUJPOJOUIF from the Doppler radar unit. This same prinTVQFSDFMMUIVOEFSTUPSN5IFSJHIUJNBHFTIPXTNPUJPOPGUIFQSFDJQJUBUJPOBMPOHUIF ciple allows police radar to determine the SBEBSCFBNUIBUJT IPXGBTUSBJOPSIBJMJTNPWJOHUPXBSEPSBXBZGSPNUIFSBEBS speed of moving cars. Unfortunately, a single *OUIJTFYBNQMF UIFSBEBSXBTVOVTVBMMZDMPTFUPUIFUPSOBEPDMPTFFOPVHIUPNBLF Doppler radar unit cannot detect air movePVUUIFTJHOBUVSFPGUIFUPSOBEPJUTFMG .PTUPGUIFUJNFPOMZUIFXFBLFSBOEMBSHFS NFTPDZDMPOFJTEFUFDUFE "GUFS/0""
ments that occur parallel to it. Therefore, when a more complete picture of the winds within a cloud mass is desired, it is necessary to use two or Apparent wavelength Approaching ambulance more Doppler units.
Students Sometimes Ask… How dangerous is it to be in a mobile home during a tornado? Mobile homes represent a relatively small fraction of all residences in the United States. Yet, according to the National Weather Service, during the span 2000–2010, 52 percent of all tornado fatalities (314 of 604) occurred in mobile homes.
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Doppler radar can detect the initial formation and subsequent development of the mesocyclone within a severe thunderstorm that frequently precedes tornado development. Almost all (96 percent) mesocyclones produce damaging hail, severe winds, or tornadoes. Those that produce tornadoes (about 50 percent) can sometimes be distinguished by their stronger wind speeds and their sharper gradients of wind speeds. Mesocyclones can sometimes be identified within parent storms 30 minutes or more before tornado formation, and if a storm is large, at distances up to 230 kilometers (140miles). In addition, when close to the radar, individual tornado circulations may sometimes be detected. Ever since the implementation of the national Doppler network, the average lead time for tornado warnings has increased from less than 5 minutes in the late 1980s to about 13 minutes today. Doppler radar is not without problems. One concern relates to the weak tornadoes that rank at or near the bottom of the Enhanced Fujita Intensity Scale. Because Doppler radar makes forecasting and detection of these tornadoes possible, the potential exists for numerous warnings being issued for tornadoes that do little or no damage. This could desensitize the public to the dangers of more rare, life-threatening tornadoes.
It should also be pointed out that not all tornado-bearing storms have clear-cut radar signatures and that other storms can give false signatures. Detection, therefore, is sometimes a subjective process, and a given display could be interpreted in several ways. Consequently, trained observers will continue to form an important part of the warning system in the foreseeable future. Although some operational problems exist, the benefits of Doppler radar are many. As a research tool, it is not only providing data on the formation of tornadoes but is also helping meteorologists gain new insights into thunderstorm development, the structure and dynamics of hurricanes, and air-turbulence hazards that plague aircraft. As a practical tool for tornado detection, it has significant advantages over a system that uses conventional radar.
Concept Check 10.10 1 %JTUJOHVJTICFUXFFOBUPSOBEPXBUDIBOEBUPSOBEPXBSOJOH 2 8IBUBEWBOUBHFTEPFT%PQQMFSSBEBSIBWFPWFS DPOWFOUJPOBMSBEBS
Give It Some Thought 1. If you are a resident of central Ohio and hear that a
cyclone is approaching, should you immediately seek shelter? What if you live in western lowa? 2. Which one of the locations shown on the accompanying map is more likely to have dryline thunderstorms? Why is this the case?
conditions. Explain the connection between these two phenomena. 5. The accompanying table lists the number of tornadoes reported in the United States by decade. Propose a reason that might explain why the total for the 2000s was so much higher than for the 1950s. Number of U.S. Tornadoes Reported, by Decade 1950–1959
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8579
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8196
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6. Figure 10–28 shows that the number of tornado
3. Sinking air warms by compression (adiabatically), yet
thunderstorm downdrafts are usually cold. Explain this apparent contradiction. 4. Studies have linked the formation of supercell thunderstorms to the existence of temperature inversions. However, cumulonimbus clouds form in an unstable environment, whereas temperature inversions are associated with very stable atmospheric
deaths in the United States in the 2000s was less than 40percent the number that occurred in the 1950s, even though there was a significant rise in the population during that span. To what can you attribute this decline in the death toll? 7. As you will learn in Chapter 11, the intensity of a hurricane is monitored and reported as the storm approaches. However, the intensity of a tornado is not determined and reported until after the storm passes. Why is this the case?
Chapter 10 Thunderstorms and Tornadoes
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THUNDERSTORMS AND TORNADOES IN REVIEW r Although tornadoes and hurricanes are, in fact, cyclones, the
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vast majority of cyclones are not hurricanes or tornadoes. The term cyclone refers to the circulation around any low-pressure center, no matter how large or intense it is. Dynamic thermal instability occurs during the development of thunderstorms, which form when warm, humid air rises in an unstable environment. A number of mechanisms, suchas unequal heating of Earth’s surface or lifting of warm air along a front or mountain slope, can trigger the upward air movement needed to create thunderstorm-producing cumulonimbus clouds. Air-mass thunderstorms frequently occur in maritime tropical (mT) air during the spring and summer. Generally, three stages are involved in the development of these storms—the cumulus stage, mature stage, and dissipating stage. Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms than do the Plains states. Many thunderstorms that form over the eastern two-thirds of the United States occur as part of the general convergence and frontal wedging that accompany passing midlatitude cyclones. Severe thunderstorms are capable of producing heavy downpours and flash flooding as well as strong, gusty, straight-line winds. They are influenced by strong vertical wind shear—that is, changes in wind direction and/or speed between different heights and updrafts that become tilted and continue to build upward. Downdrafts from the thunderstorm cells reach the surface and spread out to produce anadvancing wedge of cold air, called a gust front. Some of the most dangerous weather is produced by a type of thunderstorm called a supercell, a single, very powerful thunderstorm cell that at times may extend to heights of 20kilometers (65,000 feet) and persist for many hours. These cells may produce a mesocyclone, a column of cyclonically rotating air, within which tornadoes sometimes form. Squall lines are relatively narrow, elongated bands of thunderstorms that develop in the warm sector of a midlatitude cyclone, usually in advance of a cold front. A mesoscale convective complex (MCC) consists of many individual thunderstorms that are organized into a large oval to circular cluster. They form most frequently in the Great Plains from groups of afternoon air-mass thunderstorms. Thunder is produced by lightning. Lightning equalizes the electrical difference associated with the formation of a large cumulonimbus cloud by producing a negative flow of current from the region of excess negative charge to the region with
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excess positive charge or vice versa. The most common type of lightning, often called sheet lightning, occurs within and between clouds. The less common, but more dangerous, type of lightning is cloud-to-ground lightning. The origin of charge separation in clouds, although not fully understood, hinges on rapid vertical movements within a cloud. The lightning we see as single flashes is really several very rapid strokes between the cloud and the ground. When air is heated by the electrical discharge of lightning, it expands explosively and produces the sound waves we hear as thunder. Tornadoes are violent windstorms that take the form of a rotating column of air, or vortex, that extends downward from a cumulonimbus cloud. Some tornadoes consist of a single vortex. Within many stronger tornadoes, called multiple-vortex tornadoes, are smaller intense whirls called suction vortices that rotate within the main vortex. Pressures within some tornadoes have been estimated to be as much as 10percent lower than immediately outside the storm. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds approach 480 kilometers (300 miles) per hour. Severe thunderstorms, and hence tornadoes, are most often spawned along the cold front or squall line of a midlatitude cyclone or in association with supercell thunderstorms. Tornadoes can also form in association with tropical cyclones (hurricanes). April through June is the period of greatest tornado activity, but tornadoes occur during every month of the year. The average tornado has a diameter between 150 and 600 meters, travels across the landscape toward the northeast at approximately 45 kilometers per hour, and cuts a path about 26 kilometers long. Most tornado damage is caused by tremendously strong winds. One commonly used guide to tornado intensity is the Enhanced Fujita Intensity Scale (EF-scale). A rating on the EF-scale is determined by assessing damages produced by a storm. Because severe thunderstorms and tornadoes are small and short-lived phenomena, they are among the most difficult weather features to forecast precisely. When weather conditions favor the formation of tornadoes, a tornado watch is issued to alert the public to the possibility of tornadoes over a specified area for a particular time interval. A tornado warning is issued by local offices of the National Weather Service when a tornado has been sighted in an area or is indicated by weather radar. With its ability to detect the movement of precipitation within a cloud, Doppler radar technology has greatly advanced the accuracy of tornado warnings.
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VOCABULARY REVIEW air-mass thunderstorm (p. 274) cumulus stage (p. 275) dart leader (p. 286) dissipating stage (p. 275) Doppler radar (p. 296) dryline (p. 280) Enhanced Fujita intensity scale (EF-scale) (p. 293) entrainment (p. 275) flash (p. 284)
squall line (p. 278) step leader (p. 284) stroke (p. 284) supercell (p. 277) thunder (p. 286) thunderstorm (p. 273) tornado (p. 286) tornado warning (p. 296) tornado watch (p. 296)
gust front (p. 277) leader (p. 284) lightning (p. 281) mature stage (p. 275) mesocyclone (p. 278) mesoscale convective complex (MCC) (p. 281) multiple-vortex tornado (p. 287) return stroke (p. 284) severe thunderstorm (p. 276)
PROBLEMS 1. If thunder is heard 15 seconds after lightning is seen, about how far away was the lightning stroke?
public. Which four states experience the greatest number of tornadoes? Are these the states with the greatest tornado threat? Which map is most useful for depicting the tornado hazard in the United States? Does the map in Figure 10–22 have an advantage over either or both of these maps?
2. Examine the upper-left portion of Figure 10–23 and determine the percentage of tornadoes that exhibited directions of movement toward the E through NNE. 3. Figures 10–32 and 10–33 represent two common ways that U. S. tornado statistics are graphically presented to the
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Chapter 10 Thunderstorms and Tornadoes
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Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes, and a Pearson eText version of this book to enhance your study of Thunderstorms and Tornadoes.
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Hurricanes The whirling tropical cyclones that occasionally have wind speeds exceeding 300kilometers (185 miles) per hour are known in the United States as hurricanes— the greatest storms on Earth. Hurricanes are among the most destructive of natural disasters. When a hurricane reaches land, it is capable of annihilating low-lying coastal areas and killing thousands of people. On the positive side, hurricanes provide essential rainfall over many areas they cross. Consequently, a resort owner along the Florida coast may dread the coming of hurricane season, whereas a farmer in Japan may welcome its arrival.
Aftermath of Hurricane Ike, September 14, 2008. The eye of the storm passed directly over Galveston, Texas. (Photo by Smiley N. Pool/Rapport Press/Newcom)
After completing this chapter, you should be able to: r Define hurricane and describe the basic structure and characteristics of this storm.
r Use the Saffir–Simpson scale and explain how hurricane intensity is determined.
r Identify areas of hurricane formation on a world map and discuss the conditions that promote hurricane formation.
r Discuss the three broad categories of hurricane destruction, including an example of each.
r Distinguish among tropical depression, tropical storm, and hurricane. r List and discuss the factors that cause hurricanes to diminish in intensity.
r List four tools that provide data used to track hurricanes and develop forecasts. r Contrast hurricane watch and hurricane warning and relate these concepts to hurricane forecasts. 303
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Profile of a Hurricane Many view the weather in the tropics with favor—and rightfully so. Places such as islands in the South Pacific and the Caribbean are known for their lack of significant day-to-day variations. Warm breezes, steady temperatures, and rains that come as heavy but brief tropical showers are expected. It is ironic that these relatively tranquil regions occasionally produce some of the most violent storms on Earth (Figure 11–1). Hurricanes are intense centers of low pressure that form over tropical or subtropical oceans and are characterized by intense convective (thunderstorm) activity and strong
Hurricane Igor
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cyclonic circulation. Sustained winds must equal or exceed 119 kilometers (74 miles) per hour. Unlike midlatitude cyclones, hurricanes lack contrasting air masses and fronts. Rather, the source of energy that produces and maintains hurricane-force winds is the huge quantity of latent heat liberated during the formation of the storm’s cumulonimbus towers. These intense tropical storms are known in variousparts of the world by different names. In the northwestern Pacific, they are called typhoons, and in the southwestern Pacific and
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Figure 11–1 Satellite image of Hurricane Igor on September 16, 2010. Maximum sustained winds were 213 kilometers (132 miles) per hour. The inset image in the upper right is a digital photograph of Igor’s well-developed eye, taken from the International Space Station. The storm had its greatest impact in Newfoundland, where most of the damage resulted from flooding triggered by Igor’s heavy rains. (NASA)
Chapter 11 Hurricanes
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Figure 11–2 This world map shows the regions where most hurricanes form as well as their principal months of occurrence and the most common tracks they follow. Hurricanes do not develop within about 5° of the equator because the Coriolis force is too weak in that region. Because warm surface ocean temperatures are necessary for hurricane formation, hurricanes seldom form poleward of 20° latitude nor over the cool waters of the South Atlantic and the eastern South Pacific.
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the Indian Ocean, they are called cyclones. In the following discussion, these storms will be referred to as hurricanes. The term hurricane is derived from Huracan, a Carib god ofevil. Most hurricanes form between the latitudes of 5° and 20° over all the tropical oceans except only rarely in the South Atlantic and the eastern South Pacific (Figure 11– 2). The western North Pacific has the greatest number of storms, averaging 20peryear. Fortunately for those living in the coastal regions of the southern and eastern United States, only about five hurricanes, on average, develop each year in the warm sector of the North Atlantic. Although many tropical disturbances develop each year, only a few reach hurricane status. By international
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agreement, a hurricane has sustained wind speeds of at least 119 kilometers (74 miles) per hour and a rotary circulation.* Mature hurricanes average about 600 kilometers (375 miles) across, although they can range in diameter from 100 kilometers (60 miles) up to about 1500 kilometers (930 miles). From the outer edge of a hurricane to the center, the barometric pressure has sometimes dropped 60 millibars, from 1010 to 950 millibars. A steep pressure gradient like that shown in Figure11–3 generates the rapid, inward spiraling winds of a hurricane. As the air moves closer to the center of the storm, its ve-
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Figure 11–3 Weather maps showing Hurricane Fran at 7 AM, on two successive days, September 5 and 6, 1996. On September 5, winds exceeded 190 kilometers per hour. As the storm moved inland, heavy rains caused flash floods, killed 30 people, and caused more than $3 billion in damages. The station information plotted off the Gulf and Atlantic coasts is from data buoys, which are remote floating instrument packages.
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Box 11–1
The Conservation of Angular Momentum In a similar manner, when a parcel of air moves toward the center of a storm, the product of its distance and velocity must remain unchanged. Therefore, as air moves inward from the outer edge, its rotational velocity must increase. Let us apply the law of conservation of angular momentum to the horizontal movement of air in a hypothetical hurricane. Assume that air with a velocity of 5 kilometers per hour begins 500 kilometers from the center of the storm. By the time it reaches a point 100 kilometers from the center, it will have a velocity of 25 kilometers per hour (assuming that there is no friction). If this same parcel of air were to continue to advance toward the storm’s center until its radius was just 10 kilometers, it would be traveling at 250 kilometers per hour. Friction reduces these values somewhat.
Why do winds blowing around a storm move faster near the center and more slowly near the edge? To understand this phenomenon, we must examine the law of conservation of angular momentum. This law states that the product of the velocity of an object around a center of rotation (axis) and the distance of the object from the axis is constant. Picture an object on the end of a string being swung in a circle. If the string is pulled inward, the distance of the object from the axis of rotation decreases and the speed of the spinning object increases. The change in radius of the rotating mass is balanced by a change in its rotational speed. Another common example of the conservation of angular momentum occurs when a figure skater starts whirling on the ice with both arms extended (Figure 11–A). Her arms are traveling in a circular path about an axis (her body). When the skater pulls her arms inward, she decreases the radius of the circular path of her arms. As a result, her arms go faster and the rest of her body must follow, thereby increasing her rate of spinning.
FIGURE 11–A When the skater’s arms are extended, she spins slowly. When her arms are pulled in, she spins much faster.
locity increases. This acceleration is explained by the law of conservation of angular momentum (Box 11–1). As the inward rush of warm, moist surface air approaches the core of the storm, it turns upward and ascends in a ring of cumulonimbus towers (Figure 11–4a). This doughnut-shaped wall of intense convective activity surrounding the center of the storm is called the eye wall. It is here that the greatest wind speeds and heaviest rainfall occur. Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion. Near the top of the hurricane the airflow is outward, carrying the rising air away
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Figure 11–4 (a) Cross section of a hurricane. Note that the vertical dimension is greatly exaggerated. The eye, the zone of relative calm at the center of the storm, is a distinctive hurricane feature. Sinking air in the eye warms by compression. Surrounding the eye is the eye wall, the zone where winds and rain are most intense. Tropical moisture spiraling inward creates rain bands that pinwheel around the storm center. Outflow of air at the top of the hurricane is important because it prevents the convergent flow at lower levels from “filling in” the storm. (After NOAA) (b)Measurements of surface pressure and wind speed during the passage of Cyclone Monty at Mardie Station in Western Australia between February 29 and March 2, 2004. The strongest winds are associated with the eye wall, and the weakest winds and lowest pressure are found in the eye. In this part of the world the term cyclone is used instead of hurricane. (Data from World Meteorological Organization)
Chapter 11 Hurricanes
Concept Check 11.1 1 Define hurricane. What other names are used for this storm? 2 In what latitude zone do hurricanes develop? 3 Distinguish between the eye and the eye wall of a hurricane. How do conditions differ in these zones?
Hurricane Formation andDecay A hurricane is a heat engine that is fueled by the latent heat liberated when huge quantities of water vapor condense. The amount of energy produced by a typical hurricane in just a single day is truly immense. The release of latent heat warms the air and provides buoyancy for its upward flight. The result is to reduce the pressure near the surface, which in turn encourages a more rapid inflow of air. To get this engine started, a large quantity of warm, moist air is required, and a continuous supply is needed to keep it going. As the graph in Figure 11–5 illustrates, hurricanes most often form in late summer and early fall. It is during this span that sea-surface temperatures reach 27°C (80°F) or higher and are thus able to provide the necessary heat and moisture to the air (Figure 11–6). This ocean-water temperature requirement accounts for the fact that hurricane formation over the relatively cool waters of the South Atlantic and the eastern South Pacific is extremely rare. For the same reason, few hurricanes form poleward of 20° latitude (see Figure11–2). Although water temperatures are sufficiently high, hurricanes do not form within 5° of the equator because the Coriolis force is too weak in that region to initiate the necessary rotary motion.
110 100 Number of storms per 100 years
from the storm center, thereby providing room for more inward flow at the surface. At the very center of the storm is the eye of the hurricane. This well-known feature is a zone where precipitation ceases and winds subside. The graph in Figure 11–4b shows changes in wind speed and air pressure as Cyclone Monty came ashore at Mardie Station in Western Australia between February 29 and March 2, 2004. The very steep pressure gradient and strong winds associated with the eye wall are evident, as is the relative calm of the eye. The eye offers a brief but deceptive break from the extreme weather in the enormous curving wall clouds that surround it. The air within the eye gradually descends and heats by compression, making it the warmest part of the storm. Although many people believe that the eye is characterized by clear blue skies, this is usually not the case because the subsidence in the eye is seldom strong enough to produce cloudless conditions. Although the sky appears much brighter in this region, scattered clouds at various levels are common.
