Contents. 1 Composition and Structure of the Atmosphere 2. 2 Solar Radiation and the Seasons 38. Part One Energy and Mass 1

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Preface xvi Part One Energy and Mass 1 1 Composition and Structure of the Atmosphere 2 The Thickness of the Atmosphere 5 Composition of the Atmosphere 5 The Permanent Gases 6 / Variable Gases 7

1 Preface xvi Part One Energy and Mass 1 1 Composition and Structure of the Atmosphere 2 The Thickness of the Atmosphere 5 Composition of the Atmosphere 5 The Permanent Gases 6 / Variable Gases 7 Aerosols 11 Vertical Structure of the Atmosphere 13 Density 14 / Layering Based on Temperature Profiles 14 A Layer Based on Electrical Properties: The Ionosphere 18 Evolution of the Atmosphere 19 Some Weather Basics 20 Atmospheric Pressure and Wind 20 / Temperature 24 Humidity 25 Planetary Atmospheres 25 Weather Forecasting Both Art and Science Focus on the Environment: Photosynthesis, Respiration, and Carbon Dioxide Focus on the Environment: Depletion of the Ozone Layer Focus on the Environment: Aerosols and Climate Special Interest: Recent Severe Weather 22 2 Solar Radiation and the Seasons 38 Energy 39 Kinds of Energy 40 / Energy Transfer Mechanisms 41 Radiation 43 Radiation Quantity and Quality 43 Intensity and Wavelengths of Emitted Radiation 44 The Solar Constant 52 The Causes of Earth s Seasons 53 Earth s Revolution and Rotation Physical Principles: The Three Temperature Scales 47 v

vi 2 2 Physical Principles: The Nature of Radiation, Absorption, and Emission 48 2 3 Physical Principles: The Sun 50 3 Energy Balance and Temperature 66 Atmospheric Influences on Insolation 68

2 vi 2 2 Physical Principles: The Nature of Radiation, Absorption, and Emission Physical Principles: The Sun 50 3 Energy Balance and Temperature 66 Atmospheric Influences on Insolation 68 Absorption 68 / Reflection and Scattering 69 / Transmission 72 The Fate of Solar Radiation 72 Energy Transfer Processes between the Surface and the Atmosphere 74 Surface Atmosphere Radiation Exchange 74 / Conduction 77 Convection 77 / Net Radiation and Temperature 80 Latitudinal Variations 81 The Greenhouse Effect 82 Global Temperature Distributions 83 Influences on Temperature 85 Latitude 85 / Altitude 85 / Atmospheric Circulation 87 Contrasts Between Land and Water 87 / Warm and Cold Ocean Currents 88 / Local Impacts on Temperature 88 Measurement of Temperature 89 Instrument Shelters 92 Temperature Means and Ranges 93 Global Extremes 94 Temperature and Human Comfort 95 Thermodynamic Diagrams and Vertical Temperature Profiles Physical Principles: Selective Absorption by Water Vapor and Carbon Dioxide Physical Principles: Earth s Equilibrium Temperature Special Interest: Recent Severe Heat Waves Forecasting: Effects of Cloud Cover and Wind Speed on Temperature 94 4 Atmospheric Pressure and Wind 104 The Concept of Pressure 105 Vertical and Horizontal Changes in Pressure 106 The Equation of State 109 Measurement of Pressure 110 Mercury Barometers 110 / Corrections to Mercury 110 Barometer Readings 110 / Aneroid Barometers 112 The Distribution of Pressure 112 Pressure Gradients 113 / Hydrostatic Equilibrium 114 Horizontal Pressure Gradients in the Upper Atmosphere 115

vii Forces Affecting the Speed and Direction of the Wind 118 The Coriolis Force 118 / Friction 121 Winds in the Upper Atmosphere 121 Supergeostrophic and Subgeostrophic Flow 122 Near-Surface Winds

