Understanding Weather and Climate

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

2 Contents Preface xiv Parti Energy and Mass 1 7 Composition and Structure of the Atmosphere The Thickness of the Atmosphere 3 Composition of the Atmosphere 4 The Permanent Gases 5 / Variable Gases 6 Aerosols 11 Evolution of the Atmosphere 11 Vertical Structure of the Atmosphere 14 Density 14 / Layering Based on Temperature Profiles 15 A Layer Based on its Chemical Characteristic: The Ionosphere 18 Planetary Atmospheres 20 Box 1-1 Focus on the Environment: Photosynthesis and Carbon Dioxide 8 Box 1-2 Focus on the Environment: Depletion of the Ozone Layer 10 Box 1-3 Focus on the Environment: Aerosols and Climate 13 Z. Solar Radiation and the Seasons 24 Energy 25 Kinds of Energy 26 / Energy Transfer Mechanisms 27 Radiation 29 Radiation Quantity and Quality 29 Intensity and Wavelengths of Emitted Radiation 30 The Solar Constant 38 The Causes of Earth's Seasons 39 Earth's Revolution and Rotation 39 Box 2-1 Physical Principles: The Three Temperature Scales 33 Box 2-2 Physical Principles: The Nature of Radiation, Absorption, and Emission 35 Box 2-3 Physical Principles: The Sun 36

3 Energy Balance and Temperature 50 Atmospheric Influences on Insolation 51 Absorption 51 / Reflection and Scattering 52 Transmission 56 The Fate of Solar Radiation 56 Energy Transfer Processes Between the Surface and the Atmosphere 57 Surface-Atmosphere Radiation Exchange 57 Conduction 60 / Convection 61 Net Radiation and Temperature 64 Latitudinal Variations 66 Global Temperature Distributions 67 Influences on Temperature 69 Latitude 69 / Altitude 70 Atmospheric Circulation 70 Contrasts Between Land and Water 71 Warm and Cold Ocean Currents 72 Local Impacts on Temperature 72 Measurement of Temperature 74 Instrument Shelters 76 Temperature Means and Ranges 76 Global Extremes 77 Box 3-1 Special Interest: A Look at the Sky 54 Box 3-2 Special Interest: Selective Absorption by Water Vapor and Carbon Dioxide 60 Box 3-3 Physical Principles: The Greenhouse Effect 65 Box 3-4 Physical Principles: Earth's Equilibrium Temperature 69 Part II Water in the Atmosphere 79 Atmospheric Moisture 80 Water Vapor and Liquid Water 81 Evaporation and Condensation 81 Indices of Water Vapor Content 83 Vapor Pressure 83 / Absolute Humidity 84 Specific Humidity 84 / Mixing Ratio 85 Relative Humidity 85 / Dew Point 87 Methods of Achieving Saturation 88 The Effects of Curvature and Solution 93 Effect of Curvature 93 / Effect of Solution 94 Cooling the Air to the Dew or Frost Point 96 Diabatic Processes 96 / Adiabatic Processes 96 The Environmental Lapse Rate 98

4 Forms of Condensation 98 Dew 99 / Frost 99 / Frozen Dew 100 / Fog 101 Formation and Dissipation of Cloud Droplets 104 Box 4-1 Special Interest: Measuring Humidity 90 Box 4-2 Physical Principles: The Varying Value of the Saturated Adiabatic Lapse Rate 99 D Cloud Development and Forms 108 Mechanisms That Lift Air 109 Orographic Uplift 109 / Frontal Lifting 111 Convergence 112 / Localized Convection 112 Static Stability and the Environmental Lapse Rate 112 Absolutely Unstable Air 113 / Absolutely Stable Air 114 Conditionally Unstable Air 114 Factors Influencing the Environmental Lapse Rate 116 Heating or Cooling of the Lower Atmosphere 116 Advection of Cold and Warm Air at Different Levels 117 Advection of an Air Mass with a Different ELR 118 Limitations on the Lifting of Unstable Air 118 A Layer of Stable Air 119 / Entrainment 120 Extremely Stable Air: Inversions 120 Cloud Types 122 High Clouds 123 / Middle Clouds 125 / Low Clouds 127 Clouds with Vertical Development 127 / Unusual Clouds 130 Cloud Coverage 133 Box 5-1 Special Interest: Radiation Inversions and Human Activities 121 Box 5-2 Focus on the Environment: The Unknown Influence of Clouds on Climatic Change 123 Box 5-3 Special Interest: Why Clouds Have Clearly Defined Boundaries 131 Box 5-4 Physical Principles: The Surprising Composition of Clouds Precipitation Processes 136 Growth of Cloud Droplets 137 Growth by Condensation 138 / Growth in Warm Clouds 138 Growth in Cool and Cold Clouds 139 Forms of Precipitation 141 Snow 141 / Rain 144 / Graupel and Hail 147 Sleet 147 / Freezing Rain 148 Measuring Precipitation 150 Raingages 150 / Snow Measurement 153 Cloud Seeding 154 Box 6-1 Physical Principles: Why Cloud Droplets Don't Fall 142 Box 6-2 Physical Principles: The Effect of Hail Size on Damage 149