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Figure 11–5 Frequency of tropical storms and hurricanes from May 1 through December 31 in the Atlantic basin. The graph shows the number of storms to be expected over a span of 100years. The period from late August through October is clearly the most active. (After National Hurricane Center/NOAA)
Hurricane Formation Many tropical storms achieve hurricane status in the western parts of oceans, but their origins often lie far to the east. In such locations, disorganized arrays of clouds and thunderstorms, called tropical disturbances, sometimes develop and exhibit weak pressure gradients and little or no rotation. Most of the time these zones of convective activity die out. However, tropical disturbances occasionally grow larger and develop strong cyclonic rotation. Several different situations can trigger tropical disturbances. They are sometimes initiated by the powerful
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Figure 11–6 Among the necessary ingredients for a hurricane is warm ocean temperatures above 27°C (80°F). This color-coded satellite image from June 1, 2010, shows sea-surface temperatures at the beginning of hurricane season. (NASA)
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convergence and lifting associated with the intertropical convergence zone (ITCZ). Others form when a trough from the middle latitudes intrudes into the tropics. Tropical disturbances that produce many of the strongest hurricanes that enter the western North Atlantic and threaten North America often begin as large undulations or ripples in the trade winds known as easterly waves, so named because they gradually move from east to west. Figure 11–7 illustrates an easterly wave. The lines on this simple map are not isobars. Rather, they are streamlines, lines drawn parallel to the wind direction used to depict surface airflow. When middle-latitude weather is analyzed, isobars are usually drawn on the weather map. By contrast, in the tropics the differences in sea-level pressure are quite small, so isobars are not always useful. Streamlines are helpful because they show where surface winds converge and diverge. To the east of the wave axis the streamlines move poleward and get progressively closer together, indicating that the surface flow is convergent. Convergence encourages air to rise and form clouds. Therefore, the tropical disturbance is located on the east side of the wave. To the west of the wave axis, surface flow diverges as it turns toward the equator. Consequently, clear skies are the rule here. Easterly waves frequently originate as disturbances in Africa. As these storms head westward with the prevailing trade winds, they encounter the cold Canary Current (see Figure 3–9, p. 72). If the disturbance survives the trip across the cold stabilizing waters of the current, it is rejuvenated by the heat and moisture of the warmer water of the midAtlantic. From this point on, a disturbance may develop
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Figure 11–7 Easterly wave in the subtropical North Atlantic. Streamlines show low-level airflow. To the east of the wave axis, winds converge as they move slightly poleward. To the west of the axis, flow diverges as it turns toward the equator. Tropical disturbances are associated with the convergent flow of the easterly wave. Easterly waves extend for 2000 to 3000 kilometers (1200 to 1800miles) and move from east to west with the trade winds at rates between 15 and 35 kilometers (10 and 20 miles) per hour. At this rate, it takes an imbedded tropical disturbance a week or 10 days to move across the North Atlantic.
into a more intense and organized system; some of these may reach hurricane status. Even when conditions seem to be right for hurricane formation, many tropical disturbances do not strengthen. One circumstance that may inhibit further development is a temperature inversion called a trade wind inversion. It forms in association with the subsidence that occurs in the region influenced by the subtropical high.* A strong inversion diminishes the ability of air to rise and thus inhibits the development of strong thunderstorms. Another factor that works against the strengthening of tropical disturbances is strong upper-level winds. When such winds are present, a strong flow aloft disperses the latent heat released from cloud tops—heat that is essential for continued storm growth and development. What happens on those occasions when conditions favor hurricane development? As latent heat is released from the clusters of thunderstorms that make up the tropical disturbance, areas within the disturbance get warmer. As a consequence, air density lowers and surface pressure drops, creating a region of weak low pressure and cyclonic circulation. As pressure drops at the storm center, the pressure gradient steepens. In response, surface wind speeds increase and bring additional supplies of moisture to nurture storm growth. The water vapor condenses, releasing latent heat, and the heated air rises. Adiabatic cooling of rising air triggers more condensation and the release of more latent heat, which causes a further increase in buoyancy. And so it goes. Meanwhile, higher pressure develops at the top of the developing tropical depression.** This causes air to flow outward (diverge) from the top of the storm. Without this outward flow up top, the inflow at lower levels would soon raise surface pressures and thwart further storm development. Although many tropical disturbances occur each year, only a few develop into full-fledged hurricanes. Recall that tropical cyclones are called hurricanes only when their winds reach 119 kilometers (74 miles) per hour. By international agreement, lesser tropical cyclones are given different names, based on the strength of their winds. When a cyclone’s strongest winds do not exceed 63 kilometers (39miles) per hour, it is called a tropical depression. When sustained winds are between 63 and 119 kilometers (39 and 74 miles) per hour, the cyclone is termed a tropical storm. It is during this phase that a name is given (Andrew, Fran, Opal, and so on). If the tropical storm becomes a hurricane, the name remains the same (Box 11–2). Each year between 80 and 100 tropical storms develop around the world. Of them, usually half or more eventually reach hurricane status.
*In the troposphere, a temperature inversion exists when temperatures in a layer of air increase with an increase in altitude rather than decrease with height, which is usually the case. For more on how subsidence can produce an inversion, see the section “Inversions Aloft” in Chapter 13, p.370. **See Figure 6–11 and the discussion of the sea–land breeze in the section “Pressure Gradient Force” in Chapter 6.
Chapter 11 Hurricanes
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Box 11–2 Naming Tropical Storms and Hurricanes Tropical storms are named to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Tropical storms and hurricanes can last a week or longer, and two or more storms can occur in the same region at the same time. Thus, names can reduce the confusion about what storm is being described. During World War II, tropical storms were informally assigned women’s names (perhaps after wives and girlfriends) by U.S. Army Corps and Navy meteorologists who were monitoring storms over the Pacific. From 1950 to 1952, tropical storms in the North Atlantic were identified by the phonetic alphabet—Able, Baker, Charlie, and so forth. In 1953 the U.S. Weather Bureau (now the National Weather Service) switched to women’s names. The practice of using feminine names continued until 1978, when a list containing both male and female names was adopted for tropical cyclones in the eastern Pacific. In the same year the World Meteorological Organization (WMO) accepted a proposal that both male and female names be adopted for Atlantic hurricanes, beginning with the 1979 season. The WMO has created six lists of names for tropical storms over ocean areas. The names used for Atlantic, Gulf of Mexico, and Caribbean hurricanes are shown in Table11–A. The names are ordered alphabetically and do not contain names that begin with the letters Q, U, X, Y, and Z because of the scarcity of names beginning with those letters. When a tropical depression reaches tropical storm status, it is assigned the next unused name on the list. At the beginning of the next hurricane season,
names from the next list are selected, even though many names may not have been used the previous season. The names for Atlantic storms are used over again at the end of each six-year cycle unless a hurricane was particularly noteworthy. This is to avoid confusion when
storms are discussed in future years. For example, five names from the list for the extraordinary 2005 Atlantic hurricane season were retired—Dennis, Katrina, Rita, Stan, and Wilma. They were replaced by Don, Katia, Rina, Sean, and Whitney on the list for the year 2011.
TABLE 11–A Tropical Storm and Hurricane Names for the Atlantic, Gulf of Mexico, and Caribbean Sea* 2011
2012
2013
2014
2015
2016
Arlene
Alberto
Andrea
Arthur
Ana
Alex
Bret
Beryl
Barry
Bertha
Bill
Bonnie Colin
Cindy
Chris
Chantal
Cristobal
Claudette
Don
Debby
Dorian
Dolly
Danny
Danielle
Emily
Ernesto
Erin
Edouard
Erika
Earl
Franklin
Florence
Fernand
Fay
Fred
Fiona
Gert
Gordon
Gabrielle
Gonzalo
Grace
Gaston
Harvey
Helene
Humberto
Hanna
Henri
Hermine
Irene
Isaac
Ingrid
Isaias
Ida
Ian
Jose
Joyce
Jerry
Josephine
Joaquin
Julia
Katia
Kirk
Karen
Kyle
Kate
Karl
Lee
Leslie
Lorenzo
Laura
Larry
Lisa
Maria
Michael
Melissa
Marco
Mindy
Matthew
Nate
Nadine
Nestor
Nana
Nicholas
Nicole
Ophelia
Oscar
Olga
Omar
Odette
Otto
Philippe
Patty
Pablo
Paulette
Peter
Paula
Rina
Rafael
Rebekah
Rene
Rose
Richard
Sean
Sandy
Sebastien
Sally
Sam
Shary
Tammy
Tony
Tanya
Teddy
Teresa
Tobias
Vince
Valerie
Van
Vicky
Victor
Virginie
Whitney
William
Wendy
Wilfred
Wanda
Walter
* If the entire alphabetical list of names for a given year is exhausted, the naming system moves on to using letters of the Greek alphabet (alpha, beta, gamma, and so on). This issue never arose until the record-breaking 2005 hurricane season, when Tropical Storm Alpha, Hurricane Beta, tropical storms Gamma and Delta, Hurricane Epsilon, and Tropical Storm Zeta occurred after Hurricane Wilma.
Students Sometimes Ask…
Hurricane Decay
When is hurricane season?
Hurricanes diminish in intensity whenever they (1) move over ocean waters that cannot supply warm, moist tropical air; (2) move onto land; or (3) reach a location where the large-scale flow aloft is unfavorable. Richard Anthes describes the possible fate of hurricanes in the first category as follows:
Hurricane season is different in different parts of the world. People in the United States are usually most interested in Atlantic storms. The Atlantic hurricane season officially extends from June through November. More than 97 percent of tropical activity in that region occurs during this six-month span. The “heart” of the season is August through October (see Figure 11–5, p. 307). During these three months, 87 percent of the minor hurricane (category 1 and 2) days and 96 percent of the major hurricane (category 3, 4, and 5) days occur. Peak activity is in early to mid-September.
Many hurricanes approaching the North American or Asian continents from the southeast are turned toward the northeast, away from the continents, by the steering effect of an upper-level trough. This recurvature carries the storms toward higher latitudes where the ocean temperatures are
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cooler and an encounter with cool, dry polar air masses is more likely. Often the tropical cyclone and a polar front interact, with cold air entering the tropical cyclone from the west. As the release of latent heat is diminished, the upper-level divergence weakens, mean temperatures in the core fall and the surface pressure rises.*
Whenever a hurricane moves onto land, it loses its punch rapidly. For example, in Figure 11–3 notice how the isobars show a much weaker pressure gradient on September 6, after Hurricane Fran moved ashore, than on September 5, when it was over the ocean. The most important reason for this rapid demise is the fact that the storm’s source of warm, moist air is cut off. When an adequate supply of water vapor does not exist, condensation and the release of latent heat must diminish. In addition, the increased surface roughness over land results in a rapid reduction in surface wind speeds. This factor causes the winds to move more directly into the center of the low, thus helping to eliminate the large pressure differences.
Concept Check 11.2 1 What is the source of energy that drives a hurricane? 2 Why do hurricanes not form near the equator? Explain the lack of hurricanes in the South Atlantic and eastern South Pacific.
3 During what months do most tropical storms and hurricanes in
This hurricane occurred in the Atlantic in 2004. (NASA) Question 1 Did the storm occur in the North Atlantic or the South Atlantic? How did you figure this out? Question 2 Did the storm more likely occur in March or September? Question 3 Are hurricanes in this region common or rare? Explain.
the Atlantic basin occur? Why are these months favored?
4 List two factors that inhibit the strengthening of tropical disturbances.
5 Distinguish between tropical depression, tropical storm, and hurricane.
TABLE 11–1
6 Why does the intensity of a hurricane diminish rapidly when it moves onto land?
Rank
Hurricane
Year
Category
1.
Texas (Galveston)
1900
4
8000*
2.
Southeastern Florida (Lake Okeechobee)
1928
4
2500–3000
3.
Katrina
2005
4
1833
4.
Audrey
1957
4
At least 416
5.
Florida Keys
1935
5
408
6.
Florida (Miami)/ Mississippi/ Alabama/Florida (Pensacola)
1926
4
372
7.
Louisiana (Grande Isle)
1909
4
350
8.
Florida Keys/ South Texas
1919
4
287
9. (tie)
Louisiana (New Orleans)
1915
4
275
9. (tie)
Texas (Galveston)
1915
4
275
Hurricane Destruction The vast majority of hurricane-related deaths and damage are caused by relatively infrequent yet powerful storms. Table 11–1 lists the deadliest hurricanes to strike the United States between 1900 and 2011. The storm that pounded an unsuspecting Galveston, Texas, in 1900 was not just the deadliest U.S. hurricane ever but the deadliest natural disaster of any kind to affect the United States. Of course, the deadliest and most costly storm in recent memory occurred in August 2005, when Hurricane Katrina devastated the Gulf Coast of Louisiana, Mississippi, and Alabama.** Although hundreds of thousands fled before the storm made landfall, thousands of others were caught by the storm. In addition to the human suffering and *Tropical Cyclones: Their Evolution, Structure, and Effects, Meteorological Monographs, Vol. 19, no. 41 (1982), p. 61. Boston: American Meteorological Society. **Many images of this storm can be seen in “Severe and Hazardous Weather: Hurricane Katrina from Space,” which begins on p. 318.
The 10 Deadliest Hurricanes to Strike the U.S. Mainland 1900–2011 Deaths
Source: National Weather Service/National Hurricane Center, NOAA Technical Memorandum NWS TPC-5. *This number may actually have been as high as 10,000–12,000.
Chapter 11 Hurricanes
tragic loss of life that were left in the wake of Hurricane Katrina, the financial losses caused by the storm are practically incalculable, Up until August 2005, the $25 billion in damages associated with Hurricane Andrew in 1992 represented the costliest natural disaster in U.S. history. This figure was exceeded many times over when Katrina’s economic impact was calculated. The final accounting likely exceeded $100 billion. Although the amount of damage caused by a hurricane depends on several factors, including the size and population density of the area affected and the nearshore bottom configuration, certainly the most significant factor is the strength of the storm itself.
Saffir–Simpson Scale Based on the study of past storms, the Saffir–Simpson scale was established to rank the relative intensities of hurricanes (Table 11–2). Predictions of hurricane severity and damage are usually expressed in terms of this scale. When a tropical storm becomes a hurricane, the National Weather Service assigns it a scale (category) number. Category assignments are based on observed conditions at a particular stage in the life of a hurricane and are viewed as estimates of the amount of damage a storm would cause if it were to make landfall without changing size or strength. As conditions change, the category of a storm is reevaluated so that public safety officials can be kept informed. By using the Saffir–Simpson scale, the disaster potential of a hurricane can be monitored and appropriate precautions can be planned and implemented.
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A rating of 5 on the scale represents the worst storm possible, and a 1 is least severe. Storms that fall into category 5 are rare. Only three storms this powerful are known to have hit the continental United States: Andrew struck Florida in 1992, Camille pounded Mississippi in 1969, and a Labor Day hurricane struck the Florida Keys in 1935. Damage caused by hurricanes can be divided into three classes: (1) storm surge, (2) wind damage, and (3) inland freshwater flooding.
Storm Surge Without question, the most devastating damage in the coastal zone is caused by storm surge. It not only accounts for a large share of coastal property losses but is also responsible for 90 percent of all hurricane-caused deaths. A storm surge is a dome of water 65 to 80 kilometers (40 to 50 miles) wide that sweeps across the coast near the point where the eye makes landfall. If all wave activity were smoothed out, the storm surge would be the height of the water above normal tide level (Figure 11–8). In addition, tremendous wave activity is superimposed on the surge. We can easily imagine the damage that this surge of water could inflict on lowlying coastal areas (Figure 11–9). The worst surges occur in places like the Gulf of Mexico, where the continental shelf is very shallow and gently sloping. In addition, local features such as bays and rivers can cause the surge height to double and increase in speed. In the delta region of Bangladesh, for example, most of the land is less than 2 meters (6.5 feet) above sea level. When a storm surge superimposed on normal high tide
Students Sometimes Ask… Are larger hurricanes stronger than smaller hurricanes? Not necessarily. Actually, there is very little correlation between intensity (either measured by maximum sustained winds or by central pressure) and size (either measured by the radius of galeforce winds or the radius of the outer closed isobar). Hurricane Andrew is a good example of a very intense storm (category 5) that was also relatively small (gale-force winds extended only 150 kilometers [90miles] from the eye). Research has shown that changes in intensity and size are essentially independent of one another.
TABLE 11–2 Scale Number (category)
Mean sea level
Saffir–Simpson Hurricane Scale* Central Pressure (millibars)
Winds (km/hr)
Storm Surge (meters)
Damage
1
≥980
119–153
1.2–1.5
Minimal
2
965–979
154–177
1.6–2.4
Moderate
3
945–964
178–209
2.5–3.6
Extensive
4
920–944
210–250
3.7–5.4
Extreme
5
Normal high tide
920
250
5.4
Catastrophic
*A more complete version of the Saffir–Simpson hurricane scale can be found in Appendix F, p. 481.
Height of storm surge Normal high tide
Figure 11–8 Superimposed upon high tide, a storm surge can devastate a coastal area. The worst storm surges occur in coastal areas where there is a very shallow and gently sloping continental shelf extending from the beach. The Gulf Coast is such a place.
Figure 11–9 Crystal Beach, Texas, on September 16, 2008, three days after Hurricane Ike came ashore. At landfall the storm had sustained winds of 165 kilometers (105 miles) per hour. The extraordinary storm surge caused much of the damage pictured here. (Photo by Earl Nottingham/Associated Press)
inundated that area on November 13, 1970, the official death toll was 200,000; unofficial estimates ran to 500,000. It was one of the worst natural disasters of modern times. In May 1991 a similar event again struck Bangladesh. This time the storm took the lives of at least 143,000 people and devastated coastal towns in its path. A common misconception about the cause of hurricane storm surges is that the very low pressure at the cen-
North Carolina
South Carolina
nd d a of ee tion e p S rec an t di urric men h ve mo kph 50
Georgia
nd wi h ne t Net 5 kp he a t st 12 rric en Hu vemph = fromthwe r o mo50 k n on nd e Wi ft sidph le 5 k 17
Florida
312
e an t rric en Hu vemph mo50 k n d n i do e t w kph = Win t sid h e h p N 25 the 2 m est rig 75 k 1 fro thw u so
ter of the storm acts as a partial vacuum that allows the ocean to rise up in response. However, this effect is relatively insignificant. The most important factor responsible for the development of a storm surge is the piling up of ocean water by strong onshore winds. Gradually the hurricane’s winds push water toward the shore, causing sea level to elevate while also churning up violent wave activity. As a hurricane advances toward the coast in the Northern Hemisphere, storm surge is always most intense on the right side of the eye, where winds are blowing toward the shore. In addition, on this side of the storm the forward movement of the hurricane also contributes to the storm surge. In Figure 11–10, assume that a hurricane with peak winds of 175 kilometers (109 miles) per hour is moving toward the shore at 50 kilometers (31 miles) per hour. In this case, the net wind speed on the right side of the advancing storm is 225 kilometers (140 miles) per hour. On the left side, the hurricane’s winds are blowing opposite the direction of storm movement, so the net winds are away from the coast, at 125 kilometers (78 miles) per hour. Along the shore facing the left side of the oncoming hurricane, the water level may actually decrease as the storm makes landfall.
Figure 11–10 Winds associated with a Northern Hemisphere hurricane that is advancing toward the coast. This hypothetical storm, with peak winds of 175 kilometers (109 miles) per hour, is moving toward the coast at 50 kilometers (31 miles) per hour. On the right side of the advancing storm, the 175-kilometer-per-hour winds are in the same direction as the movement of the storm (50kilometers per hour). Therefore, the net wind speed on the right side of the storm is 225 kilometers (140 miles) per hour. On the left side, the hurricane’s winds are blowing opposite the direction of storm movement, so the net winds of 125 kilometers (78 miles) per hour are away from the coast. Storm surge will be greatest along the part of the coast hit by the right side of the advancing hurricane.
Chapter 11 Hurricanes
Wind Damage Destruction caused by wind is perhaps the most obvious of the classes of hurricane damage. Debris such as signs, roofing materials, and small items left outside become dangerous flying missiles in hurricanes. For some structures, the force of the wind is sufficient to cause total ruin. Just read the descriptions of category 3, 4, and 5 storms in Appendix F, p. 481. Mobile homes are particularly vulnerable. High-rise buildings are also susceptible to hurricane-force winds. Upper floors are most vulnerable because wind speeds usually increase with height. Recent research suggests that people should stay below the tenth floor but remain above any floors at risk for flooding. In regions with good building codes, wind damage is usually not as catastrophic as storm-surge damage. However, hurricane-force winds affect a much larger area than storm surge and can cause huge economic losses. For example, in 1992 it was largely the winds associated with Hurricane Andrew that produced more than $25 billion of damage in southern Florida and Louisiana. A hurricane may produce tornadoes that contribute to the storm’s destructive power. Studies have shown that more than half of the hurricanes that make landfall produce at least one tornado. In 2004 the number of tornadoes associated with tropical storms and hurricanes was extraordinary. Tropical Storm Bonnie and five landfalling hurricanes— Charley, Frances, Gaston, Ivan, and Jeanne—produced nearly 300 tornadoes that affected the southeast and midAtlantic states (Table 11–3). Hurricane Frances produced the most tornadoes ever reported from one hurricane. The large number of hurricane-generated tornadoes in 2004 helped make this a record-breaking year for tornadoes—surpassing the previous record by more than 300.*
Heavy Rains and Inland Flooding The torrential rains that accompany most hurricanes represent a third significant threat—flooding. The 2004 hurricane season was very deadly, with a loss of life that exceeded 3000 people. Nearly all of the deaths occurred in Haiti, as a result of flash floods and mudflows caused by the heavy rains associated with then Tropical Storm Jeanne.