3 vii Forces Affecting the Speed and Direction of the Wind 118 The Coriolis Force 118 / Friction 121 Winds in the Upper Atmosphere 121 Supergeostrophic and Subgeostrophic Flow 122 Near-Surface Winds 123 Cyclones, Anticyclones, Troughs, and Ridges 124 Measuring Wind Physical Principles: Velocity, Acceleration, Force, and Pressure Physical Principles: Variations in Density Physical Principles: The Hydrostatic Equation Physical Principles: The Coriolis Effect Physical Principles: The Equation of Motion Special Interest: Strong Winds and the Little Rock, Arkansas, Air Disaster 127 Part Two Water in the Atmosphere Atmospheric Moisture 132 Water Vapor and Liquid Water 133 Evaporation and Condensation 134 Indices of Water Vapor Content 135 Vapor Pressure 135 / Absolute Humidity 136 Specific Humidity 136 / Mixing Ratio 137 Relative Humidity 137 / Dew Point 139 Distribution of Water Vapor 141 Methods of Achieving Saturation 141 The Effects of Curvature and Solution 145 Effect of Curvature 145 / Effect of Solution 147 Ice Nuclei 147 Measuring Humidity 148 High Humidities and Human Discomfort 149 Cooling the Air to the Dew or Frost Point 152 Diabatic Processes 153 / Adiabatic Processes 153 The Environmental Lapse Rate 154 Forms of Condensation 156 Dew 156 / Frost 157 / Frozen Dew 157 / Fog 158 Distribution of Fog 161 Formation and Dissipation of Cloud Droplets Forecasting: Dew Point and Nighttime Minimum Temperatures Forecasting: Vertical Profiles of Moisture Physical Principles: The Varying Value of the Saturated Adiabatic Lapse Rate 155

viii 6 Cloud Development and Forms 168 Mechanisms That Lift Air 170 Orographic Uplift 170 / Frontal Lifting 171 / Convergence 172 Localized Convection 173 Static Stability and the Environmental Lapse

Advection of Cold and Warm Air at Different Levels 177 Advection of an Air Mass with a Different ELR 179 Limitations on the Lifting of Unstable Air 179 A Layer of Stable Air 180 / Entrainment 181

4 viii 6 Cloud Development and Forms 168 Mechanisms That Lift Air 170 Orographic Uplift 170 / Frontal Lifting 171 / Convergence 172 Localized Convection 173 Static Stability and the Environmental Lapse Rate 173 Absolutely Unstable Air 173 / Absolutely Stable Air 175 Conditionally Unstable Air 176 Factors Influencing the Environmental Lapse Rate 177 Heating or Cooling of the Lower Atmosphere 177 Advection of Cold and Warm Air at Different Levels 177 Advection of an Air Mass with a Different ELR 179 Limitations on the Lifting of Unstable Air 179 A Layer of Stable Air 180 / Entrainment 181 Extremely Stable Air: Inversions 181 Cloud Types 182 High Clouds 185 / Middle Clouds 187 / Low Clouds 188 Clouds with Vertical Development 189 Unusual Clouds 193 Cloud Coverage Forecasting: Determining Stability from Thermodynamic Diagrams Special Interest: Radiation Inversions and Human Activities Focus on the Environment: The Unknown Influence of Clouds on Climatic Change Special Interest: Why Clouds Have Clearly Defined Boundaries Physical Principles: The Surprising Composition of Clouds Precipitation Processes 200 Growth of Cloud Droplets 202 Growth by Condensation 204 / Growth in Warm Clouds 204 Growth in Cool and Cold Clouds 205 Forms of Precipitation 207 Snow 207 / Rain 211 / Graupel and Hail 212 / Sleet 213 Freezing Rain 214 Measuring Precipitation 215 Raingages 216 / Snow Measurement 218 Cloud Seeding Physical Principles: Why Cloud Droplets Don t Fall Physical Principles: The Effect of Hail Size on Damage 215

Part Three Distribution and Movement of Air 225 8 Atmospheric Circulation and Pressure Distributions 226 ix Single-Cell Model 228 The Three-Cell Model 228 The Hadley Cell 228 / The Ferrel and Polar

5 Part Three Distribution and Movement of Air Atmospheric Circulation and Pressure Distributions 226 ix Single-Cell Model 228 The Three-Cell Model 228 The Hadley Cell 228 / The Ferrel and Polar Cells 232 The Three-Cell Model vs. Reality: The Bottom Line 232 Semipermanent Pressure Cells 232 The Upper Troposphere 234 Westerly Winds in the Upper Atmosphere 235 The Polar Front and Jet Streams 237 / Troughs and Ridges 238 Rossby Waves 240 The Oceans 242 Ocean Currents 242 / Upwelling 244 Major Wind Systems 246 Monsoons 246 / Foehn, Chinook, and Santa Ana Winds 249 Katabatic Winds 252 / Sea and Land Breeze 252 Valley and Mountain Breeze 254 Air Sea Interactions 254 El Niño, La Niña, and the Walker Circulation 255 Pacific Decadal Oscillation 258 / Arctic Oscillation Physical Principles: Problems with the Single-Cell Model Physical Principles: The Movement of Rossby Waves Physical Principles: The Dishpan Experiment Focus on the Environment: Wildfires Air Masses and Fronts 264 Formation of Air Masses 265 Source Regions 266 / Continental Polar (cp) and Continental Arctic (ca) Air Masses 267 / Maritime Polar (mp) Air Masses 269 Continental Tropical (ct) Air Masses 274 / Maritime Tropical (mt) Air Masses 274 Fronts 275 Cold Fronts 275 / Warm Fronts 278 / Stationary Fronts 279 Occluded Fronts 280 / Drylines Special Interest: Maritime Air Masses Invade Eastern North America Special Interest: The Pineapple Express 276