5 Part III Distribution and Movement of Air Atmospheric Pressure and Wind 158 The Concept of Pressure 159 Vertical and Horizontal Changes in Pressure 160 The Equation of State 162 Measurement of Pressure 164 Mercury Barometers 164 / Corrections to Mercury Barometer Readings 165 / Aneroid Barometers 165 The Distribution of Pressure 166 Pressure Gradients 166 / Hydrostatic Equilibrium 168 Horizontal Pressure Gradients in the Upper Atmosphere 169 Forces Affecting the Speed and Direction of the Wind 171 The Coriolis Force 171 / Friction 175 Winds in the Upper Atmosphere 175 Supergeostrophic and Subgeostrophic Flow 177 Near-Surface Winds 178 Cyclones, Anticyclones, Troughs, and Ridges 179 Measuring Wind 182 Box 7-1 Physical Principles: Velocity, Acceleration, Force, and Pressure 162 Box 7-2 Special Interest: How Does Humidity Affect Density? 164 Box 7-3 Physical Principles: The Hydrostatic Equation 170 Box 7-4 Physical Principles: The Equation of Motion Atmospheric Circulation and Pressure Distributions 184 Single-Cell Model 185 The Three-Cell Model 187 The Hadley Cell 187 / The Ferrel and Polar Cells 190 The Three-Cell Model vs. Reality: The Bottom Line 190 Semi-permanent Pressure Cells 191 The Upper Troposphere 193 Westerly Winds in the Upper Atmosphere 195 / The Polar Front and Jet Streams 196 / Troughs and Ridges 196 / Rossby Waves 198 The Oceans 199 Ocean Currents 200 / Upwelling 204 Major Wind Systems 205 Monsoons 205 / Foehn, Chinook, and Santa Ana Winds 210 Katabatic Winds 212 / Sea and Land Breeze 213 Valley and Mountain Breeze 214 Box 8-1 Physical Principles: Problems with the Single-Cell Model 188 Box 8-2 Physical Principles: The Movement of Rossby Waves 200 Box 8-3 Special Interest: The Dishpan Experiment 203 Box 8-4 Special Interest: El Nino and the Walker Circulation 206

6 9 Air Masses and Fronts 218 Formation of Air Masses ~ 219 Source Regions 219 / Continental Polar (cp) and Continental Arctic (ca) Air Masses 220 Maritime Polar (mp) Air Masses 222 Continental Tropical (ct) Air Masses 222 Maritime Tropical (mt) Air Masses 228 Fronts 228 Cold Fronts 229 / Warm Fronts 232 / Stationary Fronts 233 Occluded Fronts 234 / Drylines 235 Box 9-1 Special Interest: Maritime Air Masses Invade Eastern North America 224 Box 9-2 Special Interest: The Pineapple Express 229 Part IV Disturbances Mid-latitude Cyclones 240 The Life Cycle of a Mid-latitude Cyclone 241 Cyclogenesis 241 / Mature Cyclones 243 Occlusion 244 / Evolution and Movement of Cyclones 244 Processes of the Middle and Upper Troposphere 246 Rossby Waves and Vorticity 246 The Effect of Fronts on Upper-Level Patterns 250 Cold Fronts and the Formation of Upper-Level Troughs 251 Interaction of Surface and Upper-Level Patterns 252 An Example of a Mid-latitude Cyclone 254 April / April / April / April Flow Patterns and Large-Scale Weather 258 The Modern View Mid-latitude Cyclones and Conveyor Belts 258 Migration of Surface Cyclones Relative to Rossby Waves 261 Anticyclones 262 Box 10-1 Physical Principles: Vorticity and Rossby Waves 248 Box 10-2 Special Interest: A Closer Look at Divergence and Convergence 252 Box 10-3 Special Interest: Short Waves in the Upper Atmosphere and Their Effect on Surface Conditions Lightning, Thunder, and Tornadoes 266 Processes of Lightning Formation 267 Charge Separation 268 / Leaders, Strokes, and Flashes 269 Types of Lightning 271 / Thunder 271 Lightning Safety 272