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Hurricane Agnes (1972) illustrates that even modest storms can have devasting results. Although it was only a category 1 storm on the Saffir–Simpson scale, it was one of the costliest hurricanes of the twentieth century, creating more than $2 billion in damage and taking 122 lives. The greatest destruction was attributed to flooding in the northeastern portion of the United States, especially in Pennsylvania, where record rainfalls occurred. Harrisburg received nearly 32 centimeters (12.5 inches) in 24 hours, and western Schuykill County measured more than 48 centimeters (19 inches) during the same span. Agnes’s rains were not as devastating elsewhere. Prior to reaching Pennsylvania, the storm caused some flooding in Georgia, but most farmers welcomed the rain because dry conditions had been plaguing them earlier. In fact, the value of the rains to crops in the region far exceeded the losses caused by flooding. Another well-known example is Hurricane Camille (1969). Although this storm is best known for its exceptional storm surge and the devastation it brought to coastal areas, the greatest number of deaths associated with this storm occurred in the Blue Ridge Mountains of Virginia two days after Camille’s landfall. Many areas received more than 25centimeters (10 inches) of rain, and severe flooding took more than 150 lives. To summarize, extensive damage and loss of life in the coastal zone can result from storm surge, strong winds, and torrential rains. When loss of life occurs, it is commonly caused by storm surge, which can devastate entire barrier islands or zones within a few blocks of the coast. Although wind damage is usually not as catastrophic as storm surge, it affects a much larger area. Where building codes are inadequate, economic losses can be especially severe. Because hurricanes weaken as they move inland, most wind damage occurs within 200 kilometers (125 miles) of the coast. Far from the coast, a weakening storm can produce extensive flooding long after the winds have diminished below hurricane levels. Sometimes the damage from inland flooding exceeds storm-surge destruction.
Concept Check 11.3 1 What is the purpose of the Saffir–Simpson scale? 2 What are the three broad categories of hurricane damage? Provide brief examples of each.
TABLE 11–3
Number of Tornadoes Spawned by Hurricanes and Tropical Storms in the United States, 2004
Tropical Storm Bonnie
30
Hurricane Charley
25
Hurricane Frances
117
Hurricane Gaston Hurricane Ivan Hurricane Jeanne
1 104 16
*K. L. Gleason, et al., “U.S. Tornado Records” in Bulletin of the American Meteorological Society, Vol. 86, No. 6 ( June 2005), p. 551.
3 Which side of an advancing hurricane in the Northern Hemisphere has the strongest winds and highest storm surge—right or left? Why does one side of the storm have stronger winds than the other side?
Estimating the Intensity ofaHurricane The Saffir–Simpson hurricane scale appears to be a straightforward tool. However, the accurate observations needed to correctly portray hurricane intensity at the surface are
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Cyclone Nargis
A
ll three classes of hurricane damage came into play in May2008, when the first cyclone of the season in the northern Indian Ocean struck Myanmar (also called Burma). The satellite image in Figure 11–B shows the storm over the Bay of Bengal and the map shows the path of the storm and the rainfall it produced. The storm devastated much of the fertile Irrawaddy Delta (Figure 11–C). According to a United Nations estimate, more than 90percent of the dwellings were destroyed in the hardest-hit areas. In addition, death and destruction also occurred in portions of Yangon, the nation’s largest city. In all, about 30,000 square kilometers (nearly 11,600 square miles) were significantly affected—an area that was home to nearly one-quarter of Myanmar’s 57 millionpeople.
More than 90percent of the dwellings were destroyed in the hardest-hit areas. A storm surge of 3.6 meters (12 feet) and torrential rains (up to 600 mm [2feet]
FIGURE 11–B The small satellite image shows Cyclone Nargis hovering over the Bay of Bengal on May 1, 2008. At one point Nargis was a category 4 storm on the Saffir–Simpson scale, with winds of 210 kilometers (131 miles) per hour. Although it lost strength before coming ashore on May 2 as a category 2 storm, Nargis still had powerful winds and torrential rains. (NASA) The map shows rainfall accumulations along the path of Cyclone Nargis between April27 and May 4, 2008, using data from the Tropical Rainfall Measuring Mission Satellite. Rainfall totals range up to 600millimeters (nearly 2 feet). The path of the storm and its strength on various dates are shown as colored line segments. (NASA)
China Bangladesh Myanmar (Burma)
Vietnam
May 4 Bay of Bengal
Laos
May 2 Thailand
India
April 30 Cambodia
April 27
N Total Rainfall (mm) 0
300
Storm Intensity (Category) 600
TS 1
2
3
4
in some places) brought widespread flooding. Powerful winds contributed to the destruction. The fact that this low-lying region was densely populated and had a high proportion of poorly constructed dwellings exacerbated the situation. The government reported the official death toll to be nearly 85,000, with an additional 54,000 people unaccounted for. It is likely that more than 100,000 perished. It was the deadliest cyclone to hit Asia since 1991, when 143,000people died in Bangladesh. Many who survived the storm lost their homes; an estimated 2million people were displaced. FIGURE 11–C The storm surge and heavy rains produced by Cylone Nargis combined to cause extensive flooding on the low-lying Irrawaddy Delta, killing thousands and displacing as many as 2 million people. (Photo by Mandalay Gazette, HO/AP Photo)
Chapter 11 Hurricanes
Mosambique Channel
Zimbabwe A B
Square-cone parachute increases stability of dropsonde
315
Vents fill chute within 10 seconds after release from aircraft
Shock cord reduces stress when chute opens C
GPS antenna
D
GPS receiver collects data from GPS satellites used to calculate wind speed and direction
South Africa
Pressure sensor (NASA)
N
This satellite image shows Tropical Cyclone Favio as it came ashore along the coast of Mozambique, Africa, on February 22, 2007. This powerful storm was moving from east to west. Portions of the cyclone had sustained winds of 203 kilometers (126 miles) per hour as it made landfall. Question 1 Identify the eye and eye wall of the storm. Question 2 Based on wind speed, classify the storm using the Saffir–Simpson scale. Question 3 Which one of the lettered sites should experience the strongest storm surge? Explain.
sometimes difficult to obtain. Estimating hurricane intensity is difficult because direct surface observations in the eye wall are rarely available. Therefore, winds in this most intense part of the storm have to be estimated. One of the best ways to estimate surface intensity is to adjust the wind speeds measured by reconnaissance aircraft. Winds aloft are stronger than winds at the surface. Therefore, the adjustment of values determined for winds aloft to values expected at the surface involves reducing the measurements made aloft. However, until the late 1990s, determining the proper adjustment factor was problematic because surface observations in the eye wall were too limited to establish a broadly accepted relationship between flight-level and surface winds. In the early 1990s the reduction factors commonly used ranged from 75 to 80 percent (that is, surface wind speeds were assumed to be between 75 and 80 percent of the speed at 3000 meters [10,000 feet]). Some scientists and engineers even maintained that surface winds were as low as 65 percent of flight-level winds. Beginning in 1997 a new instrument, called a Global Positioning System (GPS) dropwindsonde, came into use (sometimes just called a dropsonde). After being released from the aircraft, this package of instruments, slowed by a small parachute, drifts downward through the storm (Figure 11–11).
Humidity sensors and temperature sensor
Microprocessor controls the transmitter and digitizes data from the sensors Battery pack provides power for at least one hour Radio transmitter sends temperature, humidity, pressure, and GPS (wind) data to the aircraft every 0.5 second Fall speed ranges from 36 mph at 20,000 feet to 24 mph at sea level. A drop from 20,000 feet lasts 7 minutes.
Figure 11–11 The Global Positioning System (GPS) dropwindsonde is frequently just called a dropsonde. This cylindrical instrument package is roughly 7 centimeters (2.75inches) in diameter, 40 centimeters (16 inches) long, and weighs about 0.4 kilogram (0.86 pound). The instrument package is released from an aircraft and falls through the storm via a parachute, making and transmitting measurements of temperature, pressure, winds, and humidity every half-second. (After NASA)
During the descent, it continuously transmits data on temperature, humidity, air pressure, wind speed, and wind direction. The development of this technology provided, for the first time, a way to accurately measure the strongest winds in a hurricane, from flight level all the way to the surface. Over a span of several years, hundreds of GPS dropwindsondes were released in hurricanes. The data accumulated from these trials showed that the speed of surface winds in the eye wall averaged about 90 percent of the flight-level winds, not 75 to 80 percent. Based on this new understanding, the National Hurricane Center now uses the 90 percent figure to estimate a hurricane’s maximum surface winds from flight-level observations. This means that the winds in some storms in the historical record were underestimated. For example, in 1992 the surface winds in Hurricane Andrew were estimated to be 233 kilometers (145 miles) per hour. This was 75 to 80 percent of the value measured at 3000 meters by the reconnaissance aircraft. When scientists at the National Hurricane Center reevaluated the storm using the 90 percent value, they concluded that the maximum-sustained surface winds had been 266 kilometers (165 miles)
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per hour—33 kilometers (20 miles) per hour faster than the original 1992 estimate. Consequently, in August 2002 the intensity of Hurricane Andrew was officially changed from category 4 to category 5. The upgrade makes Hurricane Andrew only the third category 5 storm on record to strike the continental United States. (The other two were the Florida Keys 1935 Hurricane and Hurricane Camille in 1969.)
Concept Check 11.4 1 Why is estimating the surface intensity of a hurricane difficult? 2 What is a GPS dropwindsonde? 3 The intensity of which hurricane was upgraded from category 4 to category 5, using data provided by GPS dropwindsondes?
Detecting, Tracking, and Monitoring Hurricanes Figure 11–12 shows the paths followed by some notable Atlantic hurricanes. What determines these tracks? The storms can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere. The movement of hurricanes has been likened to a leaf being carried along by the currents in a stream, except that for a hurricane the “stream” has no set boundaries. In the latitude zone equatorward of about 25° north, tropical storms and hurricanes commonly move to the west, with a slight poleward component. This occurs because of the semipermanent cell of high pressure (called
the Bermuda High) that is positioned poleward of the storm (see Figure 7–10b, p. 200). On the equatorward side of this high-pressure center, easterly winds prevail, guiding the storms westward. If this high-pressure center is weak in thewestern Atlantic, the storm often turns northward. On the poleward side of the Bermuda High, westerly winds prevail, steering the storm back toward the east. Often it is difficult to determine whether the storm will curve back out to sea or whether it will continue straight ahead and make landfall. A location only a few hundred kilometers from a hurricane—just a day’s striking distance away—may experience clear skies and virtually no wind. Before the age of weather satellites, such a situation made it difficult to warn people of impending storms. The worst natural disaster in U.S. history came as a result of a hurricane that struck an unprepared Galveston, Texas, on September8, 1900. The strength of the storm, together with the lack of adequate warning, caught the population by surprise and cost the lives of 6000people in the city and at least 2000 more elsewhere (Figure 11–13).* In the United States, early warning systems have greatly reduced the number of deaths caused by hurricanes. At the same time, however, an astronomical rise has occurred in the amount of property damage. The primary reason for this latter trend is the rapid population growth and accompanying development in coastal areas.
The Role of Satellites
Today many different tools provide data that are used to detect and track hurricanes. This information is used to develop forecasts and to issue watches and warnings. The greatest single advancement in tools used for observing tropical cyclones has been the development of meteorological satellites. Galveston 1900 Because the tropical and subCamille 1969 tropical regions that spawn hurAndrew 1992 ricanes consist of enormous areas 50° Opal 1995 of open ocean, conventional obFloyd 1999 servations are limited. The need Katrina 2005 for meteorological data from these Ike 2008 vast regions is now met primarily 40° by satellites. Even before a storm begins to develop cyclonic flow and the spiraling cloud bands so ATLANTIC OCEAN typical of a hurricane, the storm 30° can be detected and monitored by satellites. 20°
10°
GULF OF MEXICO
100°
90°
80°
70°
60°
50°
40°
*For a fascinating account of the Galveston storm, read Isaac’s Storm by Erik Larson (New York: Crown Publishers, 1999).
Figure 11–12 This map shows a variety of tracks for some memorable hurricanes. To examine the history of many important storms, visit this interesting interactive site: http:// www.csc.noaa.gov/hurricanes/.
Chapter 11 Hurricanes
317
Figure 11–13 Aftermath of the Galveston hurricane of 1900. Entire blocks were swept clean, and mountains of debris accumulated around the few remaining buildings. (AP Photos)
The advent of weather satellites has largely solved the problem of detecting tropical storms and has significantly improved monitoring. The box “Severe and Hazardous Weather: Hurricane Katrina from Space” (p. 318) provides excellent examples. However, satellites are remote sensors, and it is not unusual for wind-speed estimates to be off by tens of kilometers per hour and for storm-position estimates to have errors. It is still not possible to precisely determine detailed structural characteristics. A combination of observing systems is necessary to provide the data needed for accurate forecasts and warnings.
Aircraft Reconnaissance Aircraft reconnaissance represents a second important source of information about hurricanes. Ever since the first experimental flights into hurricanes were made in the 1940s, the aircraft and the instruments employed have become quite sophisticated (Figure 11–14). When a hurricane is
within range, specially instrumented aircraft can fly directly into a threatening storm and accurately measure details of its position and current state of development. Data transmission can be made directly from an aircraft in the midst of a storm to the forecast center, where input from many sources is collected and analyzed. Measurements from reconnaissance aircraft are limited because they cannot be taken until a hurricane is relatively close to shore. Moreover, measurements are not taken continuously or throughout the storm. Rather, the aircraft provides sample “snapshots” of small parts of the hurricane. Nevertheless, the data collected are critical in analyzing the current characteristics needed to forecast the future behavior of a storm. A major contribution to hurricane forecasting and warning programs has been an improved understanding of the structure and characteristics of these storms. Although advancements in remote sensing from satellites have been made, measurements from reconnaissance aircraft will be Figure 11–14 In the Atlantic basin, most operational hurricane reconnaissance is carried out by the U.S. Air Force Reconnaissance Squadron based at Keesler AFB, Mississippi. Pilots fly through the hurricane to its center, measuring all basic weather elements as well as providing an accurate location of the eye. They use planes like the one in the background of this image. The National Oceanic and Atmospheric Administration uses small, speciallyequipped jets (foreground) mostly on research missions to aid scientists in better understanding these storms. (Photo by NOAA)
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Hurricane Katrina from Space
S
atellites allow us to track the formation, movement, and growth of hurricanes. In addition, their specialized instruments provide data that can be transformed into images that allow scientists to analyze the internal structure and workings of these huge storms. The images and captions in this box provide a unique perspective of Hurricane Katrina, the most devastating storm to strike the United States in more than a century. Figure 11–D is from NASA’s Terra satellite and is a relatively “traditional” image that shows Katrina approaching the Gulf coast. Figure 11–E is a color-enhanced infrared (IR) image from the GOES-East satellite. Recall from Chapter 2 that the wavelengths of radiation emitted by an object are temperature dependent. Longer IR wavelengths indicate cooler temperatures, and shorter wavelengths are associated with warmer temperatures. The high tops of towering cumulonimbus clouds are colder than the tops
Alabama Florida Mississippi
Texas
FIGURE 11–E Color-enhanced infrared image from the GOES-East satellite of Hurricane Katrina several hours before it made landfall on August 29, 2005. The most intense activity is associated with red and orange. (NOAA)
Louisiana
FIGURE 11–D This image shows Katrina at 1 PM on Sunday, August 28, 2005, as a massive storm covering much of the Gulf of Mexico. After passing over Florida as a category 1 hurricane, Katrina entered the Gulf and intensified into a category 5 storm with winds of 257 kilometers (160 miles) per hour and even stronger gusts. Air pressure at the center of the storm measured 902 millibars. When Katrina came ashore the next day, it was a slightly less vigorous category 4 storm. (NASA image)
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FIGURE 11–F NASA’s QuikSCAT satellite was the source of data for this image of Hurricane Katrina on August 28, 2005. The image depicts relative wind speeds. The strongest winds, shown in shades of purple, circle a well-defined eye. The barbs show wind direction. (NASA image)
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3.2 6.4 9.6 12.8 in. FIGURE 11–G Storm track and rainfall values for Hurricane Katrina for the period August 23 to 31, 2005. Rainfall amounts are derived from satellite data. The highest totals (dark red) exceeded 30 centimeters (12 inches) over northwestern Cuba and the Florida Keys. Amounts over southern Florida (green to yellow) were 12–20 centimeters (5–8 inches). Rainfall along the Mississippi coast (yellow to orange) was between 15–23 centimeters (6–9 inches). After coming ashore, Katrina moved through Mississippi, western Tennessee, and Kentucky and into Ohio. Because the storm moved rapidly, rainfall totals (green to blue) in these areas were generally less than 13 centimeters (5 inches). (NASA image)
of clouds at lower altitudes (less vertically developed clouds). In this image ofKatrina taken a few hours before landfall, the highest (coldest) cloud tops and thus the most intense storms are easily seen. Meteorologists use color-enhanced imagery to aid with satellite interpretation. The colors enable them to easily and quickly see features that are of special interest. Figure 11–F from NASA’s QuikSCAT satellite is very different in appearance. It provides a detailed look at Katrina’s surface winds shortly before the storm made landfall. This image depicts relative wind speeds rather than actual values. The satellite sends out high-frequency radio waves, some of which bounce off the ocean and return to the satellite. Rough, storm-tossed seas create a strong signal, whereas a smooth surface returns a weaker signal. In order to match wind speeds with the type of signal that returns to the satellite, scientists compare wind measurements taken by data buoys in the ocean to the strength of the signal received by the satellite. When there are too few data buoy measurements to compare to the satellite data, exact wind speeds cannot be determined. Instead, the image provides a clear picture of relative wind speeds. Figure 11–G shows the Multi-satellite Precipitation Analysis (MPA) of the storm. This image, which also depicts the track of the storm, shows the overall pattern of rainfall. It was constructed from data collected over several days by the Tropical Rainfall Measuring Mission (TRMM) satellite and other satellites. Figure 11–H provides yet another satellite perspective. Seeing the pattern of rainfall in different parts of a hurricane is very useful to forecasters because it helps determine the strength of the storm. Scientists have developed a way to process data from the Precipitation Radar (PR) aboard the TRMM satellite within three hours and display it in 3-D. Every time the satellite passes over a named tropical cyclone anywhere in the world, the PR instrument sends data to create a 3-D snapshot of the storm. Such an image provides information on how heavily the rain is falling in different parts of the storm, such
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as the eye wall versus the outer rain bands. It also gives a 3-D look at the cloud heights and “hot towers” inside the storm. Higher hot towers in the eye wall usually indicate a strengthening storm.
FIGURE 11–H Tropical Rainfall Measuring Mission (TRMM) Satellite image of Hurricane Katrina early on August 28, 2005. This cutaway view of the inner portion of the storm shows cloud height on one side and rainfall rates on the other. The tall rain columns provide a clue that the storm is strengthening. Two isolated towers (in red) are visible: one in an outer rain band and the other in the eye wall. The eye wall tower rises 16 kilometers (10 miles) above the ocean surface and is associated with an area of intense rainfall. Towers this tall near the core are often an indication that a storm is intensifying. Katrina grew from a category 3 to a category 4 storm soon after this image was received. (NASA image)
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required for the foreseeable future to maintain the present level of accuracy for forecasts of potentially dangerous tropical storms.
Radar and Data Buoys Radar is a third basic tool in the observation and study of hurricanes (Figure 11–15). When a hurricane nears the coast, it is monitored by land-based Doppler weather radar.* Doppler radar provides detailed information on hurricane wind fields, rainfall intensity, and storm movement. As a result, local National Weather Service offices are able to provide short-term warnings for floods, tornadoes, and high winds for specific areas. Sophisticated mathematical calculations provide forecasters with important information derived from the radar data, such as estimates of rainfall amounts. A limitation of radar is *For a more complete discussion of this important tool, see the section “Doppler Radar” in Chapter 10.
Figure 11–15 Doppler radar image of Hurricane Charley over Charlotte Harbor, Florida, just after landfall on August 13, 2004. The range of coastal radar is about 320 kilometers (200miles). (NOAA/National Weather Service)
Chapter 11 Hurricanes
that it cannot “see” farther than about 320 kilometers (200 miles) from the coast, and hurricane watches and warnings must be issued long before a storm comes into range.
Data Buoys Data buoys represent a fourth method of gathering data for the study of hurricanes (see Figure 12–3, p. 330). These remote, floating instrument packages are positioned in fixed locations all along the Gulf Coast and Atlantic Coast of the United States. When you examine the weather maps in Figure 11–3, you can see data buoy information plotted at several offshore stations. Ever since the early 1970s, data provided by these units have become a dependable and routine part of daily weather analysis aswell as an important element of the hurricane warning system. The buoys represent the only means of making nearly continuous direct measurements of surface conditions over ocean areas.
