Atmospheric Moisture

1. Atmospheric Moisture GEOG 1112- Lecture 12

2. Chapter 6: Atmospheric Moisture McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess

3. 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

4. Atmospheric Moisture • Adiabatic Processes • Clouds • The Buoyancy of Air • Precipitation • Atmospheric Lifting and Precipitation • Global Distribution of Precipitation • Acid Rain

5. 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

6. 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 Figure 6-2

7. 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

8. Six Unique Properties of Water • High boiling point • High heat of vaporization • High heat capacity • Superior solvent • High surface tension • Expands when frozen

9. 1- High boiling point • Water exists in liquid phase at normal Earth temperatures • Allows oceans, lakes, rivers • And it also exists as a solid and gas

10. 2- High Heat of Vaporization • 590 cal/g • Water requires a large amount of energy to evaporate • Good for cooling (perspiration) • It plays a very important role in global heat transfer

11. 3- High Heat Capacity • 1 cal/g/°C • Water has the highest heat capacity • It moderates local and global temperatures • Useful for coolant • Good cooking fluid

12. 4- Superior Solvent • Water is both Acid and Base (H-OH) • Water is an excellent cleanser • Over time, it erodes almost anything • Because of this, it is easily polluted SOME PROPERTIES OF SOLVENTS

13. 5- High Surface Tension • Force that causes surface to contract • Water has high wetting ability • Allows capillarity: water moves nutrients through plants and animals

14. 6- Expands when frozen • Water is densest at 4° C • Ice (below 0° C) floats • Oceans don’t freeze from bottom up, which would mean that they would freeze solid • Crucial for Oxygen exchange with atmosphere

15. Water and the Hydrosphere • Three States of Water: • Solid (ice) • Liquid (water) • Gas (vapor)

16. Water and the Hydrosphere • Hydrosphere: Total realm of water at Earth’s surface. • Oceans • Ice • Surface water • Groundwater • Atmospheric water • Soil moisture • Biota

17. Water and the Hydrosphere • Hydrologic Cycle: • Water moves among the ocean, atmosphere and land • Evaporation • Precipitation • Transpiration from plants • Runoff • Sinks into soil • Recharge of groundwater

18. The Hydrologic Cycle Figure 6-1

19. The Hydrologic Cycle

21. Water and the Hydrosphere Distribution of water in the hydrosphere • Oceans: 97.5% • Fresh water: 2.5%

22. Water on Earth

24. Water Budget • Relationship between evaporation, condensation, and storage of water within the Earth system

25. 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 Figure 6-4

26. 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 Figure 6-5

27. Atmospheric Moisture • Bottom 18,000’ (5500 m) of the atmosphere • Source of condensation and precipitation • Absorbs and reflects incoming and outgoing radiation • Helps to maintain Earth temperature • Water and the Heat Budget • Latent heat provides the major source of power for Earth’s storms • Temperature determines the water vapor capacity of air • Polar • Cold ~ 0.2% by volume • Tropical • Warm ~ 5.0% by volume

28. Water Vapor and Evaporation • Properties of water vapor • Colorless, odorless, invisible • Air feels sticky • Evaporation • Warmer temperatures evaporate more water • Vapor pressure • Windiness increases evaporation • Evapotranspiration Figure 6-6

29. Humidity Humidity: the amount of water vapor in the air The maximum quantity of moisture that can be held in the air depends on air temperature

30. 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 Figure 6-7

31. Atmospheric Humidity • Amount of water vapor in an air mass at a given time • Absolute and Specific Humidity • Absolute humidity: mass of water vapor present per unit volume of air, expressed as grams per cubic meter, or grains per cubic foot (g/m3) • Specific humidity: mass of water vapor present per unit mass of air, expressed as grams per kilogram of moist air (g/kg) (mixing ratio) • Vapor pressure- that part of the total atmospheric pressure attributable to its water vapor content

32. 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 Figure 6-8

33. 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% Figure 6-8 Saturation specific humidity at temperature of 20°C

34. Relative Humidity • Compares the amount of water vapor present in the air to the maximum amount that the air can hold at that temperature • Usually expressed as a percentage • 100% is saturated • Relates to water source and temperature • Evaporation is higher when the air is dry (low vapor pressure) than when the air is humid (high vapor pressure) • Air above water is likely to contain more moisture

35. Humidity • Relative Humidity changes when: • Atmosphere gains or loses water vapor • Evaporation • Temperature changes • Lower temperature relative humidity rises • Raise temperature relative humidity decreases

36. Humidity • Specific Humidity: actual quantity of water held by a parcel of air • Grams of water vapor per kilogram of air (g/kg) • Highest in equatorial zones • Lowest near poles

37. Saturation • Saturation: point at which sufficient cooling has occurred so that an air mass contains the maximum amount of water vapor it can hold • Further cooling produces condensation of excess water vapor • Capacity: the maximum amount of water vapor that can be contained in a given quantity of air at a given temperature

38. Humidity • Dew-point temperature: temperature at which air with a given humidity will reach saturation when cooled without changing its pressure • Any further cooling will cause condensation of water vapor in the air

39. Measures of Humidity • Temperature and relative humidity are inversely related • Dewpoint temperature • Sensible temperature Figure 6-9

40. Sources of Atmospheric Moisture • Transpiration and Evapotranspiration • Evaporation: process by which a liquid is converted to the gaseous (or vapor) state by the addition of latent heat • Transpiration: transfer of moisture from living plants to the atmosphere by the emission of water vapor, primarily from leaf pores • Evapotranspiration: combined water loss to the atmosphere from ground and water surfaces by evaporation and, from plants, by transpiration

41. What Determines the Rate of Evaporation? • Amount of available water • Greater over oceans • And the Tropical Rainforest • Temperature of surface and air • Warmer surface has more energy for evaporation • Warmer air has a higher saturation vapor pressure • Vapor pressure of air • Degree of saturation affects how much more can be added • Wind • Wind increases atmospheric mixing

42. Potential Evapotranspiration for the U. S.

43. Potential Evapotranspiration • Hypothetical rate of evapotranspiration if at all times there is a more than adequate amount of soil water for growing plants • Depends on Solar Energy, Cloud Conditions, Temperature • Actual Evapotranspiration is always less than PE

44. Lysimeter

45. Condensation and Cloud Formation

46. 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 Figure 6-10

47. Condensation • Process by which a gas is changed to a liquid • Depends on: • Relative humidity • Degree of cooling • Condensation nuclei: minute particles in the atmosphere (e.g., dust, smoke, pollen, sea salt) on which condensation can take place • Condensation nucleus: a tiny bit of solid matter (aerosol) in the atmosphere, on which water vapor condenses to form a tiny water droplet

48. Fog Figure 6-18

49. Fog • Mass of suspended water droplets within the atmosphere that is in contact with the ground

50. Radiation Fog • Fog produced by cooling of air in contact with a cold ground surface (surface inversion) • Associated with valley drainage • More likely in industrial areas because of condensation nuclei