7 Thunderstorms: Self-Extinguishing vs. Self-Propagating 272 Air Mass Thunderstorms 272 / Severe Thunderstorms 274 Mesoscale Convective Complexes 275 / Squall Line Thunderstorms 278 / Supercell Storms 278 / Microbursts 280 Geographic and Temporal Distribution of Thunderstorms 280 Tornadoes 281 Tornado Characteristics and Dimensions 284 / Tornado Formation 284 / The Location and Timing of Tornadoes 286 Tornado Damage 288 / Fatalities 294 Watches and Warnings 294 / Waterspouts 296 Box 11-1 Physical Principles: Electricity in the Atmosphere 270 Box 11-2 Special Interest: Potential Instability 276 Box 11-3 Special Interest: Doppler Radar 282 Box 11-4 Special Interest: Tornado Research Tropical Storms and Hurricanes 298 Hurricanes Around the Globe 299 The Tropical Setting 301 Hurricane Characteristics 302 The Eye and the Eye Wall 304 Hurricane Formation 305 Conditions Necessary for Hurricane Formation 306 Hurricane Movement and Dissipation 307 Destruction by Hurricanes 311 Hurricane Forecasts and Advisories 313 Hurricane Watches and Warnings 314 Hurricane Intensity Scale 315 Box 12-1 Special Interest: The Naming of Hurricanes 309 Box 12-2 Special Interest: Seasonal and Long-Term Hurricane Forecasts 315 Party Human Activities Weather Forecasting and Analysis 320 Forecasting Methods 322 Types of Forecasts 323 Assessing Forecasts 323 Data Acquisition and Dissemination 325 Forecast Procedures and Products 327 Analysis Phase 327 / Prediction Phase 328 Post-Processing Phase 328 / Medium-Range Forecasts 330 Long-Range Forecasts 333

8 Weather Maps and Images 335 Surface Maps 336 / Upper-Level Maps 337 Satellite Images 342 / Radar Images 344 Thermodynamic Diagrams Lifted Index and K-Index 346 Chapter 13 Appendix: Numerical Forecast Models 349 Model Characteristics 349 Scale Considerations 349 / Horizontal Representation 350 Physical Processes 351 Measures of Forecast Accuracy and Skill 352 Box 13-1 Special Interest: Modernization of the National Weather Service Human Effects: Air Pollution and Heat Islands 356 Atmospheric Pollutants 357 Particulates 358 / Carbon Oxides 361 Sulfur Compounds 362 / Nitrogen Oxides (NO X ) 364 Volatile Organic Compounds (Hydrocarbons) 365 Photochemical Smog 366 Atmospheric Controls on Air Pollution 368 Effect of Winds on Horizontal Transport 368 Effect of Atmospheric Stability 369 Urban Heat Islands 372 Radiation Effects 372 / Changes in Heat Storage 374 Sensible and Latent Heat Transfer 375 Box 14-1 Focus on the Environment: Air Pollution- Natural or Anthropogenic? 360 Box 14-2 Focus on the Environment: The Counteroffensive on Air Pollution 366 Box 14-3 Focus on the Environment: Smog in Southern California 370 Part VI Current, Past, and Future Climates Earth's Climates 380 The Koeppen System 382 Tropical Climates 384 Tropical Wet (Af) 385 / Monsoonal (Am) 386 Tropical Wet and Dry (Aw) 388 Dry Climates 390 Subtropical Deserts (BWh) 390 / Subtropical Steppe (BSh) 392 Mid-latitude Deserts (BWk) 393 / Mid-latitude Steppe (BSk) 394 Mild Mid-latitude Climates 394

9 Mediterranean (Csa, Csb) 395 / Humid Subtropical (Cfa,Cwa) 396 / Marine West Coast (Cfb, Cfc) 397 Severe Mid-latitude Climates 398 Humid Continental (Dfa, Dfb, Dwa, Dwb) 398 Subarctic (Dfc, Dfd, Dwc, Dwd) 399 Polar Climates 399 Tundra (ET) 400 / Ice Cap (EF) 401 / Highland Climates (H) 401 Box 15-1 Special Interest: Different Climate Classification Schemes for Different Purposes Climate Changes: Past and Future 404 Defining Climate Change 405 The Time Scales of Climate Change 406 Past Climates 407 Warm Intervals and Ice Ages 407 / The Current Ice Age 409 The Last Glacial Maximum 411 / The Holocene 413 The Last Century 416 / Millennial-Scale Oscillations 416 Factors Involved in Climatic Change 417 Variations in Solar Output 417 / Changes in Earth's Orbit 418 Changes in Land Configuration and Surface Characteristics 420 Changes in Atmospheric Turbidity 421 Changes in Radiation-Absorbing Gases 423 Feedback Mechanisms 425 Ice-Albedo Feedback 425 / Evaporation of Water Vapor 428 Ocean-Atmosphere Interactions 428 Atmosphere-Biota Interactions 429 Methods for Determining Past Climates 430 Remnant Landforms 430 / Past Vegetation 432 Relic Soils 434 / Oceanic Deposits 434 / Ice Cores 434 Box 16-1 Focus on the Environment: Historic Warmth in the 1990s 415 Box 16-2 Special Interest: General Circulation Models 426 Box 16-3 Focus on the Environment: Plant Migrations and Global Change 431 Appendix A: Units of Measurement and Conversions 437 Appendix B: The Standard Atmosphere 439 Appendix C: Weather Map Symbols 441 Appendix D: Weather Extremes 447 Glossary 455 Index 465

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

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