Hurricane Watches and Warnings Using input from the observational tools that were just described in conjunction with sophisticated computer models,
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VORTRAC—A New Hurricane Tracking Technique Rapidly intensifying storms can catch vulnerable coastal areas by surprise. In 2007 Hurricane Humberto struck near Port Arthur, Texas, after unexpectedly strengthening from a tropical depression to a hurricane in less than 19 hours. In 2004, parts of the southwest coast of Florida were caught off guard when Hurricane Charley’s top winds increased from 175 to more than 230 kilometers (110 to 145 miles) per hour in just 6 hours as the storm approached land. A new technique known as VORTRAC (Vortex Objective Radar Tracking and Circulation) was successfully tested in 2007 and put into use in 2008. It is intended to help improve short-term hurricane warnings by capturing sudden intensity changes in potentially dangerous hurricanes during the critical time when storms are nearing land. Prior to the development of VORTRAC, the only way to monitor the intensity of a landfalling hurricane was to rely on aircraft that dropped instrument packages (dropwindsondes) into the storm. This can be done only every hour or two, which means that a sudden drop in barometric pressure and the accompanying increase in winds could be difficult to detect in a timely manner. VORTRAC uses that portion of the Doppler radar network that is along the Gulf and Atlantic coastline from Maine to Texas. Each unit can measure winds blowing toward or away from it, but until VORTRAC was established, no single radar could estimate a hurricane’s rotational winds and central pressure. The technique uses a series of mathematical formulas that combine data from a single radar with knowledge of Atlantic hurricane structure in order to map the storm’s rotational winds. VORTRAC also infers the barometric pressure in the eye of the hurricane, which is a reliable indicator of its strength (Figure 11–16). Each radar can sample conditions out to a distance of 190kilometers (120 miles), and forecasters using VORTRAC can update the status of a storm about every six minutes.
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Figure 11–16 This graph displays the results of a test of the VORTRAC system on Hurricane Humberto, a storm that intensified rapidly as it approached the Texas/Louisiana coast on September13, 2007. The VORTRAC estimates agreed closely with Humberto’s actual central pressure, as measured by reconnaissance planes just before landfall. VORTRAC also captured the storm’s rapid intensification, indicating that the pressure began falling quickly near the end of a four-hour span when no aircraft data were available. (After M. Bell/National Center for Atmospheric Research)
meteorologists attempt to forecast the movements and intensity of a hurricane. The goal is to issue timely watches and warnings. A hurricane watch is an announcement that hurricane conditions are possible within a specified coastal area. Because preparedness activities become difficult once winds reach tropical storm force, a hurricane watch is issued 48 hours in advance of the anticipated onset of tropical storm–force winds. By contrast, a hurricane warning is issued 36 hours in advance and indicates that hurricane conditions are expected somewhere within a specified coastal area. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force. Two factors are especially important in the watch-andwarning decision process. First, adequate lead time must be provided to protect life and, to a lesser degree, property. Second, forecasters must attempt to keep overwarning at a minimum. This, however, can be a difficult task. Clearly, the decision to issue a warning represents a delicate balance between the need to protect the public on the one hand andthe desire to minimize the degree of overwarning on the other.
Hurricane Forecasting* Hurricane forecasts are a basic part of any warning program. Several aspects can be part of such a forecast. We certainly want to know where a storm is headed. The predicted path *Based on “Hurricane Forecasting in the United States: An Information Statement of the AMS,” Bulletin of the American Meteorological Society, Vol. 88, No. 6 ( June 2007), pp. 950–953.
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Daniel Brown: Senior Hurricane Specialist, National Hurricane Center They say “to everything there is a season.” Well, to Daniel Brown there are two seasons: hurricane and non-hurricane. During hurricane season, Brown—a senior hurricane specialist and the warning coordination meteorologist at the National Hurricane Center in Miami—monitors weather conditions in the Atlantic and east Pacific basins. If a storm is brewing, Brown is one of the operational forecasters charged with determining what its track, strength, and size will be for the upcoming five days. This, he says, is the most exciting aspect of his job. “I love walking into the office every day and looking at satellite imagery to see what’s going on around the globe.” The National Hurricane Center uses data from satellites, airplane reconnaissance, and sophisticated computer models when issuing storm watches and warnings. Advances in technology have helped this effort. “We are much better than we were 20 years ago at predicting the path of a storm,” Brown says.
We are much better than we were 20 years ago at predicting the path of a storm. Even so, the National Hurricane Center can’t rely on technology alone to make good predictions. Sometimes, for example, instruments that measure winds at the surface of the ocean might conflict with satellite data. “When data conflict, that’s where the hurricane specialist’s experience comes in. Is the storm a category 1, or is it a category 2? The specialist can determine why one measurement might not agree with another and make the best determination,” says Brown. The off-season is also busy. “A lot happens outside of hurricane season,” says Brown, referring to the National Hurricane
Center’s outreach, training, and education effort. “Run from water. Hide from wind” is a mantra Brown gives to the public and emergency management personnel on hurricane preparedness. “Communicating risk to people is a difficult challenge. With our outreach and education effort, we are always looking for new ways to reach people, especially if they live on the coast or in an evacuation zone.” Social media now play a role. “Twitter and Facebook have been positive avenues for reaching thousands of people.”
Communicating risk to people is a difficult challenge. The National Hurricane Center in Miami is also a Regional Specialized Meteorological Center (RSMC), part of the United Nations’s World Meteorological Organization. Brown and his colleagues provide forecasts to all countries in the Atlantic basin and also teach courses to international forecasters and emergency management personnel. Brown credits his father for sparking a serious interest in meteorology. When Brown was growing up, his father was an avid weather watcher and never lost interest in learning more about the weather. By high school, Brown knew that he wanted to pursue a career in meteorology. Brown majored in meteorology at the University of North Carolina at Asheville. While there, he worked as an intern at NOAA’s National Climatic Data Center in Asheville for a year and a half. In 1993, one year after Hurricane Andrew struck Florida, Brown got an internship at the Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center. He also worked for three years in the Miami National Weather Service Forecast Office (WFO), where he issued the
of a storm is called the track forecast. Of course, there is also interest in knowing the intensity (strength of the winds), probable rainfall amounts, and likely size of the storm surge. Track Forecasts The track forecast is probably the most basic information because accurate prediction of other storm characteristics is of little value if there is significant uncertainty about where the storm is going. Accurate track forecasts are important because they can lead to timely evacuations from the surge zone, where the greatest number of deaths usually occur. Fortunately, track forecasts have been steadily improving. During the span 2001–2005, forecast er-
warning for a tornado that struck downtown Miami. After working for the WFO, he went back to TAFB and became a hurricane specialist in 2006. At TAFB, Brown has been involved in improving the National Hurricane Center’s storm tracking time frames. The center used to issue watches 36 hours in advance and warnings 24 hours in advance. Thanks to more sophisticated computer models, the National Hurricane Center now issues watches and warnings 12 hours earlier, helping emergency management personnel to prepare sooner. After a storm is over, Brown and his colleagues put together a Tropical Cyclone Report. “It’s basically the ‘what happened’ of the storm,” he says. They reanalyze all the data collected during the storm and compile size and intensity estimates for every six hours of the storm’s life. The report goes into the long-term climate record and helps the National Hurricane Center determine forecasting errors and areas for improvement.
DANIEL BROWN, Meteorologist Dan Brown is a Senior Hurricane Specialist at the National Hurricane Center in Miami, Florida. (NOAA)
rors were roughly half of what they were in 1990. During the very active 2004 and 2005 Atlantic hurricane seasons, 12- to 72-hour track forecast accuracy was at or near record levels. Consequently, the length of official track forecasts issued by the National Hurricane Center was extended from three days to five days (Figure 11–17). Current five-day track forecasts today are as accurate as three-day forecasts were 15 years ago. Much of the progress is due to improved computer models and a dramatic increase in the quantity of satellite data from over the oceans. Despite improvements in accuracy, forecast uncertainty still requires that hurricane warnings be issued for relatively
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Figure 11–17 Five-day track forecast for Tropical Storm Dean issued at 5 PM Tuesday, August 14, 2007. When a hurricane track forecast is issued by the National Hurricane Center, it is termed a forecast cone. The cone represents the probable track of the center of the storm and is formed by enclosing the area swept out by a set of circles along the forecast track (at 12hours, 24 hours, 36 hours, and so on). The size of each circle gets larger with time. Based on statistics from 2003–2007, the entire track of an Atlantic tropical cyclone can be expected to remain entirely within the cone roughly 60 to 70 percent of the time. (National Weather Service/National Hurricane Center)
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large coastal areas. During the span 2000–2005, the average length of coastline under a hurricane warning in the United States was 510 kilometers (316 miles). This represents a significant improvement over the preceding decade, when the average was 730 kilometers (452 miles). Nevertheless, only about one-quarter of an average warning area experiences hurricane conditions. Forecasting Other Characteristics In contrast to the improvements in track forecasts, errors in forecasts of hurricane intensity (wind speeds) have not changed significantly in 30 years. Accurate predictions of rainfall as hurricanes make landfall also remains elusive. However, accurate predictions of impending storm surge are possible when good information regarding the storm’s track and surface wind
Give It Some Thought 1. Why might people in some parts of the world welcome the arrival of hurricane season? 2. Hurricanes are sometimes referred to as “heat engines.” What is the “fuel” that provides the energy for these high-powered engines? 3. The accompanying world map shows the tracks and intensities of nearly 150 years of tropical cyclones. It is based on all storm tracks available from the National Hurricane Center and the Joint Typhoon Warning Center. a. What area has experienced the greatest number of category 4 and 5 storms? b. Why do hurricanes not form in the very heart of the tropics, astride the equator? c. Explain the absence of storms in the South Atlantic and eastern South Pacific.
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Concept Check 11.5 1 What was the worst natural disaster in United States history? Why is such an event unlikely to occur in the United States again?
2 List four tools that provide data used to track hurricanes and develop forecasts.
3 Distinguish between a hurricane watch and a hurricane warning.
4 What is a track forecast? Why are such forecasts important?
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4. Refer to the graph in Figure 11–4b. Explain why wind speeds are greatest when the slope of the pressure curve is steepest. 5. Although observational tools and hurricane forecasts continue to improve, the potential for loss of life due to hurricanes is likely growing. Suggest a reason for this apparent contradiction. 6. Assume that it is late September 2016, and Hurricane Gaston, a category 5 storm, is projected to follow the path shown on the accompanying map. Answer the following questions.
b. Should the city of Houston expect to experience Gaston’s fastest winds and greatest storm surge? Explain why or why not. c. What is the greatest threat to life and property if this storm approaches the Dallas–Fort Worth area? 7. Examine the accompanying photo showing destruction caused by a strong category 4 hurricane. Which one of the three basic classes of damage was most likely responsible? What is your reasoning?
(Barry Williams. AFP/Getty Images/Newscom)
a. Name the stages of development that Gaston must have gone through to become a hurricane. At what point did it receive its name?
8. A television meteorologist is able to inform viewers about the intensity of an approaching hurricane. However, the meteorologist can report the intensity ofa tornado only after it has occurred. Why is this true?
HURRICANES IN REVIEW ●
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Most hurricanes form between latitudes 5° and 20° over all tropical oceans except the South Atlantic and eastern South Pacific. The North Pacific has the greatest number of storms, averaging 20 per year. In the western Pacific, hurricanes are called typhoons, and in the Indian Ocean, they are referred to as cyclones. A steep pressure gradient generates the rapid, inward spiraling winds of a hurricane. As the warm, moist air approaches the core of the storm, it turns upward and ascends in a ring of cumulonimbus towers and forms a doughnutshaped wall called the eye wall. At the very center of the storm, called the eye, the air gradually descends, precipitation ceases, and winds subside. A hurricane is a heat engine fueled by the latent heat liberated when huge quantities of water vapor condense. Hurricanes develop most often in late summer, when ocean waters have reached temperatures of 27°C (80°F) or higher and are thus able to provide the necessary heat and moisture to the air. The initial stage of a tropical storm’s life cycle, called a tropical disturbance, is a disorganized array of clouds that exhibits a weak pressure gradient and little or no rotation. Tropical disturbances that produce many of the strongest hurricanes
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that enter the western North Atlantic and threaten North America often begin as large undulations or ripples in the trade winds known as easterly waves. Each year, only a few tropical disturbances develop into full-fledged hurricanes (storms that require minimum wind speeds of 119 kilometers per hour to be so termed). When a cyclone’s strongest winds do not exceed 63 kilometers per hour, it is called a tropical depression. When winds are between 63 and 119 kilometers per hour, the cyclone is termed a tropical storm. Hurricanes diminish in intensity whenever they (1) move over cool ocean waters that cannot supply warm, moist tropical air; (2) move onto land; or (3) reach a location where large-scale flow aloft is unfavorable. Although damages caused by a hurricane depend on several factors, including the size and population density of the area affected and the nearshore ocean bottom configuration, the most significant factor is the strength of the storm itself. The Saffir–Simpson scale ranks the relative intensities of hurricanes. A 5 on the scale represents the strongest storm possible, and a 1 is the least severe. Damage caused by hurricanes can be divided into three categories: (1) storm surge, (2) wind damage, and (3) inland freshwater flooding.
Chapter 11 Hurricanes ●
North Atlantic hurricanes develop in the trade winds, which generally move these storms from east to west. Because of early warning systems that help detect and track hurricanes, the number of deaths associated with these violent storms have been greatly reduced. Because the tropical and subtropical regions that spawn hurricanes consist of enormous areas of open oceans, meteorological data from these vast regions are provided primarily by
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satellites. Other important sources of hurricane information are aircraft reconnaissance, radar, and remote, floating instrument platforms called data buoys. Hurricane watches and warnings alert coastal residents to possible or expected hurricane conditions. Two important factors in the watch-and-warning decision process are (1) providing adequate lead time and (2) attempting to keep overwarning to a minimum.
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VOCABULARY REVIEW easterly wave (p. 308) eye (p. 307) eye wall (p. 306) hurricane (p. 304)
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PROBLEMS Questions 1–5 refer to the weather maps of Hurricane Fran in Figure 11–3.
b. At what rate did the storm move during this 24-hour span? Express your answers in miles per hour. 3. The midlatitude cyclone shown in Figure 9–21 has an east–west diameter of approximately 1200 miles (when the 1008-millibar isobar is used to define the outer boundary of the low). Measure the diameter (north–south) of Hurricane Fran on September 5. Use the 1008-millibar isobar to represent the outer edge of the storm. How does this figure compare to the midlatitude cyclone? 4. Determine the pressure gradient for Hurricane Fran on September 5. Measure from the 1008-millibar isobar at Charleston to the center of the storm. Express your answer in millibars per 100 miles. 5. The weather map in Figure 9–21 shows a well-developed midlatitude cyclone. Calculate the pressure gradient of this storm from the 1008-millibar isobar at the Wyoming–Idaho border to the center of the low. Assume that the pressure at the center of the storm is 986 millibars and the distance is 625 miles. Express your answer in millibars per 100 miles. How does this answer compare to your answer to Problem 4? 6. Hurricane Rita was a major storm that struck the Gulf Coast in late September 2005, less than a month after Hurricane Katrina. Figure 11–18 is a graph showing changes in air pressure and wind speed from the storm’s beginning as an unnamed tropical disturbance north of the Dominican Republic on September 18, until its last remnants faded away
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Figure 11–18 Graph to accompany Problem 6. in Illinois on September 26. Use the graph to answer these questions. a. Which line represents air pressure, and which line represents wind speed? How did you figure this out? b. What was the storm’s maximum wind speed, in knots? Convert this answer to kilometers per hour by multiplying by 1.85. c. What was the lowest pressure attained by Hurricane Rita? d. Using wind speed as your guide, what was the highest category reached on the Saffir–Simpson scale? On what day was this status reached? e. When landfall occurred, what was the category of Hurricane Rita?
Log in to www.mymeteorologyLlab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Hurricanes.
Weather Analysis and Forecasting People expect thorough and accurate weather forecasts. The desire for reliable weather predictions ranges from NASA’s need to evaluate conditions leading up to the launch of a satellite to families wanting to know if the upcoming weekend weather will be suitable for a beach outing. Such diverse industries as airlines and fruit growers depend heavily on accurate weather forecasts. In addition, the designs of buildings, oil platforms, and industrial facilities rely on a sound knowledge of the atmosphere in its most extreme forms, including thunderstorms, tornadoes, and hurricanes. We are no longer satisfied with receiving only short-range predictions but expect accurate long-range forecasts as well.
This supercell thunderstorm produced a tornado near Quinter, Kansas, in May 2008. Weather radar is a basic tool used for nowcasting, a technique commonly used for severe weather warnings. (Photo by Ryan McGinnis/ agefotostock)
After completing this chapter, you should be able to:
t Explain the mission of the National Weather Service. t Distinguish between weather analysis and weather t t t
forecasting. Describe the basis of numerical weather prediction. List and distinguish among the various methods of weather forecasting. Interpret what a particular pattern in the flow aloft indicates about surface weather.
t Differentiate between weather forecasts and t t
30- and 90-day outlooks. Explain why the percentage of correct forecasts is not always a good measure of forecast skill. Discuss the advantages and disadvantages of infrared imagery and visible imagery generated by weather satellites.
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What is a weather forecast? Simply, a weather forecast is a scientific estimate of the weather conditions at some future time (Figure 12–1). Forecasts are usually expressed in terms of the most significant weather variables, which include temperature, cloudiness, humidity, precipitation, wind speed, and wind direction. As the following quote illustrates, weather forecasting is a formidable task: Imagine a system on a rotating sphere that is 8000 miles wide, consists of different materials, different gases that have different properties (one of the most important of which, water, exists in different concentrations), heated by a nuclear reactor 93 million miles away. Then, just to make life interesting, this sphere is oriented such that, as it revolves around the nuclear reactor, it is heated differently at different times of the year. Then, someone is asked to watch the mixture of gases, a fluid only 20 miles deep, that covers an area of 250 million square miles, and to predict the state of the fluid at one point on the sphere 2 days from now. This is the problem weather forecasters face.*
*Robert T. Ryan, “The Weather Is Changing . . . or Meteorologists and Broadcasters, the Twain Meet,” Bulletin of the American Meteorological Society 63, no. 3 (March 1982), 308.
The Weather Business: ABrief Overview The U.S. governmental agency responsible for gathering and disseminating weather-related information is the National Weather Service (NWS), a branch of the National Oceanic and Atmospheric Administration (NOAA). The mission of the NWS is as follows: The National Weather Service (NWS) provides weather, hydrologic and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy. NWS data and products form a national information database and infrastructure that can be used by other governmental agencies, the private sector, the public, and the global community.
Perhaps the most important services provided by the NWS are forecasts and warnings of hazardous weather, including thunderstorms, floods, hurricanes, tornadoes, winter weather, and extreme heat. According to the Federal Emergency Management Agency, 80 percent of all declared emergencies are weather related. In a similar vein, the
Figure 12–1 Accurate weather forecasts are important to many human activities, such as the launch of the space shuttle Atlantis on July 8, 2011. (NASA)
Chapter 12 Weather Analysis and Forecasting
Department of Transportation reports that more than 6000 vehicular fatalities per year can be attributed to weather. As global population increases, the economic impact of weatherrelated phenomena also escalates. As a result, the NWS is under greater pressure to provide more accurate and longerrange forecasts. To produce even a short-range forecast is an enormous task. It involves complicated and detailed procedures, including collecting weather data, transmitting those data to a central location, and compiling them on a global scale. After processing, these data are analyzed so that an accurate assessment of the current conditions can be made. In the United States, weather information from around the world is collected by the National Centers for Environmental Prediction, located in Camp Springs, Maryland, near Washington, D.C. This branch of the NWS prepares weather maps, charts, and forecasts on national as well as global scales. These forecasts are disseminated to regional Weather Forecast Offices, where they are used to produce local and regional weather forecasts. The final phase in the weather business is the dissemination of a wide variety of forecasts. Each of the 125 Weather Forecast Offices regularly issues regional and local forecasts, aviation forecasts, and warnings covering its forecast area. The local forecast seen on The Weather Channel or your local TV station is derived from a forecast issued by one of the offices operated by the NWS (Figure 12–2). Further, all the data and products (maps, charts, and forecasts) provided by the NWS are available at no cost to the general public and to private forecasting services, such as AccuWeather and WeatherData.
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The demand for highly visual forecasts containing computer-generated graphics has increased proportionately with the use of personal computers and the Internet. The animated depictions of the weather that appear on most local newscasts are produced by the private sector because this task is outside the mission of the NWS. The private sector has also assumed responsibility for customizing NWS products to create a variety of specialized weather reports tailored for specific audiences. In a farming community, for example, the weather reports might include frost warnings, while winter forecasts in Denver, Colorado, include the snow conditions at area ski resorts. It is important to note that despite the valuable role that the private sector plays in disseminating weather-related information to the public, the NWS is the official voice in the United States for issuing warnings during hazardous and life-threatening weather situations. Two major weather centers operated by the NWS serve critical functions in this regard. The Storm Prediction Center in Norman, Oklahoma, maintains a constant vigil for severe thunderstorms and tornadoes (see Chapter 10). Hurricane watches and warnings for the Atlantic, Caribbean, Gulf of Mexico, and eastern Pacific are issued by the National Hurricane Center/Tropical Prediction Center in Miami, Florida (see Chapter 11). In summary, the process of providing weather forecasts and warnings throughout the United States occurs in three stages. First, to create an image of the current state of the atmosphere, data are collected and analyzed on a global scale. Second, the NWS employs a variety of techniques to establish the future state of the atmosphere; a process called weather forecasting. Third, forecasts are disseminated to the public, mainly through the private sector. The National Weather Service serves as the soleentity responsible for issuing watches and warnings of extreme weather events.