x Part Four Disturbances 287 10 Mid-latitude Cyclones 288 Polar Front Theory 289 The Life Cycle of a Mid-latitude Cyclone 290 Cyclogenesis 290 / Mature Cyclones 290 / Occlusion 292 Evolution and

6 x Part Four Disturbances Mid-latitude Cyclones 288 Polar Front Theory 289 The Life Cycle of a Mid-latitude Cyclone 290 Cyclogenesis 290 / Mature Cyclones 290 / Occlusion 292 Evolution and Movement of Cyclones 293 Processes of the Middle and Upper Troposphere 294 Rossby Waves and Vorticity 294 Surface Fronts and Upper-Level Patterns 297 Cold Fronts and the Formation of Upper-Level Troughs 299 Interaction of Surface and Upper-Level Conditions 300 An Example of a Mid-latitude Cyclone 302 April / April / April / April Flow Patterns and Large-scale Weather 305 The Steering of Mid-latitude Cyclones 307 Migration of Surface Cyclones Relative to Rossby Waves 308 The Modern View Mid-latitude Cyclones and Conveyor Belts 309 Anticyclones Physical Principles: Vorticity and Rossby Waves Physical Principles: A Closer Look at Divergence and Convergence Forecasting: Short Waves in the Upper Atmosphere and Their Effect on Surface Conditions Lightning, Thunder, and Tornadoes 316 Processes of Lightning Formation 318 Charge Separation 318 / Leaders, Strokes, and Flashes 319 Types of Lightning 321 / Thunder 322 Lightning Safety 323 Thunderstorms: Self-Extinguishing vs. Self-Propagating 324 Air Mass Thunderstorms 324 / Severe Thunderstorms 325 Downbursts and Microbursts 333 Geographic and Temporal Distribution of Thunderstorms 334 Tornadoes 335 Tornado Characteristics and Dimensions 335 / Tornado Formation 336 The Location and Timing of Tornadoes 339 / Tornado Damage 344 Fatalities 345 / Watches and Warnings 346 The Tornado Outbreak of May 3 6, / Waterspouts Physical Principles: Electricity in the Atmosphere Special Interest: A Personal Account of Ball Lightning 323

xi 11 3 Forecasting: Potential Instability 327 11 4 Forecasting: Doppler Radar 332 11 5 Special Interest: Tornado Research 348 12 Tropical Storms and Hurricanes 356 Hurricanes Around the Globe 358

7 xi 11 3 Forecasting: Potential Instability Forecasting: Doppler Radar Special Interest: Tornado Research Tropical Storms and Hurricanes 356 Hurricanes Around the Globe 358 The Tropical Setting 358 Hurricane Characteristics 359 The Eye and the Eye Wall 361 Hurricane Formation 362 Conditions Necessary for Hurricane Formation 363 Hurricane Movement and Dissipation 364 Hurricane Destruction and Fatalities 367 Hurricane Forecasts and Advisories 371 Hurricane Watches and Warnings 374 / Hurricane Intensity Scale Special Interest: The Naming of Hurricanes Special Interest: Recent Notable Hurricanes Forecasting: Seasonal and Long-Term Hurricane Forecasts Special Interest: The Galveston Hurricane of Part Five Human Activities Weather Forecasting and Analysis 384 Why Is Weather Forecasting Imperfect? 385 Forecasting Methods 386 Types of Forecasts 387 Assessing Forecasts 388 Data Acquisition and Dissemination 389 Forecast Procedures and Products 392 Analysis Phase 392 / Prediction Phase 393 Post-Processing Phase 393 / Medium-Range Forecasts 396 Long-Range Forecasts 398 Weather Maps and Images 401 Surface Maps 401 / Upper-Level Maps 402 Satellite Images 407 / Radar Images 408 Thermodynamic Diagrams 410 Lifted Index and K-Index 412 Chapter 13 Appendix: Numerical Forecast Models 417 Model Characteristics 417

xii Scale Considerations 417 / Horizontal Representation 418 Physical Processes 419 Measures of Forecast Accuracy and Skill 420 13 1 Special Interest: Modernization of the National Weather Service

