Mpg 10e lecture_ch06

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Mpg 10e lecture_ch06

  1. 1. Chapter 6: Atmospheric Moisture McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess
  2. 2. © 2011 Pearson Education, Inc. Atmospheric Moisture • The Impact of Moisture on the Landscape • The Hydrologic Cycle • The Nature of Water: Commonplace but Unique • Phase Changes of Water • Water Vapor and Evaporation • Measures of Humidity • Condensation 2
  3. 3. © 2011 Pearson Education, Inc. Atmospheric Moisture • Adiabatic Processes • Clouds • The Buoyancy of Air • Precipitation • Atmospheric Lifting and Precipitation • Global Distribution of Precipitation • Acid Rain 3
  4. 4. © 2011 Pearson Education, Inc. The Impact of Moisture on the Landscape • Formation of fog, haze, clouds, and precipitation • Short term impacts of precipitation—floods • Longer term impacts (i.e., caves) on Earth’s surface 4
  5. 5. © 2011 Pearson Education, Inc. The Hydrologic Cycle 5 Figure 6-1
  6. 6. © 2011 Pearson Education, Inc. The Nature of Water: Commonplace but Unique • Chemistry of water – Atoms and molecules – Two hydrogen and one oxygen molecule (H2O) – Covalent bonds – Electrical polarity of water molecule – Hydrogen bonds 6 Figure 6-2
  7. 7. © 2011 Pearson Education, Inc. The Nature of Water: Commonplace but Unique • Important properties of water – Exists as a liquid at most points on Earth’s surface – Expands when it freezes; less dense than liquid water; ice floats in water – Hydrogen bonding creates surface tension, a “skin” of molecules giving water a stickiness quality – Capillarity – Good solvent – High specific heat 7
  8. 8. © 2011 Pearson Education, Inc. Phase Changes of Water • Water typically exists in three states – Solid: ice – Liquid: liquid water – Gas: water vapor • Latent heat is required to convert water to its different phases 8 Figure 6-4
  9. 9. © 2011 Pearson Education, Inc. Phase Changes of Water • Phase change processes – Condensation: gas to liquid – Evaporation: liquid to gas – Freezing: liquid to solid – Melting: solid to liquid – Sublimation: solid to gas and gas to solid • Latent heat required for each process • Latent heat as a source of atmospheric energy 9 Figure 6-5
  10. 10. © 2011 Pearson Education, Inc. Water Vapor and Evaporation • Properties of water vapor – Colorless, odorless, invisible – Air feels sticky • Evaporation – Warmer temperatures evaporate more water – Vapor pressure – Windiness reduces evaporation – Evapotranspiration 10 Figure 6-6
  11. 11. © 2011 Pearson Education, Inc. Measures of Humidity • Humidity—amount of water vapor in the air • Absolute humidity—mass of vapor for a given volume of air • Specific humidity—mass of water vapor for a given mass of air • Vapor pressure—contribution of water vapor to total atmospheric pressure 11 Figure 6-7
  12. 12. © 2011 Pearson Education, Inc. Measures of Humidity • Relative humidity—how close the air is to saturation • Saturation represents the maximum amount of water vapor the air can hold • Saturation depends on temperature • Saturation vapor pressure 12 Figure 6-8
  13. 13. © 2011 Pearson Education, Inc. Measures of Humidity • Relative humidity—example calculation – Assume air at 20°C has 10 g of water vapor per kg of dry air – To calculate relative humidity, use the curve to get saturation conditions at 20°C (15 g/kg) – RH = (10g/15g) X 100% = 66.7% 13 Figure 6-8 Saturation specific humidity at temperature of 20°C
  14. 14. © 2011 Pearson Education, Inc. Measures of Humidity • Temperature and relative humidity are inversely related • Dewpoint temperature • Sensible temperature 14 Figure 6-9
  15. 15. © 2011 Pearson Education, Inc. Condensation • Conversion of vapor to liquid water • Surface tension makes it nearly impossible to grow pure water droplets • Supersaturated air • Need particle to grow droplet around, a cloud condensation nuclei • Liquid water can persist at temperatures colder than 0°C without a nuclei—supercooled 15 Figure 6-10
  16. 16. © 2011 Pearson Education, Inc. Adiabatic Processes • Definition of adiabatic process • Dry adiabatic lapse rate • Lifting condensation level (LCL) • Saturated adiabatic lapse rate • Parcel lapse rates versus environmental lapse rate 16 Figure 6-13
  17. 17. © 2011 Pearson Education, Inc. Clouds • Definition of clouds • Influence on radiant energy • Classification (3 primary cloud forms) – Cirrus clouds 17 Figure 6-15a