Concept Check 12.1 1 What is the mission of the National Weather Service? 2 Approximately what percentage of all declared emergencies are weather related?
3 What branch of the National Weather Service produces local and regional weather forecasts?
4 List the three steps involved in providing weather forecasts.
Weather Analysis
Figure 12–2 The forecast seen on your local TV station is derived from one issued by a NWS Weather Forecast Office. (Photo by Exactostock/SuperStock)
Before weather can be predicted, the forecaster must have an accurate picture of current atmospheric conditions. This formidable task, called weather analysis, involves collecting, transmitting, and compiling billions of pieces of observational data. Because the atmosphere is ever changing, this must be accomplished quickly. High-speed supercomputers have greatly aided weather analysts.
database. The Federal Aviation Administration (FAA), in cooperation with the NWS, also operates observation stations at most metropolitan airports. Together, the NWS and the FAA operate nearly 900 Automated Surface Observing Systems (ASOS). These automated systems provide weather observations that include temperature, dew point, wind, visibility, and sky conditions and can also detect conditions such as rain or snow (Figure 12–4). Automation often assists, or, in some cases, replaces human observers because it can provide information from remote areas. However, some research has shown that human observers are more reliable for determining certain weather elements, such as cloud cover and sky conditions. Observations Aloft Upper-air observations are essential for producing reliable forecasts (see Figure 1–24, p. 28). Worldwide, nearly 900 balloon-borne instrument packages, called radiosondes, are launched twice daily at 0000 and 1200 Greenwich Mean Time (6:00 pm and 6:00 am Central Standard Time). Most of the upper-air observation stations are located in the Northern Hemisphere, with 92 stations operated by the National Weather Service. Figure 12–3 Data buoy used to record atmospheric conditions over a section of the global ocean. The data this buoy collects are transmitted via satellite to a land-based station. (Photo by Michael Dwyer/Alamy)
Wind tower (speed and direction)
Gathering Data A vast network of weather stations is required to collect the data needed for generating even the shortest-range forecasts. On a global scale the World Meteorological Organization (WMO), an agency of the United Nations, is responsible for the international exchange of weather data. This task requires that data from the more than 185 participating nations and territories be standardized. Surface Observations Worldwide, more than 10,000 observation stations on land, 7000 ships at sea, and hundreds of data buoys and oil platforms report atmospheric conditions four times daily, at 0000, 0600, 1200, and 1800 Greenwich Mean Time (Figure 12–3). These data are rapidly sent around the globe, using a communications system dedicated to weather information. In the United States, 125 Weather Forecast Offices are responsible for gathering and transmitting weather information to a central
Precipitation identification sensor
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Figure 12–4 Automated Surface Observing System (ASOS) equipped to sample the sky for cloud coverage; take temperature and dew-point measurements; determine wind speed and direction; and even detect present weather—such as whether it is raining or snowing. (Photo by Bobbé Christopherson)
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aircraft contribute upper-air information over the ocean by regularly reporting wind, temperature, and, occasionally, turbulence along their flight routes. A number of technical advances have been made that improved our ability to make observations aloft. Special radar units called wind profilers are used to measure wind speed and direction up to 10 kilometers (6 miles) above the surface. These measurements can be taken every 6minutes, in contrast to the 12-hour interval between balloon launches. In addition, satellites and weather radar have become invaluable tools for making weather observations. The importance of these modern technologies is considered later in this chapter. Despite the advances being made in the collection of weather data, two difficulties remain. First, observations may be inaccurate due to instrument 10 10 malfunctions or data transmission 16 12 1012 39 errors. Second, observations are dif21 1016 ficult, or in some cases impossible, to obtain in some regions—particularly 37 1020 40 34 over oceans and in mountainous areas. 33
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A radiosonde is a lightweight instrument package containing sensors that measure temperature, humidity, and pressure as a weather balloon rises through the atmosphere. By tracking the radiosonde, wind speed and direction at various altitudes can be determined. A radiosonde flight typically lasts about 90 minutes, during which it may ascend to heights of more than 35 kilometers (about 115,000 feet). Because pressure decreases with an increase in altitude, the balloon eventually stretches to its breaking point and bursts. When this occurs, a small parachute opens, and the instrument package slowly descends to Earth. If you find a radiosonde, follow the mailing instructions as the instruments can be reused. Acquiring upper-air data over the ocean is problematic. Only a few ships launch radiosondes. Some commercial
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Once this large body of data has been collected, analysts display it in a format that can be comprehended easily by forecasters. This step is ac1020 complished by placing the information on a number of synoptic weather maps (Box12–1). These maps are called synoptic, which means “coincident in time,” because they display a synopsis of the weather conditions at a given 80 74 moment. These weather charts provide 1016 a symbolic representation of the state of the atmosphere. Thus, to the trained eye, a weather map is a snapshot that shows the status of the atmosphere, including data on temperature, humidity, pressure, and wind. More than 200 surface maps and charts covering several levels of the atmosphere are produced each day by the NWS and its forecast centers. This task was once completed by hand, but computers now analyze and plot the data systematically. Typically, lines and symbols are used to depict weather patterns (Figure 12–5a). Once a map is generated, an analyst fine-tunes it and corrects any errors or omissions. In addition to the surface map, twice-daily upper-air charts are drawn at 850-, 700-, 500-, 300-, and 200-millibar levels. Recall that on these charts, height contours (in meters or tens of meters) instead Figure 12–5 Simplified synoptic weather maps. (a) Surface weather map for 7:00AM Eastern Standard Time depicting a well-developed middle-latitude cyclone. (b) A 500-millibarlevel map, with height contours in tens of meters, for the same time.
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Box 12–1 Constructing a Synoptic Weather Chart Production of surface weather charts starts with plotting the data from selected observing stations. By international agreement, data are plotted using the symbols illustrated in Figure 12–A. Data that are usually plotted include temperature, dew point, pressure and its tendency, cloud cover (height, type, and amount), wind speed and direction, and weather, both current and past. These data are always plotted in the same position around the station symbol for consistent reading. (The only exception to this arrangement is the wind arrow because it is oriented with the direction of airflow.) Using Figure 12–A, for example, you can see that the temperature is plotted in the upper-left corner of the sample model, and it will always appear in that location. A more complete weather station model and a key for decoding weather symbols are found in Appendix B. The data are plotted as shown in Figure12–B (left). Once data have been plotted, isobars and fronts are added to the weather chart (see Figure 12–B right). Isobars are usually plotted on surface maps at intervals of 4 millibars (1004, 1008, 1012, etc.). The positions of the isobars are estimated as accurately as possible from the pressure readings available. Note in
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FIGURE 12–A A specimen station model showing the placement of commonly plotted data. (Abridged from the International Code)
Figure 12–B (right) that the 1012-millibar isobar is about halfway between two stations that report 1010 millibars and 1014 millibars. Frequently, observational errors and other complications require an analyst to
of isobars are used to depict the pressure field. These charts also contain isotherms (equal temperature lines) shown as dashed lines labeled in degrees Celsius. This series of upper-air charts provides a three-dimensional view of the atmosphere. Figure 12–5 shows a simplified version of a surface map, as well as a 500-millibar-height contour chart covering the same time period. In summary, the analysis phase involves collecting and compiling billions of pieces of observational data describing the current state of the atmosphere. These data are then displayed on a number of different weather maps that show current weather patterns at predetermined levels throughout the atmosphere.
Concept Check 12.2 1 What does the process of weather analysis involve? 2 Weather Forecast Offices are responsible for collecting and transmitting weather data. In what other ways are these tasks accomplished?
3 What role do radiosondes play in weather forecasting? 4 Briefly explain the role of synoptic weather maps in weather forecasting.
Cloud type (Middle altocumulus)
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smooth the isobars so that they conform to the overall picture. Many irregularities in the pressure field are caused by local influences that have little bearing on the larger circulation depicted on the charts. Once the isobars
Students Sometimes Ask… Who was the first weather forecaster? In the United States, Benjamin Franklin is often credited with making the first long-term weather predictions in his Poor Richard’s Almanac, beginning in 1732. However, these forecasts were based primarily on folklore rather than weather data. Nevertheless, Franklin may have been the first to document that storm systems move. In 1743, while living in Philadelphia, rainy weather prevented Franklin from viewing an eclipse. Through later correspondence with his brother, he learned that the eclipse had been visible in Boston, but within a few hours that city also experienced rainy weather conditions. These observations led Franklin to the conclusion that the storm, which prevented his viewing of the eclipse in Philadelphia, moved up the east coast to Boston.
Weather Forecasting Using Computers Until the late 1950s, all weather maps and charts were plotted manually and served as the primary tools for making weather forecasts. Forecasters used various techniques to
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are drawn, centers of high and low pressure are designated. Because fronts are boundaries that separate contrasting air masses, they can often be identified on weather charts by locating zones exhibiting abrupt changes in conditions. Because several elements change across a front, all are examined so that the frontal position is located most accurately. Some of the most easily recognized changes that can aid in identifying a front on a surface chart are as follows:
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Notice in Figure 12–B that FIGURE 12–B Simplified weather charts. (left) Stations, with data for temperature, dew point, wind direction, all theconditions listed are easily wind speed, sky cover, and barometric pressure. (right) Same chart showing isobars and fronts. detected across the frontal zone. However, not all fronts are as easily defined as the one on our sample opposite sides of the front are subdued. air, where flow is less complex, become an map. In some cases, surface contrasts on When this happens, charts of the upper important tool for detecting fronts.
extrapolate future conditions from the patterns depicted on the most recent weather charts. One method involved matching current conditions with similar, well-established patterns from the past. From such comparisons, meteorologists predicted how current systems might change in the hours and days to come. As this practice evolved, “rules of thumb” were established to aid forecasters. Applying these rules to current weather charts became the foundation of weather forecasting and still plays an important role in making short-range (24 hours or less) predictions. Later, computers were used to plot data and produce surface and upper-air charts. As technologies improved, computers eventually replaced manual creation of weather forecasts. Computers have played a key role in improving the accuracy and detail of weather forecasts and in lengthening the period for which useful guidance can begiven.
Numerical Weather Prediction Numerical weather prediction is the technique used to forecast weather using mathematical models designed to represent atmospheric processes. (The word numerical is somewhat misleading here because all types of weather
forecasting are based on some quantitative data and therefore could fit under this heading.) Numerical weather prediction relies on the fact that the behavior of atmospheric gases is governed by a set of physical principles or laws that can be expressed as mathematical equations (Box 12–2, page 337). If we can solve these equations, we will have a description of the future state (a forecast) of the atmosphere, derived from the current state, which we can interpret in terms of weather—temperature, moisture, cloud cover, and wind. This method is analogous to using a computer to predict the future positions of Mars, using Newton’s laws of motion and knowing the planet’s current position. Highly refined computer models that attempt to mimic the behavior of the “real” atmosphere are used in numerical weather prediction. All numerical simulations are based on the same governing equations but differ in the way the equations are applied and in the parameters that are used. For example, some models use a very closely spaced set of data points and cover a specific concentrated area, whereas others describe the atmosphere more broadly on a global scale. In the United States, several different numerical models are used. Numerical weather forecasting begins by entering current atmospheric variables (temperature, wind speed,
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humidity, and pressure) into a computer simulation. This set of values represents the atmosphere at the start of the forecast. After literally billions of calculations, a forecast of how these basic elements are expected to change over a short time frame (perhaps only 5 to 10 minutes) is generated. When the new values have been calculated, the process begins again with the next 5 to 10 minute forecast being established. By repeating this procedure many times over, a forecast for six or more days is created. Using mathematical models, the NWS produces a variety of generalized forecast charts. Because these machine-generated maps predict atmospheric conditions at some future time, they are called prognostic charts, or simply progs. Most numerical models are designed to generate prognostic charts that predict changes in the flow pattern aloft. In addition, some models create forecasts for other conditions, including maximum and minimum temperatures, wind speeds, and precipitation probabilities. Even the simplest models require such a vast number of calculations that they could not have been used prior to the development of supercomputers. Once generated, a statistical analysis is used to modify these machine-generated forecasts by making comparisons of how accurate previous forecasts have been. This approach, known as Model Output Statistics (MOS), (pronounced “moss”), corrects for errors the model tends to make consistently. For example, certain forecast models may predict too much rain, overly strong winds, or temperatures that are too high or too low. MOS forecasts form the baseline on which forecasters from the NWS, as well as private forecast companies, try to make improvements. This final step is performed by humans, using their knowledge of meteorology and making allowances for known model shortcomings (Figure 12–6). In summary, meteorologists use equations to create mathematical models of the atmosphere. By utilizing data on initial atmospheric conditions, they solve these equations to predict a future state. Of course, this is a complicated process because Earth’s atmosphere is a very complex, dynamic system that can only be roughly approximated using mathematical models. Further, because of the nature of the equations, tiny differences in the data can yield huge differences in outcomes. Nevertheless, these models produce surprisingly good results—much better than those made withoutthem.
time, develop into two very different weather patterns. One may intensify, becoming a major disturbance, while the other withers and dissipates. To demonstrate this point, Edward Lorenz at the Massachusetts Institute of Technology employed a metaphor known as the butterfly effect. Lorenz described a butterfly in the Amazon rain forest fluttering its wings and setting into motion a subtle breeze that travels and gradually magnifies over time and space. According to Lorenz’s metaphor, two weeks later this faint breeze has grown into a tornado over Kansas. Obviously, by stretching the point considerably, Lorenz tried to illustrate that a very small change in initial atmospheric conditions can dramatically affect the resulting weather pattern elsewhere. To deal with the inherent chaotic behavior of the atmosphere, forecasters rely on a technique known as ensemble forecasting. Simply, this method involves producing a number of forecasts using the same computer model but slightly altering the initial conditions, while remaining within an error range of the observational instruments. Essentially, ensemble forecasting attempts to assess how the inevitable errors and omissions in weather measurements might affect the result. One of the most important outcomes of ensemble forecasts is the information they provide about forecast uncertainty. For example, assume that a prognostic chart that was generated using the best available weather data predicts the occurrence of precipitation over a wide area of the southeastern United States within 24 hours. Now let’s say that the same calculations are performed several times in succession, each time making minor adjustments to the initial conditions. If most of these prognostic charts also predict a pattern of precipitation in the Southeast, the forecaster will place a high degree of confidence in the forecast. On the
Ensemble Forecasting One of the most significant challenges for weather forecasters is the apparently chaotic behavior of the atmosphere. Specifically, two very similar atmospheric disturbances may, over
Figure 12–6 Forecasters at the National Weather Service provide nearly 2 million predictions annually to the public and commercial interests. (Photo by Ryan McGinnis/Alamy)
Chapter 12 Weather Analysis and Forecasting
other hand, if the prognostic charts generated by the ensemble method differ significantly, there will be far less confidence in theforecast.
Role of the Forecaster Despite faster computers, constant improvements in numerical models, and significant technological advances, prognostic charts provide only a generalized picture of atmospheric behavior. As a result, human forecasters, using their knowledge of meteorology as well as judgments based on experience, continue to serve a vital role in creating weather forecasts, particularly short-range forecasts. When formulated, a variety of prognostic charts are sent to the regional weather forecast offices of the NWS. The responsibility of the forecasters is to blend numerical predictions with local conditions and regional weather quirks to produce site-specific forecasts. This task is further complicated by the availability of multiple prognostic charts. For example, generally two different numerical models are employed to predict the minimum temperature for a given day. One method works better on some days than on others and performs better in some locales than in others. It is up to the forecaster to select the “best” model each day, or perhaps to blend the data from both models. Often, forecasters add extra detail to the model forecast. Isolated summer thunderstorms, for example, are on a scale too small for the computer models to adequately resolve. In addition, weather phenomena such as tornadoes and thunderstorm downbursts cannot be predicted using available forecasting techniques (Figure 12–7). Therefore, emphasis is placed on using satellites and weather radar to detect and track these phenomena.
Concept Check 12.3 1 Briefly describe the basis of numerical weather prediction.
2 How are prognostic charts different from synoptic weathermaps?
3 What do computer-generated numerical models try to predict?
4 What is ensemble forecasting? 5 What additional information does an ensemble forecastprovide over a traditional numerical weather prediction?
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Other Forecasting Methods Although machine-generated prognostic charts form the basis of modern forecasting, other methods are available to meteorologists. These methods, which have “stood the test of time,” include persistence forecasting, climatological forecasting, analog methods, and trend forecasting.
Persistence Forecasting Perhaps the simplest forecasting technique, called persistence forecasting, is based on the tendency of weather to remain unchanged for several hours or even days. If it is raining at a particular location, for example, it might be reasonable to assume that it will still be raining in a few hours. PersisFigure 12–7 Mesoscale phenomena such as this tornado are too small to appear on computer-generated prognostic charts. Detection of such events relies heavily on weather radar and geostationary satellites. (Photo by A. T. Wilett/Alamy)
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This wintertime satellite image shows a large commashaped cloud pattern moving from the Pacific toward California. The green and turquoise water color off the southern coast of California indicates the presence of a bloom of phytoplankton (microscopic floating aquatic plants). Also notice the greenish-colored area south of the Salton Sea, a region classified as having a desert climate. (Image courtesy of NASA) Question 1 What name is given to the storm system that resulted in the comma-shaped cloud pattern in the image? Sie
Question 2 Describe the likely weather in the Monterey, CA area as this system passed by.
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Question 4 The phytoplankton bloom that is visible in this image is located in the area of the cold California current. How may this current, which is moving southward along the California coast, have contributed to this biological activity near the ocean surface?
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tence forecasts do not account for changes that might occur in the intensity or direction of a weather system, nor can they predict the formation or dissipation of storms. Because of these limitations and the rapidity with which weather systems change, persistence forecasts usually diminish in accuracy within 6 to 12 hours, or one day at the most.
Climatological Forecasting Another relatively simple way of generating forecasts uses climatological data—average weather statistics accumulated over many years. This method is known as climatological forecasting. Consider, for example, that Yuma, Arizona, experiences sunshine approximately 90 percent of its daylight hours; thus, forecasters predicting sunshine every day of the year would be correct about 90 percent of the time. Likewise, forecasters in Portland, Oregon, would be correct about 90 percent of the time by predicting overcast skies in December. Climatological forecasting is particularly useful when making agricultural business decisions. For example, in the relatively dry north-central portion of Nebraska known as the Sand Hills, the implementation of center-pivot irrigation made growing corn more feasible. However, farmers were faced with the question of which corn hybrid to plant. A high-yield variety widely used in southeastern Nebraska (its warmest region) seemed to be the logical choice. However, review of local climatological data showed that because of the cooler temperatures in the Sand Hills, corn planted in late April would not mature until late September, when there is a
Question 5 What is a possible explanation for the greenish-colored area south of the Salton Sea?
50 percent probability of an autumn frost. Farmers used this important climate information to select a hybrid that was better suited for the shorter growing season of the Sand Hills. One interesting use of climatological data is the prediction of a “white Christmas”—that is, a Christmas with 1 inch or more of snow on the ground. As Figure 12–8 illustrates, northern Minnesota and northern Maine have more than a 90 percent chance of experiencing a “white Christmas.” By contrast, those who are in southern Florida for the holidays have a minuscule chance of experiencing snow. 90
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Box 12–2 Numerical Weather Prediction Gregory J. Carbone* current numerical models. Errors in the initial observations fed into the computer will be amplified over time because numerical weather prediction models include many nonlinear relationships. Finally, physical conditions at all spatial scales can influence atmospheric changes. Yet the spatial resolution of numerical models is too coarse to capture many important processes. In fact, the atmospheric system moves at scales too small to ever be observed and incorporated explicitly into models. A simple example illustrates how misrepresentation of physical processes or observation error might lead to inaccurate predictions. Figure 12–C is based on an equation used to predict future values of a given variable Y. The equation is written as Yt 11 5 1 a 3 Yt 2 2 Y 2t where Y represents the value of some variable at time t. Yt 11 represents the same variable at the next time step, and arepresents a constant coefficient. Notice that each predicted value serves as the initial value for the next calculation, the same way in which output from a
numerical weather-prediction model provides input for subsequent computations. The purple line in Figure 12–C shows the equation solution over a number of time steps, given an initial value of (for example, a meteorological observation) Yt 50 5 1.5 and a coefficient value of a 5 3.75. The graph illustrates how the precision of our equations describing the evolving state of the atmosphere may affect predictions. The blue line represents values of Y that result from the adjustment of a from 3.75 to 3.749. Similarly, we can demonstrate how a very small observation error could amplify over time by adjusting Yt 5 0 from 1.5 to 1.499. The red line in the graph shows how an incremental change in the initial value affects predictions. Small errors may make very little difference in the early stages of our prediction, but such errors amplify dramatically over time. Because we cannot observe many small-scale features of the atmosphere, nor incorporate all of its processes into computer models, weather forecasts have a theoretical limit. *Professor Carbone is a faculty member in the Department of Geography at the University of South Carolina.