8 xii Scale Considerations 417 / Horizontal Representation 418 Physical Processes 419 Measures of Forecast Accuracy and Skill Special Interest: Modernization of the National Weather Service Human Effects: Air Pollution and Heat Islands 424 Atmospheric Pollutants 425 Particulates 426 / Carbon Oxides 428 / Sulfur Compounds 429 Nitrogen Oxides (NO x ) 432 / Volatile Organic Compounds (Hydrocarbons) 432 / Photochemical Smog 433 Atmospheric Controls on Air Pollution 435 Effect of Winds on Horizontal Transport 436 Effect of Atmospheric Stability 436 Urban Heat Islands 437 Radiation Effects 439 / Changes in Heat Storage 440 Sensible and Latent Heat Transfer Focus on the Environment: Severe Pollution Episodes Focus on the Environment: The Counteroffensive on Air Pollution Focus on the Environment: Smog in Southern California 440 Part Six Current, Past, and Future Climates Earth s Climates 448 Defining Climate 449 The Koeppen System 450 Tropical Climates 453 Tropical Wet (Af) 453 / Monsoonal (Am) 455 Tropical Wet and Dry (Aw) 456 Dry Climates 458 Subtropical Deserts (BWh) 458 / Subtropical Steppe (BSh) 460 Mid-latitude Deserts (BWk) 460 / Mid-latitude Steppe (BSk) 462 Mild Mid-latitude Climates 462 Mediterranean (Csa, Csb) 463 / Humid Subtropical (Cfa, Cwa) 464 Marine West Coast (Cfb, Cfc) 465 Severe Mid-latitude Climates 466 Humid Continental (Dfa, Dfb, Dwa, Dwb) 466 Subarctic (Dfc, Dfd, Dwc, Dwd) 466 Polar Climates 467 Tundra (ET) 467 / Ice Cap (EF) 468 / Highland Climates (H) Special Interest: Different Climate Classification Schemes for Different Purposes 454

xiii 16 Climate Changes: Past and Future 474 Defining Climate Change 475 The Time Scales of Climate Change 476 Past Climates 477 Warm Intervals and Ice Ages 477 / The Current Ice Age 479 The Last

Changes in Land Configuration and Surface Characteristics 490 Changes in Atmospheric Turbidity 491 Changes in Radiation-Absorbing Gases 493 Feedback Mechanisms 495 Ice-Albedo Feedback 496 /

9 xiii 16 Climate Changes: Past and Future 474 Defining Climate Change 475 The Time Scales of Climate Change 476 Past Climates 477 Warm Intervals and Ice Ages 477 / The Current Ice Age 479 The Last Glacial Maximum 481 / The Holocene 483 The Last Century 484 / Millennial-Scale Oscillations 484 Factors Involved in Climatic Change 485 Variations in Solar Output 486 / Changes in Earth s Orbit 488 Changes in Land Configuration and Surface Characteristics 490 Changes in Atmospheric Turbidity 491 Changes in Radiation-Absorbing Gases 493 Feedback Mechanisms 495 Ice-Albedo Feedback 496 / Evaporation of Water Vapor 497 Ocean Atmosphere Interactions 497 Atmosphere Biota Interactions 498 General Circulation Models 499 Methods for Determining Past Climates 501 Oceanic Deposits 502 / Ice Cores 502 / Remnant Landforms 504 Past Vegetation 505 / Relic Soils Focus on the Environment: Historic Warmth in the 1990s and Early 2000s Focus on the Environment: Plant Migrations and Global Change 503 Part Seven Special Topics and Appendices Atmospheric Optics 512 Clear Air Effects 513 Refraction and the Setting or Rising Sun 514 / Mirages 514 Cloud and Precipitation Optics 517 Rainbows 517 / Halos, Sundogs, and Sun Pillars 519 Coronas and Glories 521 Appendix A: Units of Measurement and Conversions 525 Appendix B: The Standard Atmosphere 526 Appendix C: Weather Map Symbols 527 Glossary 533 Credits 543 Index 547

Understanding Weather and Climate

Understanding Weather and Climate Edward Aguado San Diego State University James E. Burt University of Wisconsin-Madison Prentice Hall Upper Saddle River, NJ 07458 Contents Preface xiv Parti Energy and