  18. 18. © 2011 Pearson Education, Inc. Clouds – Stratus clouds – Cumulus clouds 18 Figure 6-15b Figure 6-15c
  19. 19. © 2011 Pearson Education, Inc. Clouds • Cloud types – High clouds (over 6 km) – Middle clouds (from 2 to 6 km) – Low clouds (less than 2 km) – Clouds of vertical development • Grow upward from low bases to heights of over 15 km occasionally 19 Figure 6-16
  20. 20. © 2011 Pearson Education, Inc. Fog 20 Figure 6-18
  21. 21. © 2011 Pearson Education, Inc. Dew and Frost • Dew – Usually originates from terrestrial radiation – Moisture condensation on surfaces that have been cooled to saturation – Will appear as water droplets • Frost – Simply a cloud on the ground – Occurs when air temperature lowers to saturation point, when the saturation point is below 0°C (32°F) – Will appear as large numbers of small white crystals 21 Figure 6-20
  22. 22. © 2011 Pearson Education, Inc. The Buoyancy of Air • Definition of buoyancy • Stable air—parcel is negatively buoyant, will not rise without an external force • Unstable air—parcel is positively buoyant, will rise without an external force • Conditional instability 22 Figure 6-21
  23. 23. © 2011 Pearson Education, Inc. The Buoyancy of Air • Determination of stability via temperature and lapse rate • Stable • Unstable 23 Figure 6-23 Figure 6-24
  24. 24. © 2011 Pearson Education, Inc. The Buoyancy of Air • Conditional instability • Visual determination of instability 24 Figure 6-26 Figure 6-25
  25. 25. © 2011 Pearson Education, Inc. Precipitation • Originates from clouds • Condensation insufficient to form raindrops • Other processes important • Collision/coalescence—tiny cloud drops collide and merge to form larger drops 25 Figure 6-27
  26. 26. © 2011 Pearson Education, Inc. Precipitation • Ice crystal formation – Bergeron process – Ice crystals and supercooled droplets coexist in cold clouds – Ice crystals attract vapor, supercooled drops evaporate to replenish the vapor – Ice crystals fall as snow or rain 26 Figure 6-28
  27. 27. © 2011 Pearson Education, Inc. Precipitation • Types of precipitation – Rain: liquid water – Snow: cloud ice crystals – Sleet: snow melted and frozen again before hitting land, ice pellets – Glaze (Freezing Rain): water falls as liquid, freezes to surfaces – Hail: strong updrafts are required 27 Figure 6-30
  28. 28. © 2011 Pearson Education, Inc. Atmospheric Lifting • Four types of atmospheric lifting 28 Figure 6-32
  29. 29. © 2011 Pearson Education, Inc. Global Distribution of Precipitation • High precipitation regions, tropics • Low precipitation regions, deserts and poles 29 Figure 6-34
  30. 30. © 2011 Pearson Education, Inc. Global Distribution of Precipitation 30 Figure 6-35
  31. 31. © 2011 Pearson Education, Inc. Global Distribution of Precipitation 31 Figure 6-37
  32. 32. © 2011 Pearson Education, Inc. Acid Rain • Definition of acid rain • Sources of acid rain • Principal acids— sulfuric and nitric • Number of hydrogen ions—pH 32 Figure 6-38
  33. 33. © 2011 Pearson Education, Inc. Acid Rain • Distribution of acid rain in the United States 33 Figure 6-39
  34. 34. © 2011 Pearson Education, Inc. Summary • Moisture can impact the landscape in a variety of ways, including fog, haze, and precipitation • The hydrologic cycle shows the balance between water removed from the oceans and water returned by precipitation • Water has a number of unique properties • Water vapor is the gas form of water • Evaporation rates change as surrounding atmospheric conditions change • There are several measures of vapor content in the atmosphere 34
  35. 35. © 2011 Pearson Education, Inc. Summary • There are several measures of vapor content in the atmosphere, called humidity measurements • Condensation is the process by which vapor is converted to liquid • Adiabatic processes explain changes in parcel temperature without the addition or subtraction of heat to the parcel • Clouds are a visual identification of saturation • Air has buoyancy associated with it that describes its stability 35
  36. 36. © 2011 Pearson Education, Inc. Summary • Many processes are responsible for precipitation • There are five primary types of precipitation • Atmospheric lifting occurs through four primary mechanisms • The most highly variable rainfall worldwide occurs over deserts • Tropical regions are generally wet • Acid rain affects the Northeast and results from compounds released into the air by humans 36

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