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During the past several centuries, the physical laws governing the atmosphere have been refined and expressed through mathematical equations. In the early 1950s, meteorologists began using computers, which provided an efficient means of solving these mathematical equations, to forecast the weather. The goal of such numerical weather prediction is to predict changes in large-scale atmospheric flow patterns. The equations relate to many of the processes already discussed in this book. Two equations of motion describe how horizontal air motion changes over time, taking into account pressure gradient, Coriolis, and frictional forces. The hydrostatic equation describes vertical motion in the atmosphere. The first law of thermodynamics is used to predict changes in temperature that result from the addition or subtraction of heat or from expansion and compression of air. Two equations account for the conservation of mass and the conservation of water. Finally, the ideal gas law or equation of state shows the relationship among three fundamental variables— temperature, density, and pressure. Weather-prediction models begin with observations describing the current state of the atmosphere. They use the equations to compute new values for each variable of interest, usually at 5- to 10-minute intervals, called the time step. Predicted values serve as the initial conditions for the next series of computations and are made for specific locations and levels of the atmosphere. Each model has a spatial resolution that describes the distance between prediction points. Based on solving fundamental equations repeatedly, a model predicts the future state of the atmosphere. The model output is provided to weather forecasters for fixed intervals, such as 12, 24, 36, 48, and 72 hours in the future. Despite the sophistication of numerical weather-prediction models, most still produce forecast errors. Three factors in particular restrict their accuracy: inadequate representation of physical processes, errors in initial observations, and inadequate model resolution. Whereas the models are grounded on sound physical laws and capture the major characteristics of the atmosphere, they necessarily simplify the workings of a very complex system. The representation of land–surface processes and topography are just two examples of features that are incompletely treated in
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Analog Method A somewhat more complex way to predict the weather is the analog method, which is based on the assumption that weather repeats itself, at least in a general way. Thus, forecasters attempt to find well-established weather patterns from the past that match (are analogous to) a current event. From such a comparison, forecasters predict how the current weather might evolve. Prior to the advent of computer modeling, the analog method was the backbone of weather forecasting, and an analog method called pattern recognition remains an important tool with which short-range machine-generated forecasts are improved.
Upper Airflow and Weather Forecasting In Chapter 9 the strong connection between cyclonic disturbances at the surface and the wavy flow of the westerlies aloft was established. The importance of this connection cannot be overstated, particularly as it applies to weather forecasting. In order to understand the development of thunderstorms or the formation and movement of midlatitude cyclones, meteorologists must know what is occurring aloft as well as at the surface.
Upper-Level Maps Trend Forecasting Another related method, trend forecasting, involves determining the speed and direction of features such as fronts, cyclones, and areas of clouds and precipitation. Using this information, forecasters attempt to extrapolate the future position of these weather phenomena. For example, if a line of thunderstorms is moving toward the northeast at 35 miles per hour, a trend forecast would predict that the storm will reach a community located 70 miles to the northeast in about twohours. Because weather events tend to increase or decrease in speed and intensity or change direction, trend forecasting is most effective over periods of just a few hours. Thus, it works particularly well for forecasting severe weather events that are expected to be short-lived, such as hailstorms, tornadoes, and downbursts, because warnings for such events must be issued quickly and must be site specific. The techniques used for this work, often called nowcasting, are heavily dependent on weather radar and geostationary satellites. These tools are important in detecting areas of heavy precipitation or clouds that are capable of triggering severe conditions. Nowcasting techniques use highly interactive computers capable of integrating data from a variety of sources. Prompt forecasting of tornadic winds is one example of the critical nature of nowcasting techniques.
Concept Check 12.4 1 If it is snowing today, what weather might be predicted for tomorrow if the technique called persistence forecasting is employed?
2 What type of weather phenomena are typically forecast using nowcasting techniques?
3 Briefly describe the basis of the analog method of weather forecasting.
4 What term is applied to the very short-range forecasting technique that relies heavily on weather radar and satellites?
5 What do forecasters call the technique that predicts the weather based on average weather statistics accumulated over many years?
Upper-air maps are generated twice daily, at 0000 and 1200 GMT (midnight and noon GMT). Recall that these charts are drawn at 850-, 700-, 500-, 300-, and 200-millibar (mb) levels as height contours (in meters or tens of meters), which are analogous to isobars used on surface maps. These maps also contain isotherms, depicted as dashed lines, and some show humidity as well as wind speed and direction. 850- and 700-Millibar Maps Figure 12–9a illustrates a standard 850-mb map, which is similar in general layout to a 700-mb map. Both maps show height contours using solid black lines at 30-meter intervals and isotherms labeled in degrees Celsius. If relative humidity is included, levels above 70 percent are shown in green. Wind data are plotted using black arrows. The 850-mb map depicts the atmosphere at an average height of about 1500 meters (1 mile) above sea level. In nonmountainous areas, this level is above the layer where daily temperature fluctuations are strongly influenced by the warming and cooling of Earth’s surface. (In areas where elevations are high, such as Denver, Colorado, the 850-mb level represents surface conditions.) Forecasters regularly examine the 850-mb map to find areas of cold-air and warm-air advection. Cold-air advection occurs where winds blow across isotherms from colder areas toward warmer areas. The 850-mb map in Figure 12–9b shows a blast of cold air moving south from Canada into the Mississippi Valley. Notice that the isotherms (red dashed lines) in this region of the country are packed tightly together, which means the affected area will experience a rapid drop in temperature. By contrast, warm-air advection is occurring along the eastern seaboard of the United States. In addition to moving warm air into a cooler region, warm-air advection is generally associated with widespread lifting in the lower troposphere. If the humidity in the region of warm-air advection is relatively high, then lifting could result in cloud formation and possible precipitation (see Figure 12–9c). Temperatures at the 850-mb level also provide useful information. During the winter, for example, forecasters use the 0°C isotherm as the boundary between areas of rain and areas of snow or sleet. Further, because air
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at the 850-mb level does not experience the daily cycle of temperature changes that occurs at the surface, these maps provide a way to estimate the daily maximum surface temperature. In summer, the maximum surface temperature is usually 15°C (27°F) higher than the temperature at the 850-mb level. In winter, maximum surface
temperatures tend to be about 9°C (16°F) warmer than at the 850-mb level. The 700-mb flow, which occurs about 3 kilometers (2 miles) above sea level, serves as the steering mechanism for air mass thunderstorms. Thus, winds at this level are used to predict the movement of these weather producers.
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One rule of thumb used with these charts is that when temperatures at the 700-mb level are 14°C (57°F) or higher, thunderstorms will not develop. A warm 700-mb layer acts as a lid inhibiting the upward movement of warm, moist surface air that might otherwise rise to generate towering cumulonimbus clouds. The warm conditions aloft are caused by general subsidence associated with a strong highpressure center. 500-Millibar Maps The 500-mb level is found approximately 5.5 kilometers (18,000 feet) above sea level—where about half of Earth’s atmosphere is below and half is above. Notice in the example shown in Figure 12–10 that a large trough occupies much of the eastern United States, whereas a ridge is influencing the West. Troughs indicate the presence of a storm at midtropospheric levels, whereas ridges are associated with calm weather. When a prog predicts that a trough will increase in magnitude, it is an indication that a storm is likely to intensify. To estimate the movement of surface cyclonic storms, which tend to travel in the direction of the flow at the 500-mb level but at roughly a quarter to half the speed, forecasters find it useful to rely on 500-mb maps. Sometimes an upperlevel low (shown as a closed contour in Figure 12–10) forms within a trough. When present, these features are associated with counterclockwise rotation and significant vertical lifting that typically results in heavy precipitation. 300- and 200-Millibar Maps Two of the regularly generated upper-level maps, the 300-mb and the 200-mb charts, represent zones near the top of the troposphere. Here, at altitudes above 10 kilometers (6 miles), temperatures can reach a frigid –60°C (–75°F)—levels at which the details
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of the polar jet stream can best be observed. Because the jet stream is lower in winter and higher in summer, 300-mb maps are most useful during winter and early spring, whereas the 200-mb maps are most useful during the warm season. In order to show airflow aloft, these maps often plot isotachs, which are lines of equal wind speed. Areas exhibiting the highest wind speeds may also be colored, as shown in Figure 12–11. These segments of higher-velocity winds found within the jet stream are called jet streaks. The 200- and 300-mb charts are important to forecasters for several reasons. Recall that jet streams have regions where upper-level divergence is dominant. Divergence aloft leads to rising air, which supports surface convergence and cyclonic development (see Figure 9–15, page 254). When a jet streak is strong, it tends to energize a trough, causing the pressure to drop further and the storm to strengthen. The jet stream also plays an important role in the development of severe weather and in extending the life span of supercells. Recall that thunderstorms have areas of updrafts that feed moisture into the storm and are situated side by side with downdrafts, which cause entrainment of cool, dry air into the storm. Typically, thunderstorms are relatively short-lived events that dissipate because downdrafts grow in size and eventually dominate the entire cloud (see Figure 10–4, page 274). Supercells tend to develop near the jet stream, where winds near the top of the thunderstorm may be two to three times as fast as winds near the base. This tilts the thunderstorm as it grows vertically, separating the area of updrafts from the area of downdrafts. When the updrafts are displaced from the downdrafts, the rising cells are not canceled by the downdrafts, and the storm grows inintensity.
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Figure 12–11 A 200-mb map showing the location of the jet stream (pink) and a jet streak (red). Height contours are spaced at 120-meter intervals.
Chapter 12 Weather Analysis and Forecasting
The Connection Between Upper-Level Flow and Surface Weather Thus far, we have considered the relationship between upper-level flow and short-lived weather phenomena—for example, how winds aloft steer and support the growth of thunderstorms. Next, we will examine long-period changes in the wavy flow of the westerlies and see how these changes impact our weather. Recall that midlatitude winds are generated by temperature contrasts that are most extreme where warm tropical air clashes with cold polar air. The boundary between these two contrasting air masses is the site of the polar jet stream, which is embedded in the meandering westerly flow near the top of the troposphere. Occasionally, the flow within the westerlies is directed along a relatively straight path from west to east, a pattern referred to as zonal. Storms embedded in this zonal flow move quickly across the country, particularly in winter. This situation results in rapidly changing weather conditions in which periods of light to moderate precipitation are followed by brief periods of fair weather.
More often, however, the flow aloft consists of longwave troughs and ridges that have large components of north–south flow, a pattern that meteorologists refer to as meridional. Typically, these airflow patterns slowly drift from west to east, but occasionally they stall and sometimes may even reverse direction. As this wavy pattern gradually migrates eastward, cyclonic storms embedded in the flow are carried across the country. Figure 12–12 shows a trough that on February 1, 2003, was centered off the Pacific Coast of the United States. Over the next four days the trough grew in strength as it moved eastward into the Ohio Valley. This change in intensity is shown by the height contours, which are more closely packed around the trough on February 4 than on February 1. Embedded in this upper-level trough was a cyclonic system, which grew into the major storm that is shown on the surface map in Figure 12–12. By February 5, this disturbance generated considerable precipitation over much of the eastern United States. (Notice that the center of the surface low is located somewhat to the east of the 500-mb trough—as is typical of these weather patterns.)
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Figure 12–12 The movement and intensification of an upper-level trough over a four-day period from February 1to February4, 2003. The surface map shows a strong cyclonic storm that formed and moved in conjunction with this trough.
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This is a whole-disk satellite image of Earth obtained using infrared imagery. On this type of image, high cold clouds that tend to produce precipitation appear to be white, whereas lower clouds appear in gray tones. The warmest cloud-free areas appear the darkest. (Courtesy of NOAA) Question 1 What phenomenon produced the band of clouds that is centered on the dashed line that encircles Earth? Question 2 Based on the position of this band of clouds, is it winter or summer in the Northern Hemisphere? Question 3 What weather phenomena are associated with the four groups of clouds that are numbered? Question 4 What region of Earth was warm and cloud free when this image was obtained?
In general, high-amplitude patterns have the potential for generating extreme conditions. During winter, strong troughs tend to spawn large snowstorms, whereas in summer they are associated with severe thunderstorms and tornado outbreaks. By contrast, high-amplitude ridges are associated with record heat in summer and tend to bring mild conditions in winter. Sometimes these “looped” patterns stall over an area, causing surface patterns to change minimally from one day to the next or, in extreme cases, from one week to the next. Locations in and just east of a stationary or slow-moving trough experience extended periods of rainy or stormy conditions. By contrast, areas in and just east of a stagnant ridge experience prolonged periods of unseasonably warm, dry weather. The strength and position of the polar jet stream fluctuate seasonally. The jet is strongest (faster wind speeds)
in winter and early spring, when the contrast between the warm tropics and the cold polar realm is greatest. As summer approaches, the temperature gradient diminishes, and the westerlies weaken. The position of the jet stream also changes with the migration of the Sun’s vertical rays. With the approach of winter, the mean position of the jet moves toward the equator. By midwinter, it may penetrate as far south as central Florida. Because the paths of midlatitude cyclones shift with the flow aloft, the southern states typically encounter most of their severe weather in winter and spring. (Hurricane season is the exception.) During summer, because of a poleward shift of the jet, the northern states and Canada experience an increase in the number of severe storms and tornadoes. This more northerly storm track also carries most Pacific storms toward Alaska during the warm months, producing a rather long, dry summer season for much of the Pacific Coast to the south. The position of the jet stream relative to where you live is important for other reasons. In winter, if the jet stream moves considerably south of your location, your area will experience a “cold snap,” as frigid air moves in from Canada. During the summer, by contrast, the core of the jet tends to be positioned over Canada, while warm, moist tropical air dominates much of the United States east of the Rockies. If, during the winter or early spring, strong midlatitude cyclones carried by the flow aloft pass immediately south of your location, you can expect heavy snowfall and cooler conditions. In summer, if the jet is overhead, periods of heavy rain, and perhaps hail and tornadic winds, mayoccur. An Extreme Winter Consider an example of the influence of the flow aloft for an extended period during an atypical winter. During a normal January, an upper-air ridge tends to be situated over the Rocky Mountains, while a trough extends across the eastern two-thirds of the United States. In January 1977, however, the normal flow pattern was greatly accentuated, as illustrated in Figure 12–13. The greater amplitude of the upper-level flow caused an almost continuous influx of cold air into the Deep South, producing record-low temperatures throughout much of the eastern and central United States (Figure 12–14). Therefore, people consumed above-average amounts of natural gas to heat their homes. As a result, many industries implemented employee layoffs in order to conserve their dwindling natural-gas supplies. Ohio was severely affected—such that four-day workweeks and massive shutdowns were ordered. While most of the East and South were in the deep freeze, the westernmost states were being influenced by a strong ridge of high pressure that delivered an extended period of mild temperatures and clear skies. Unfortunately, the ridge of high pressure blocked the movement of Pacific storms that usually provide much-needed winter precipitation. The shortage of moisture was especially serious in California, where January is the middle of its three-month rainy season. Throughout most of the western states, the winter rain and snow that supply water for summer irrigation were far
Chapter 12 Weather Analysis and Forecasting
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below normal. This dilemma was compounded by the fact that many reservoirs were almost empty due to the fact that the previous year was exceptionally dry. Although much of the country was concerned about economic disaster caused either by a lack of moisture or frigid temperatures, the highly accentuated flow pattern channeled unseasonably warm air into Alaska. Even Fairbanks, which generally experiences temperatures as low as –40°C (–40°F), had a mild January, with numerous days above freezing.* In summary, the wavy flow aloft governs, to a large extent, the overall magnitude and distribution of weather disturbances observed in the midlatitudes. Thus, accurate forecasts depend on our ability to predict long- and short-term changes in the upperlevel flow. Although the task of forecasting long-term variations remains beyond the capabilities of meteorologists, it is hoped that future research will allow forecasters to answer questions such as: Will next winter be colder or warmer than recent winters? and Will the Southwest experience a drought next year?
*For an excellent review of the winter weather of 1976–1977, see Thomas Y. Canby, “The Year the Weather Went Wild,” National Geographic 152, no. 6 (1977), 798–892.
Figure 12–14 Arctic air invades the eastern United States. (Photo by Michael Stubblefield/Alamy)
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Concept Check 12.5 1 Which upper-level chart is most effective for observing the
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2 During winter, if the jet stream is considerably south of your location, will the temperatures likely be warmer or colder than normal?
3 What is a jet streak? 4 What are two pieces of information that forecasters can glean from 850-mb maps?
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Long-Range Forecasts The National Weather Service issues a number of computergenerated forecasts for time spans ranging from a few hours to more than two weeks. Beyond 7 days, however, the accuracy of these forecasts diminishes considerably. In addition, the Climate Prediction Center, a branch of the NWS, produces 30- and 90-day outlooks. These are not weather forecasts in the usual sense. Instead, they offer insights into whether it will be drier or wetter and colder or warmer than normal in a particular region of the country. A series of 13 90-day outlooks are produced in onemonth increments. The series begins with the outlook for the next three months—for example, September, October, and November. Next, a separate 90-day outlook for the period beginning one month later (October, November, and December) is constructed. Figure 12–15 shows the temperature and precipitation 90-day outlooks issued for September, October, and November 2011. The temperature outlook for this period calls for warmer-than-usual conditions across much of the southwestern, north-central, and northeastern United States (Figure 12–15a). The remainder of the country is labeled “equal chance,” which indicates that there are not climatic signals for either above- or belownormal conditions during the forecast period. The precipitation outlook shown in Figure 12–15b calls for wetter-thannormal conditions across most of Kansas, Nebraska, and South Dakota, whereas drier-than-normal conditions are expected in a portion of the Southwest from Texas to southeastern Arizona. Monthly and seasonal outlooks of this type are generated using a variety of criteria. Meteorologists consider the climatology of each region—the 30-year average of variables such as temperature and precipitation. Factors such as snow and ice cover in the winter and persistently dry or wet soils in the summer are taken into account. Forecasters also consider current patterns of temperature and precipitation. For example, during 2011 and the few preceding years, the southwestern United States experienced below-normal levels of precipitation. Based on climatological data, the weather patterns that produce these conditions tend to gradually shift toward the norm rather than changing abruptly. Thus, forecasters predicted that this below-
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normal trend would continue for at least the first several months of 2012. Recently, sea-surface temperatures in the equatorial Pacific Ocean have been shown to have predictive value in forecasting temperature and precipitation patterns in various locations around the globe—particularly in the winter season. For a discussion of this relationship, see the section “El Niño and La Niña and the Southern Oscillation” in Chapter 7. Despite improved seasonal outlooks, the reliability of extended forecasts has been disappointing. Although some forecast skill is observed during the late winter and late summer, outlooks prepared for the “transition months,” when the weather can fluctuate wildly, have shown little reliability.
Concept Check 12.6 1 What two weather elements are predicted on long-range (monthly) weather charts?
Chapter 12 Weather Analysis and Forecasting
Forecast Accuracy Never, no matter what the progress of science, will honest scientific men who have a regard for their professional reputations venture to predict the weather. Dominique François Arago French physicist (1786–1853)
Some might argue that Arago’s statement is still true! Nevertheless, a great deal of scientific and technological progress has been made in the two centuries since he made this observation. Accurate weather forecasts and warnings of extreme weather conditions are provided by the National Weather Service (Figure 12–16). Their forecasts are used by government agencies to protect life and property, by electric utilities and farmers, by the construction industry, travelers, and airlines—by nearly everyone. How does the NWS measure forecast accuracy, a concept referred to as skill? When determining the occurrence of precipitation, for example, NWS forecasts are correct more than 80 percent of the time. Does this mean the NWS
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is doing a great job? Not necessarily. When establishing the skill of a forecaster, we need to examine more than the percentage of accurate forecasts. For example, measurable precipitation in Los Angeles is recorded only 11 days each year, on average. Therefore, the chance of rain in Los Angeles is 11 out of 365, or about 3 percent. Knowing this, a forecaster could predict no rain for every day of the year and be correct 97 percent of the time! Although the accuracy would be high, it would not indicate skill. Any measure of forecasting skill must consider climatic data. Thus, for forecasters to exhibit forecasting skill, they must do better than forecasts that are based simply on climatic averages. In the Los Angeles example, the forecaster must be able to predict rain on at least a few of the rainy days to demonstrate forecasting skill. The only aspect of a weather forecast that is expressed as a percentage probability is rainfall. Statistical data are studied to determine how often precipitation occurred under similar conditions. Although the occurrence of precipitation can be forecast with more than 80 percent accuracy, predictions of the amount, time of occurrence, and duration are not as reliable.
Figure 12–16 Meteorologist at the National Hurricane Center, in Miami, Florida, examining satellite imagery of Hurricane Ivan as the eye crosses the Alabama coastline on September 16, 2004. (Photo by Andy Newman/Associated Press)
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How skillful are the weather forecasts provided by the National Weather Service? In general, very short-range forecasts (0 to 12 hours) have demonstrated considerable skill, especially for predicting the formation and movement of large weather systems such as midlatitude cyclones. Over the past 20 years, the accuracy of short-range forecasts (12 to 72 hours) at predicting precipitation has improved significantly. However, specific distribution of precipitation is tied to mesoscale structures, such as individual thunderstorms, that cannot be predicted by the current numerical models. Thus, a forecast could predict 2 inches of rain for an area, but one town might receive a trace, whereas a neighboring community may be deluged with 4 inches. By contrast, predictions of maximum and minimum temperatures and wind tend to be quite accurate. Mediumrange forecasts (3 to 7 days) have also shown significant improvement in the past few decades. Large cyclonic storms are often predicted a few days in advance. Yet, beyond day 7, the predictability of day-to-day weather using these modern methods proves no more accurate than projections made from climatic data. Several factors account for the limited range of modern forecasting techniques. As noted earlier, the network of observing stations is limited in its coverage of our vast planet. Not only are large areas of Earth’s land–sea surface monitored inadequately, but globally, data from the middle and upper troposphere are meager. Moreover, the physical laws of nature are difficult to apply to chaotic systems such as the atmosphere, and the current models of the atmosphere remain incomplete. Nevertheless, numerical weather prediction has greatly improved the forecaster’s ability to project changes in the upper-level flow. When the flow aloft can be tied more closely to surface conditions, weather forecasts are likely to demonstrate greater skill. In summary, the accuracy of weather forecasts covering shortand medium-range periods has improved steadily over the past few decades, particularly in forecasting the evolution and movement of midlatitude cyclones. Technological developments and
improved computer models, as well as an increased understanding of how the atmosphere behaves, have added greatly to this success. The ability to accurately predict daily weather beyond day 7, however, remains relatively low.
Concept Check 12.7 1 What is meant by the term forecast skill? 2 Give an example of why the percentage of correct forecasts is not always a good measure of forecasting skill.
3 The forecast for which weather element is expressed as a percentage probability?
4 List two reasons modern forecasting techniques do not accurately predict the weather beyond a few days for most regions of the United States.
Satellites in Weather Forecasting Meteorology entered the space age on April 1, 1960, when the first weather satellite, TIROS 1, was launched. (TIROS stands for Television and Infra-Red Observation Satellite.) In its short life span of only 79 days, TIROS 1 obtained thousands of images and established that satellites are useful in the study of our planet. Subsequently, more than 30 versions of TIROS have been launched by the United States— the most recent in 2009. In addition to specialized television cameras, each satellite is equipped with infrared sensors capable of “seeing” cloud coverage at night. TIROS 1, like all other weather satellites in this series, was placed into a polar orbit—one that circles Earth from north-to-south (Figure 12–17a). Polar-orbiting satellites orbit Earth at low altitudes (about 850 kilometers) and require only 100 minutes per orbit. By properly orienting the orbits, these satellites drift about 15° westward over Earth’s surface during each orbit. Thus, they are able to
Figure 12–17 Weather satellites. (a) Polar-orbiting satellites about 850 kilometers (530 miles) above Earth travel over the North and South Poles. (b) A geostationary satellite moves west to east above the equator, at a distance of 35,000 kilometers (22,300 miles) and at the same rate that Earth rotates.
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Harold Brooks: Research Meteorologist Modern weather forecasting relies on far-seeing satellites and complex computer algorithms. Yet anyone who has wished for an umbrella after a forecast called for “sunny, hot” weather knows that predicting the weather is an imperfect science. Harold Brooks, a research meteorologist at NOAA’s National Severe Storms Laboratory in Norman, Oklahoma, is helping change this scenario. His primary areas of expertise— forecast evaluation and severe thunderstorm conditions—seek to make weather forecasting both more useful and accurate.
Forecasts are not homework problems where someone knows the answers beforehand. “Forecasts are not homework problems where someone knows the answers beforehand. Sometimes we get surprised and discover we don’t know why the weather turned out as it did. The way we understood how the atmosphere works doesn’t cover this particular case. That becomes a research problem,” Brooks says. One question Brooks studies is What constitutes a good weather forecast? “There are two primary things we look at: how closely the observations look like our forecast and how much value a user gets from the forecast.” When the National Weather Service puts out a tornado warning for an area, Brooks wants to know whether the storm occurred or not. He also wants to know how people interpreted the warning. Brooks says, “The National Weather Service warning has a precise meaning. But it isn’t clear whether users of those products have the same meaning in mind. The question becomes How do we relate the forecast to what my next-door neighbor would think? That’s a hard problem— one that deals with social science.”
Getting forecasts right, however, is a high-stakes business. Accurate forecasts can save both lives and dollars, but misses can cause frustration and lead people to disregard future warnings. Brooks’s other major area of interest is where and when severe thunderstorms occur. One recent project involved estimating the probability that severe tornado conditions will occur on any day of the year anywhere in the United States. The analysis relies on storm data that go back many decades. Storm records, however, are inherently biased: More reports exist for places that are more densely populated, and population changes from decade to decade. Brooks and colleagues have developed ways to smooth the data in both space and time, using statistics. The result: a map that insurers, engineers, emergency agencies, and the public can use to estimate tornado risk. The map also broadens the reach of tornado forecasting, Brooks says. “We can draw better conclusions about the environmental conditions in which those storms form. We can use this information to estimate where severe thunderstorms have actually occurred around the world in the past and where they will happen in the future.” Brooks’s interest in tornadoes may stem from growing up in St. Louis, Missouri— tornado country. He majored in physics and math as an undergraduate, and he landed a summer internship at NASA’s Goddard Space Flight Center. While there, Brooks helped prepare models of global climate during the last ice age. The work earned him an invitation to graduate school. He earned a master’s degree and then a doctorate modeling severe thunderstorms. That’s when Brooks began working as a postdoctoral researcher at the National
obtain images of the entire planet twice each day and provide coverage of a large region in a few hours. Another class of weather satellites, geostationary satellites, were first placed in orbit over the equator in 1966 (Figure 12–17b). These satellites, as their name implies, remain fixed over a point on Earth because their rate of travel keeps pace with Earth’s rate of rotation. In order to remain positioned over a given site, however, the satellite must orbit at a greater distance from Earth’s surface (about 35,000 kilometers, [22,000 miles]) than polar-orbiting satellites. At this altitude, the speed required to keep a satellite
Severe Storm Laboratory, where he remains today. After more than 20 years in weather research, the atmosphere still surprises Brooks. “Extremely rare events of violent weather on the planet almost all come out of what are fairly normal and undramatic processes. Every once in a while, these come together to challenge your notions of how the planet functions.”
Extremely rare events of violent weather on the planet almost all come out of what are fairly normal and undramatic processes.
HAROLD BROOKS is a researcher at the National Severe Storms Laboratory in Norman, Oklahoma. (Photo courtesy of Harold Brooks)
in orbit will also keep it moving in time with the rotating Earth. At this distance, the images obtained are less detailed than those captured by satellites that orbit at lower altitudes.
What Type of Images Do Weather Satellites Provide? The most recent generation of weather satellites, known as Geostationary Operational Environmental Satellites (GOES), provide visible, infrared, and water-vapor images for North America and adjacent ocean areas. These are the
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Figure 12–18 Weather satellites are invaluable tools for tracking storms andgathering atmospheric data. This isa truecolor composite image of a maturemidlatitude cyclone produced on June 11, 2011, by an instrument called MODIS (Moderate Resolution Imaging Spectroradiometer). (Courtesy of NASA)
satellite images often used on The Weather Channel. Other countries, including the European Union, India, China, Japan, and Russia, have launched similar weather satellites. Collectively, these satellites allow meteorologists to track the movement of large weather systems that cannot be adequately followed by weather radar or polar-orbiting satellites. Swirling cloud masses are easily tracked by satellites as they migrate across even the most remote portions of our planet (Figure 12–18). These satellites also play a very important role in monitoring the development and movement of tropical storms and hurricanes. Visible Light Images GOES satellites are equipped with instruments that detect sunlight reflected from Earth, producing visible light images such as the one shown in Figure 12–19. (Visible light images, also called visible images, are similar to pictures taken with an ordinary camera.) Because visible light imagery records the intensity of light reflected from cloud tops and other surfaces, these images are similar to black-and-white photos of Earth. The bright white areas are mainly cloud tops or snow- and ice-covered areas that are strong reflectors of light. Land surfaces appear dark gray because they reflect less light than clouds, and the oceans, which reflect very little light, appear nearly black. Visible images are useful in identifying cloud shapes, organizational patterns, and thickness. In general, the thicker a cloud is, the higher its albedo and the brighter it will appear in the image. In addition, cumulonimbus clouds have a bumpy appearance, whereas stratus clouds form a continuous flat-looking blanket. Figure 12–19b is a visible light image that shows a close-up of towering cumulonimbus clouds with overshooting tops that have penetrated the temperature inversion that lies above the troposphere. Infrared Images In contrast to visible images, infrared images are obtained from radiation emitted (rather than reflected) from objects and, when compared to visible
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Figure 12–19 Visible light images. (a) Visible light image provided by GOES satellite of cloud distribution about midday on July 15, 2011. This image records sunlight reflected off various Earth surfaces and appears much like a black-and-white image. (b)Close-up visible image centered on the state of Iowa showing towering cumulonimbus clouds with overshooting tops. (Courtesy of NOAA)
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Figure 12–20 Infrared satellite images. (a) Infrared image provided by GOES satellite about midday on July 15, 2011. Numbers 1–4 in this infrared image represent areas that: (1) are cloud free; (2) contain low clouds (cumulus) with cloud topsbelow2000 meters; (3) are blanketed by cloud tops between2000–6000 meters, and; (4) the infrared sensor is measuring high cold cloud tops that are likely associated with towering cumulonimbus clouds and thunderstorms. (b) Colorenhanced infrared image for the same time period as part (a). The dark-reddish color indicates the location of towering cumulonimbus clouds. (Courtesy of NOAA)
southeastern United States are also easily identifiable on the infrared image in Figure 12–20. To help meteorologists interpret infrared images, false colorization is sometimes used. The coldest, and thus highest, cloud tops, which typically identify areas of heavy precipitation, are shown in strong colors. Notice the reddish areas over the Minnesota–Wisconsin border in Figure 12–20b. This is the same area that appears as bright white in the infrared image in Figure 12–20a.
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(b) Cloud free, very warm surface temperature
images, are especially useful in determining which clouds are most likely to produce precipitation and/or stormy conditions. Infrared images are produced by computers that are programmed to display warm objects such as Earth’s surface in black and colder objects such as high, cold cloud tops in white. Because high cloud tops are colder than low cloud tops, towering cumulonimbus clouds that usually generate heavy precipitation appear very bright, whereas midlevel clouds appear somewhat light gray. Low fairweather cumulus, stratus clouds, and fog are all relatively warm and therefore appear dark gray or may not be discernable from Earth’s surface on infrared images. Compare Figure 12–19a, a visible image, to Figure 12–20a, which is an infrared image of the same region. Both images were acquired on July 15, 2011, a very hot summer day that produced thunderstorms over parts of Minnesota and Wisconsin, as well as in the southeast. As you can see, the clouds over eastern South Dakota that appear white on the visible image appear gray on the infrared image. These areas are blanketed by middle clouds between 2000–6000 meters. By contrast, the clouds over eastern Minnesota and western Wisconsin appear bright in both the visible and infrared images. This region has towering cumulonimbus clouds and was experiencing air mass thunderstorms that brought heavy rain to the area. The areas of isolated storm cells in the
Visible Versus Infrared Imagery A major advantage of infrared imagery is its ability to detect energy radiated skyward both day and night. As a result, the movement of storms can be monitored 24 hours a day. In addition, infrared images are useful for distinguishing colder high cloud tops from warmer, lower clouds. Visible images, which require reflected sunlight, are not available at night. However, an advantage of visible images is their high resolution—which makes detection of smaller features possible. Figure 12–21 is a close-up of a portion
Figure 12–21 Close-up GOES visible image of a portion of north-central United States about midday on July 15, 2011.
MN IA
WI IL
MI IN
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OH
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The Atmosphere: An Introduction to Meteorology
of the visible image in Figure 12–19. Notice the bands of fair-weather cumulus clouds over eastern Missouri and Iowa and the bumpy tops of the developing cumulonimbus clouds over southeastern Minnesota. Visible images are ideal for showing cloud patterns and storm systems such as midlatitude cyclones and hurricanes. Water-Vapor Images Water-vapor images provide yet another way to view Earth. Most of Earth’s radiation with a wavelength of 6.7 micrometers is emitted by water vapor. Satellites equipped with detectors for this narrow band of radiation are, in effect, mapping the concentration of water vapor in the atmosphere. Bright-white regions in Figure 12–22 represent regions of high water-vapor concentration, whereas dark areas indicate drier air. Because most fronts occur between air masses having contrasting moisture conditions, water-vapor images are valuable tools for locating frontal boundaries.
Other Satellite Measurements (a)
Ingenious developments have made weather satellites more than high-tech TV cameras pointed at Earth from space. For example, some satellites are equipped with instruments designed to measure temperatures and moisture at various altitudes. Other satellites allow us to measure rainfall intensity and amounts, both at the surface and
(b)
These accompanying satellite images of the eastern Pacific and a portion of North America were obtained at the same time on July 14, 2001. One of these is an infrared image and the other is a visible light image. (Courtesy of NOAA) Question 1 On which image (a or b) are the cloud shapes and patterns more easily recognized? Question 2 Which one of these images was obtained using infrared imagery? How can you tell? Question 3 What type of clouds are found in the western Great Plains (just east of the Rockies)? Question 4 Are the clouds over the eastern Pacific mainly high clouds or middle- to low-level clouds?
Figure 12–22 Water-vapor image from the GOES for July 15, 2011. The greater the intensity of white, the greater the atmosphere’s water-vapor content. Black areas are driest. (Courtesy of NOAA)
Chapter 12 Weather Analysis and Forecasting
above the surface, and to monitor winds in regions where few, if any, conventional instruments are available. Images from the Tropical Rainfall Measuring Mission (TRMM) illustrate satellite-based precipitation measurement (see Box 1.1 page 10).
Concept Check 12.8 1 How do satellites help identify clouds that are the most likely to produce precipitation?
2 What advantage do geostationary satellites have over polarorbiting satellites? Name one disadvantage.
3 What advantages do infrared images have over visible light images? Name one disadvantage.
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Students Sometimes Ask… When were the first U.S. weather maps produced? Meteorological observations were made by the U.S. Weather Bureau, the predecessor to the National Weather Service, for the first time on November 1, 1870. These observations led to the production of the first U.S. Daily Weather Map in 1871. The early maps included isobars and described the weather conditions at select locations but had little predictive value. It wasn’t until the late 1930s that air mass and frontal-analysis techniques were used by the U.S. Weather Bureau. On August 1, 1941, a new Daily Weather Map was introduced. In addition to including various types of fronts, this map used the station model—a group of symbols indicating weather conditions at nearly 100 locations (see Box 12–1). Although the appearance has changed over the years, the basic structure of the Daily Weather Map remains the same today as it did more than 70 years ago.
Give It Some Thought 1. Discuss the difference between weather analysis and weather forecasting. 2. The accompanying radar image shows the precipitation pattern (reds and yellow indicate heavy precipitation, and greens and blues indicate light-to-moderate precipitation) associated with a strong midlatitude cyclone. Use the technique called trend forecasting and assume that the cyclone maintains its current strength for the next 24 hours to complete the following. CANADA ND
Direction of storm movement
MN
MT
WI
NY MI
SD
NE IL CO
WV
IN
VA
KY
MO
KS
PA
OH
IA
WY
c. Will Pennsylvania more likely be warmer or colder in 24 hours than it was when the image was produced? Explain. d. In which state, New York or Georgia, were cirrus clouds more likely overhead when the image was produced? 3. Explain the difference between weather forecasts and 90-day outlooks. 4. The accompanying map is a simplified upper-air chart that shows the flow aloft and the position of the jet stream. Weather patterns such as this are relatively common over North America. When answering the following questions, assume that long-range weather forecasters had evidence that this flow pattern was going to persist for much of the spring and early summer.
NC TN SC
Jet st
OK
AR
NM
ATLANTIC OCEAN LA
GA
ream
AL MS
FL
TX GULF OF MEXICO
RAIN Light
Heavy
a. What state(s) will likely experience thunderstorms associated with a cold front in the next 6 to 12 hours? b. Will the temperatures in Alabama more likely rise or fall during the next 24 hours? Explain.
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a. Would the 90-day outlook for the southeastern United States predict wetter-than-normal, drierthan-normal, or equal chance? b. Would the 90-day outlook for the southwestern United States predict wetter-than-normal, drierthan-normal, or equal chance? c. Explain how you arrived at your answers. 5. Describe the basis of numerical weather prediction. 6. Use the accompanying weather station model todetermine the following weather elements for thislocation (Hint: see Box 12–1 and Appendix B.):
a. Wind direction and speed b. Temperature c. Dew-point temperature d. Sky cover e. Barometric pressure f. Cloud type g. Current weather h. Amount of precipitation in the past six hours 7. What is the difference between numerical weather prediction and the analog methods of weather forecasting? 8. The accompanying infrared satellite image shows a portion of North America at midday. Match the numbers on the image to descriptions a–d.
a. These areas are cloud free, and the infrared sensor is measuring very warm surface temperatures. b. These areas contain low clouds (cumulus), with cloud tops below 2000 meters. c. These areas are blanketed by middle clouds between 2000 and 6000 meters. d. These are areas where the infrared sensor is measuring high, cold cloud tops that are likely associated with towering cumulonimbus clouds and thunderstorms. 9. Sketch a hypothetical orbit for a polar-orbiting and a geostationary satellite on the same diagram (two orbits around Earth). Make sure the orbits are drawn to indicate relative height above Earth’s surface. a. List one advantage of a polar-orbiting satellite. b. List one advantage of a geostationary satellite. c. Explain why geostationary satellites orbit at about 36,000 kilometers (22,000 miles) above Earth’s surface. 10. The accompanying maps show the positions of the jet stream core on two different midwinter days. On which of these days would the southeastern United States be warmer? Explain your choice.
Jet
strea
m
Jet st
re
1
am
1 3
Day 1
4
4 2 2
3 Day 2
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353
WEATHER ANALYSIS AND FORECASTING IN REVIEW r In the United States, the government agency responsible for
r
r
r r
r
gathering and disseminating weather-related information is the National Weather Service (NWS). Perhaps the most important services provided by the NWS are forecasts and warnings of hazardous weather, including thunderstorms, floods, hurricanes, tornadoes, winter storms, and extreme heat. The process of providing weather forecasts and warnings throughout the United States occurs in three stages. First, data are collected and analyzed on a global scale. Second, a variety of techniques are used to establish the future state of the atmosphere—a process called weather forecasting. Finally, forecasts are disseminated to the public, mainly through the private sector. Assessing current atmospheric conditions, called weather analysis, involves collecting, transmitting, and compiling billions of pieces of observational data. On a global scale, the World Meteorological Organization is responsible for gathering, plotting, and distributing weather data. Weather data are displayed on a synoptic weather map, which shows the current status of the atmosphere and includes data on temperature, humidity, pressure, and wind. The methods used in weather forecasting include numerical weather prediction, persistence forecasting, climatological forecasting, the analog method, and trend forecasting (nowcasting). Numerical weather prediction (NWP) is the backbone of modern forecasting. Meteorologists are aware of the strong correlation between cyclonic disturbances at the surface and the daily and seasonal
fluctuations in the wavy flow of the westerlies aloft. Upper-air maps are generated twice daily and provide forecasters with a picture of conditions at various levels in the troposphere. These maps provide information that can be used to predict daily maximum temperature, locate areas where precipitation is likely to occur, and determine whether a thunderstorm will develop into a supercell. r Long-range weather forecasting relies heavily on statistical averages obtained from past weather events, also referred to as climatic data. Weekly, monthly, and seasonal weather outlooks prepared by the National Weather Service are not weather forecasts in the usual sense. Rather, they indicate whether or not a region will experience near-normal precipitation and temperatures. r Over the past several decades, improvements in observing systems, computer models of physical processes, and assimilation of data into numerical weather prediction systems have produced steady improvement in the ability to predict the evolution of larger-scale weather systems as well as day-to-day variations in temperature, precipitation, cloudiness, and air quality. r Polar-orbiting satellites and geostationary satellites are important tools that allow meteorologists to monitor even the most remote parts of the globe and to track the movement of large weather systems. GOES satellites provide visible, infrared, and water vapor images.
VOCABULARY REVIEW analog method (p. 338) Automated Surface Observing Systems (ASOS) (p. 330) climatological forecasting (p. 336) ensemble forecasting (p. 334) geostationary satellite (p. 347) isotach (p. 340) jet streak (p. 340) Model Output Statistics (MOS) (p. 334)
National Centers for Environmental Prediction (p. 329) National Weather Service (NWS), (p. 328) nowcasting (p. 338) numerical weather prediction (p. 333) persistence forecasting (p. 335) polar-orbiting satellite (p. 346) prognostic chart (prog) (p. 334) radiosonde (p. 331)
skill (p. 345) synoptic weather map (p. 331) trend forecasting (p. 338) weather analysis (p. 329) weather forecast (p. 328) Weather Forecast Office (p. 329) wind profiler (p. 331) World Meteorological Organization (WMO) (p. 330)
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PROBLEMS 1. The map in Figure 12–23 has several weather stations plotted on it. Using the weather data for a typical day in March, which are given in Table 12–1, complete the following:
TABLE 12–1
Weather Data for a Typical March Day Temperature (°F)
Pressure (mb)
Wind Direction
Sky Cover (tenths)
Wilmington, NC
57
1009
SW
7
Philadelphia, PA
59
1001
S
10
Hartford, CT
47
1001
SE
Sky obscured
−12
1008
NE
Pittsburgh, PA
52
995
WSW
10
VA
Duluth, MN
−1
1006
N
Sioux City, IA
11
1010
NW
NC
Springfield, MO
35
1011
WNW
2
Chicago, IL
34
985
NW
10
Madison, WI
23
995
NW
10
Nashville, TN
40
1008
SW
10
Louisville, KY
40
1002
SW
5
Indianapolis, IN
35
994
W
10
Atlanta, GA
49
1010
SW
7
Huntington, WV
52
998
SW
6
Toronto, Canada
44
985
E
9
Albany, NY
50
998
SE
7
Savanna, GA
63
1012
SW
10
Jacksonville, FL
66
1013
WSW
10
Norfolk, VA
67
1005
S
10
Cleveland, OH
49
988
SW
4
Little Rock, AK
37
1014
WSW
Cincinnati, OH
41
997
WSW
10
Detroit, MI
44
984
SW
10
Montreal, Canada
42
993
E
10
Quebec, Canada
34
999
NE
Sky obscured
Location
CANADA ME ND
VT MN
NY
WI
SD
NH MA CT RI
MI PA
NJ
IA NE
IN
MD DE
OH
IL
WV
MO
KS
KY TN
OK
SC AR MS
AL
GA ATLANTIC OCEAN
LA
TX
FL GULF OF MEXICO
Figure 12–23 Map to accompany Problem 1.
a. On a copy of Figure 12–23, plot the temperature, wind direction, pressure, and sky coverage by using the international symbols given in Appendix B. b. Using Figure 12–B as a guide, complete this weather map by adding isobars at 4-millibar intervals, the cold front and the warm front, and the symbol for low pressure. c. Apply your knowledge of the weather associated with a midlatitude cyclone in the spring of the year and describe the likely clouds and precipitation (if any) at each of the following locations:
1. 2. 3. 4.
Philadelphia, Pennsylvania Quebec, Canada Toronto, Canada Sioux City, Iowa
2. Many weather reports include a seven-day outlook. Check such a report and jot down the forecast for the last (seventh) day. Then, each day thereafter, write down the forecast for the day in question. Finally, record what actually occurred on that day. Contrast the forecast seven days ahead with what actually took place. How accurate (or inaccurate) was the seven-day forecast for the day you selected? How accurate was the five-day forecast for this day? The two-day forecast?
International Falls, MN
Chapter 12 Weather Analysis and Forecasting
Log in to www.mymeteorologylab.com for animations, videos, MapMaster interactive maps, GEODe media, In the News RSS feeds, web links, glossary flashcards, self-study quizzes and a Pearson eText version of this book to enhance your study of Weather Analysis and Forecasting.
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Air Pollution Air pollution and meteorology are linked in two ways. One concerns the influence that weather conditions have on the dilution and dispersal of air pollutants. The second connection is the reverse and deals with the effect that air pollution has on weather and climate. The first of these associations is examined in this chapter. The second and equally important relationship is discussed in Chapter 14 and is the focus of several sections and special-interest boxes.*
*
See Box 3–3, “How Cities Influence Temperature: The Urban Heat Island,” and Box 13–1, “Air Pollution Changing the Climate of Cities.” In addition, see the section “Country Breezes” in Chapter 7.
This crowded freeway reminds us that cars and trucks are a major source of air pollution. (Photo by Ashley Cooper/Alamy)
After completing this chapter you should be able to:
t List several natural sources of air pollution and identify those that are human accentuated.
t Distinguish between primary and secondary pollutants.
t List the major primary pollutants and summarize their effects on people and the environment.
t Discuss different meanings for the term smog and describe the formation of photochemical smog.
t Summarize trends in air quality since 1970.
t Describe the influence of wind as a factor influencing air quality.
t Sketch a graph or diagram that shows a temperature inversion and relate it to mixing depth.
t Contrast surface temperature inversions and inversions aloft.
t Discuss the formation of acid precipitation and list some of its effects on the environment. 357
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The Threat of Air Pollution
SOURCES OF PRIMARY POLLUTANTS Created by humans
Air pollution is a continuing threat to our health and welfare. According to a National Research Council report, in the United States an estimated 1.8 to 3.1 years of life are lost by people living Roasting, Nuclear or Mining, in the most polluted cities due to chronic expoCombustion Chemical heating, atomic quarrying, processes processes refining sure to particulates. In addition, more than 4000 processes farming processes premature deaths occur each year because of the elevated surface ozone concentrations commonly observed in the United States.* Air pollutACCENTUATED BY HUMANS ants also have a negative impact on crop producBreaking Pollens, Blowing Bacteria, Wildfires tion, costing U.S. agriculture more than $1 billion Volcanoes seas terpenes dust viruses annually. In other parts of the world, especially developing countries, the negative impact of air pollution on life and agriculture is even more serious. NATURAL An average adult requires about 13.5 kilograms (30 pounds) of air each day compared with about 1.2 kilograms (2.6 pounds) of food Figure 13–2 Sources of primary pollutants. and 2 kilograms (4.4 pounds) of water. The cleanliness of air, therefore, should certainly be from forest fires and brush fires, and windblown dust are as important to us as the cleanliness of our food and water. all examples of “natural air pollution” (Figure 13–2). Ever Air is never perfectly clean. Many natural sources of since people have been on Earth, however, they have added air pollution have always existed (Figure 13–1). Ash and to the frequency and intensity of some of these natural polgases from volcanic eruptions, salt particles from breaklutants, especially the last two. For example, the dust storm ing waves, pollen and spores released by plants, smoke in Figure 13–3 occurred when strong winds raised dry soil from plowed farm fields. *As reported in Bulletin of the American Meteorological Society, Vol. 89, With the discovery of fire came an increased number No. 6, June 2008. Also note that particulate matter is discussed in the section “Primary Pollutants” and that surface ozone is a significant component of of accidental as well as intentional burnings. Even today, urban smog and is discussed in the section “Secondary Pollutants.” in many parts of the world, fire is used to clear land for agricultural purposes (the so-called slash-and-burn method), filling the air with smoke and reducing visibility. When people clear the land of its natural vegetative cover for any purpose, soil is exposed and blown into the air. Yet when we consider the air in a modern-day industrial city, these human-accentuated forms of pollution, although significant, may seem minor by comparison. Although some types of air pollution are relatively recent creations, others have been around for centuries. Smoke pollution, for example, plagued London for centuries. In 1661, when John Evelyn wrote Fumifugium, or The Inconvenience of Aer and Smoak of London Dissipated, Burn scar Together with Some Remidies Humbly Proposed, the problem of foul air obviously plagued Londoners. In his book, Evelyn notes that a traveler, although many miles from London, “sooner smells than sees the city to which he repairs.” In fact, London continued to have severe air pollution problems well into the twentieth century. It was only after a devastating smog disaster in 1952 that truly decisive action was taken to clean the air. Figure 13–1 These plumes of smoke billowing into the sky London, however, has not monopolized the air pollufrom a wildfire in southern Georgia are an example of natural air tion scene. With the coming of the Industrial Revolution, pollution. Lightning started the fire on April 28, 2011, and on many cities began to experience major air pollution. Instead May 8, when this satellite image was acquired, nearly 62,000 acres had burned. (NASA) of just simply accelerating natural sources, people found
Chapter 13 Air Pollution
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page 361. More than 4000 people died as a result of this fiveday ordeal. The people who suffered most were those with respiratory and heart problems, primarily elderly individuals. Extreme air pollution darkened London again in 1953 and 1962 and affected New York City in 1953, 1963, and 1966. Since these events, the passage of legislation, the development of regulations and standards, and the advancement of control technology have reduced the frequency and severity of such episodes. Nevertheless, health authorities are equally concerned about the slow and subtle effects on our lungs and other organs of air-pollution levels that are much lower but that are present every day, year after year.
Concept Check 13.1 Figure 13–3 An example of natural air pollution that has been accentuated by human activities. This dust storm near Elkhart, Kansas, in May 1937, occurred because the natural vegetative cover that anchored the soil had been removed from a marginal environment so that the land could be farmed. Severe drought made the plowed fields vulnerable to strong winds. It was because of storms like this that portions of the Great Plains were called the Dust Bowl in the 1930s. (Photo reproduced from the collection of the Library of Congress)
many new ways to pollute the air (see Figure 13–2) and many new things with which to pollute it. In the mid- to late nineteenth century, the populations of many American and European cities swelled as people sought work in the growing numbers of new foundries and steel mills. As a result, the urban environment became increasingly fouled by the fumes of industry. In Hard Times, Charles Dickens vividly describes the scene in a late-nineteenth-century factory town:
1 Describe the impact of air pollution on human health. 2 List several examples of natural air pollution. List three that are human accentuated.
Students Sometimes Ask… What is haze? Haze is a reduction in visibility caused when light encounters atmospheric particulate matter and gases. Some light is absorbed by the particles and gases, and some is scattered away before it reaches an observer. More pollutants mean more absorption and scattering of light, which limits the distance we can see and can also degrade the color, clarity, and contrast of what we can see. Visibility impairment is one of the most obvious effects of air pollution. It not only occurs in urban areas but is a serious issue in many of our best-known national parks and wilderness areas. The same fine particles that are linked to serious health problems can also significantly affect our ability to see.
It was a town of machinery and tall chimneys out of which interminable serpents of smoke trailed themselves forever and ever, and never got uncoiled. It had a black canal in it, and a river that ran purple with ill-smelling dye.
It is clear that poor air quality was not the only environmental pollution that plagued these places! However, it should be noted that the rapid rise in urban air pollution was not necessarily viewed with great alarm. Rather, chimneys belching forth smoke and soot were a symbol of growth and prosperity (Figure 13–4). The following quotation from an 1880 speech by the well-known lawyer and orator Robert Ingersoll, for example, is reported to have elicited great cheering and cries of “Good! Good!” from the audience: “I want the sky to be filled with the smoke of American industry and upon that cloud of smoke will rest forever the bow of perpetual promise. That is what I am for.” With the rapid growth of the world’s population and accelerated industrialization, the quantities of atmospheric pollutants increased drastically. One of the most tragic air pollution episodes ever occurred in London in December 1952 and is the focus of Severe and Hazardous Weather: The Great Smog of 1952 on
Figure 13–4 Stacks belching smoke and soot such as these were once a sign of economic prosperity. (Photo by EverettCollection/ Superstock)
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Sources and Types of Air Pollution Air pollutants are airborne particles and gases that occur in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment. Pollutants can be grouped into two categories: primary and secondary. Primary pollutants are emitted directly from identifiable sources. They pollute the air immediately upon being emitted. Secondary pollutants, in contrast, are produced in the atmosphere when certain chemical reactions take place among primary pollutants. The chemicals that make up smog are important examples. In some cases the effects of primary pollutants on human health and the environment are less severe than the effects of the secondary pollutants they form.
Primary Pollutants Figure 13–5 depicts the major primary pollutants as percentages (by weight). Sources vary for each pollutant. For example, electricity generation is the most significant source of sulfur dioxide. By contrast, on-road vehicles are the number-one source of carbon monoxide, nitrogen oxides, and volatile organic compounds. Compared to other sources, the tens of millions of cars and trucks on our streets and highways are clearly the greatest contributors. What follows is a brief survey and description of the major primary pollutants. Particulate Matter Particulate matter (PM) is the general term used for a mixture of solid particles and liquid droplets found in the air. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that they can be detected only with an electron microscope. These particles come in a wide range of sizes: Fine particles are less than 2.5 micrometers in diameter, and coarser-size particles are
Nitrogen Oxides 13.0%
Volatile Organics 12.0%
larger than 2.5 micrometers. These particles originate from many different stationary and mobile sources as well as from natural sources (Figure 13–6). Fine particles (PM2.5) result from fuel combustion from motor vehicles, power generation, and industrial facilities, as well as from residential fireplaces and woodstoves. Coarse particles (PM10) are generally emitted from sources such as vehicles traveling on unpaved roads, materials handling, and crushing and grinding operations, as well as windblown dust. Some particles are emitted directly from their sources, such as smokestacks and cars. In other cases, gases such as sulfur dioxide interact with other compounds in the air to form fine particles. Particulates are frequently the most obvious form of air pollution because they reduce visibility and leave deposits of dirt on the surfaces with which they come in contact. In addition, particulates may carry any or all of the other pollutants dissolved in or absorbed on their surfaces. Originally, total suspended particulate (TSP) was the indicator used to represent this category. It included all particles up to 45 micrometers in diameter. In 1987 the U.S. Environmental Protection Agency (EPA) set new standards related only to particles smaller than 10 micrometers (identified as PM10). Then, in 1997, the EPA revised its standards for particulate matter again so that they were based on PM2.5. This change was in response to a large amount of research that analyzed the health effects of particulates. Inhalable particulate matter includes both fine and coarse particles. These particles can accumulate in the respiratory system and are associated with numerous health effects. Exposure to coarse particles is primarily associated with the aggravation of respiratory conditions, such
Particulate matter (PM2.5) 300
Wind As a Factor
Figure 13–13 The national Air Quality Index (AQI) forecast map for August 25, 2010. Specific colors for each AQI category make interpreting the map relatively easy. To check the current map, go to www.airnow.gov.
The AQI scale runs from 0 to 500 (Figure 13–13). The higher the value, the greater the level of air pollution and the greater the health concern. An AQI value of 100 generally corresponds to the national air quality standard for the pollutant. Values below 100 are usually considered satisfactory. When values exceed 100, air quality is considered to be unhealthy—at first for sensitive groups and then, as values increase, for everyone else. Specific colors are assigned to each AQI category to make it easier for people to quickly understand whether air pollution is reaching unhealthy levels in their communities. To access the current AQI, visit www.airnow.gov.
Concept Check 13.3 1 When was the Clean Air Act established?
The manner in which wind speed influences the concentration of pollutants is shown in Figure 13–14. Assume that a burst of pollution leaves a chimney stack every second. If the wind speed were 10 meters per second (23 miles per hour), the distance between each pollution “cloud” would be 10meters. If the wind is reduced to 5 meters per second, the distance between “clouds” will be 5 meters. Consequently, because of the direct effect of wind speed, the concentration of pollutants is twice as great with the 5 meters per second wind as with the 10 meters per second wind. It is easy to understand why air pollution problems seldom occur when winds are strong but rather are associated with periods when winds are weak or calm. A second aspect of wind speed influences air quality. The stronger the wind, the more turbulent the air. Thus, strong winds mix polluted air more rapidly with the surrounding air, thereby causing the pollution to be more dilute. Conversely, when winds are light, there is little turbulence, and the concentration of pollutants remains high.
Wind 10 m/sec
What are its criteria pollutants?
2 Compare emissions of primary pollutants in 1970 with emissions in 2009.
3 What is the Air Quality Index?
Meteorological Factors Affecting Air Pollution The most obvious factor influencing air pollution is the quantity of contaminants emitted into the atmosphere. Still, experience shows that even when emissions remain relatively steady for extended periods, wide variations in air quality often occur from one day to the next. Indeed, when air pollution episodes
Wind 5 m/sec
Figure 13–14 Effect of wind speed on the dilution of pollutants. The concentration of pollutants increases as wind speed decreases.
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Chapter 13 Air Pollution
Viewing an Air Pollution Episode from Space In early October 2010, a high-pressure system settled in over eastern China, and air quality began to deteriorate. By October 9 and 10, China’s National Environmental Monitoring Center declared air quality to be poor to hazardous around Beijing and in 11eastern provinces. Citizens were advised to take measures to protect themselves. Visibility was reduced to 100 meters (330 feet) in some areas, and news outlets reported that at least 32 people died in traffic accidents caused by the poor visibility. Thousands suffered with asthma and other respiratory difficulties. Instruments on NASA’s Aqua and Terra satellites captured the natural-color view of this air pollution episode shown in Figure13–D. The milky white and gray covering the right portion of the image is smog, while the brighter white patches are clouds. Two other images from NASA’s Aura satellite show levels of aerosols (Figure 13–E)
and sulfur dioxide (Figure 13–F). The primary source of sulfur dioxide is coal-burning power plants and smelters. Peak concentrations were 6 to 8 times the normal levels for China and 20 times the normal levels for the United States. The Aerosol Index indicates the presence of ultraviolet light–absorbing particles—largely smoke from agricultural burning and industrial processes. Figure 13–E shows that some areas had an index of 3.5. At an index value of 4, aerosols are so dense that you would have difficulty seeing the midday sun. On October 11, the weather changed. The stagnant air associated with high pressure was replaced when a cold front brought cleansing rain and strong winds that cleared the sky. Clearly, this air pollution episode could not have occurred without emissions from human activities. However, it is also apparent that atmospheric conditions play a key role in causing variations in air quality.
Beijing
Zhengzhou
N
100 km Aerosol Index 0.0
1.75
3.5
FIGURE 13–E This satellite image shows the extremely high levels of aerosols associated with the October 2010 air pollution episode in China. Gray areas lack data. (NASA)
Beijing Beijing
Zhengzhou Zhengzhou N
100 km SO2 (dobson units)
N
0.0
200 km
FIGURE 13–D Serious air pollution plagues a portion of China on October 8, 2010. (NASA)
The Role of Atmospheric Stability Whereas wind speed governs the amount of air into which pollutants are initially mixed, atmospheric stability determines the extent to which vertical motions will mix the pollution with cleaner air above. The vertical distance between Earth’s surface and the height to which convectional move-
4.0
8.0
FIGURE 13–F Sulfur dioxide (SO2) levels were high during the October 2010 air pollution episode in China. Gray areas lack data. (NASA)
ments extend is called the mixing depth. Generally, the greater the mixing depth, the better the air quality. When the mixing depth is several kilometers, pollutants are mixed through a large volume of cleaner air and dilute rapidly. When the mixing depth is shallow, pollutants are confined to a much smaller volume of air, and concentrations can reach unhealthy levels.
The Atmosphere: An Introduction to Meteorology
When air is stable, convectional motions are suppressed and mixing depths are small. Conversely, an unstable atmosphere promotes vertical air movements and greater mixing depths. Because heating of Earth’s surface by the Sun enhances convectional movements, mixing depths are usually greater during the afternoon hours. For the same reason, mixing depths during the summer months are typically greater than during the winter months. Temperature inversions represent situations in which the atmosphere is very stable and mixing depths are significantly restricted. Warm air overlying cooler air acts as a lid and prevents upward movement, leaving the pollutants trapped in a relatively narrow zone near the ground. This effect is dramatically illustrated by the photograph in Figure 13–15. Most of the air pollution episodes cited earlier were linked to the occurrence of temperature inversions that remained in place for many hours or days.
Increasing altitude
370
Temperature profile at night Warm inversion layer
Cold surface
Increasing temperature (a)
Afternoon temperature profile
Increasing altitude
Surface Temperature Inversions Solar heating can result in high surface temperatures during the late morning and afternoon that increase the environmental lapse rate and render the lower air unstable. During nighttime hours, however, just the opposite situation may occur: Temperature inversions, which result in very stable atmospheric conditions, can develop close to the ground. These surface inversions form because the ground is a more effective radiator than the air
Warm surface
Figure 13–15 Air pollution in downtown Los Angeles. Temperature inversions act as lids that trap pollutants below. (Photo by Walter Bibikow/agefotostock)
Increasing temperature (b)
Figure 13–16 (a) A generalized temperature profile of a surfa