Chapter 4 Moisture and Atmospheric Stability

1. Chapter 4 Moisture and Atmospheric Stability

2. Steam Fog over a Lake

3. The Hydrologic Cycle

4. Moisture-related Terms • Transpiration • The release of water vapor into the atmosphere by plants • Latent Heat • Energy absorbed or released to change the state of water • Measured in Calories • A calorie is the amount of heat needed to raise the temperature of 1g of water by 1°C

5. States of water

6. States of Water • Ice • Frozen • Liquid Water • Liquid at room temperature • Water Vapor • Gas

7. More Moisture Terms • Evaporation • The phase change from liquid to gas • Endothermic / absorbs latent heat • Condensation • The phase change from gas to liquid • Exothermic / latent heat is released • Sublimation • Phase change of solid directly to gas • Deposition • Phase change of gas directly to solid • Frost is an example of deposition

8. Water’s Changes of State

9. Ice

10. Water

11. Condensation of Water Vapor Generates Fog

12. Humidity – Water Vapor in Air • Humidity • The general term for the amount of water vapor in the air • Absolute Humidity • Mass of water vapor in a given volume of air • Mass of water vapor (g) / volume of air (m3) • Mixing Ratio • Mass of water vapor in a unit of air compared to the remaining mass of dry air • Mass of water vapor (g) / mass of dry air (kg)

13. Humidity Is the Content of Water Vapor in the Air

14. Vapor Pressure and Saturation • Vapor Pressure • The part of total atmospheric pressure attributable to its water vapor content • Saturation • Balance between evaporation and condensation • Saturation Vapor Pressure • Pressure of water vapor in a saturated environment

17. Saturation Vapor Pressure Varies with Temperature

18. Relative Humidity • Relative Humidity • Ratio of the air’s actual water vapor content compared with the amount of water vapor needed for saturation at that temperature and pressure

19. Relative Humidity Changes with Added Moisture

20. Saturation Mixing-Ratio For every 10°C increase in temperatures, the saturation vapor pressure doubles

21. Changes with Temperature

22. Higher Temperature Lower Relative Humidity with MORE Moisture! Lower Temperature Higher Relative Humidity with LESS moisture!

23. Daily Changes in Relative Humiditywith Temperature

24. Dewpoint The temperature to which a parcel of air needs to be cooled to in order to reach saturation Dewpoint temperature Sling Psychrometer

25. Cold Drinking-glasses Chill Surrounding Air to the Dew-Point Condensation on Cold Drinking-glasses

27. Dew Point Temperatures

28. Dew Point Temperatures

30. Adiabatic Temperature Changes • The Basis of Cloud Formation • Adiabatic Temperature Changes mean no heat added or subtracted • When air expands, it cools • When air is compressed it warms • Example: Pumping up a tire • Air compressed in tire causes it to warm • Escaping air is cool

31. Adiabatic Temperature Changes

32. Adiabatic Cooling • Parcel • A volume of air • Dry Adiabatic Rate • Unsaturated air / 10°C per 1000 m (1km) • Lifting Condensation Level • Parcel reaches saturation / condensation begins • Wet Adiabatic Rate • Latent heat absorbed from the evaporation processed is released at the LCL due to condensation– this reduces the adiabatic rate / BTW 5°C and 9°C per 1000 m (1km)

33. Dry & Wet Adiabatic Rates

34. Processes that lift air • Orographic Lifting • Air is forced to rise over mountains • Frontal Wedging • Warmer, less dense air is forced over cooler, denser air • Convergence • A “pile-up” of horizontal air flow results in upward movement • Localized Convective Lifting • Unequal surface heating causes small parcels to rise due to buoyancy

35. Lifting Processes Orographic

36. Heavy Precipitation in Mountains Snow Pack in the Rocky Mountains

37. Rain Shadow Desert • When air climbs over a mountain, most of the moisture is lost as precipitation on the windward side • The air descends on the leeward side • As it descends, it compresses, warms, and becomes very dry • Very little precipitation occurs in the “Rain Shadow”

38. Death Valley Rain Shadow Desert

39. Wetter Windward Locations and Leeward Rain Shadows

40. Rain Shadow Deserts • On the windward side of the Sierra Nevada in California, giant sequoias and Douglas firs are found • On the leeward side, you can find Death Valley • In addition, fast moving windward systems can produce downslope warm dry winds • These winds are often called “Chinook” and can warm the adjacent areas by 10°C (18°F) during winter

41. Frontal Wedging • Masses of warm and cold air collide, producing a front • Warmer, less dense, air rises over colder, denser, air

42. Frontal Wedging

43. Convergence • When air flows in from more than one direction • Air ascends, cools, and forms clouds • Florida provides an excellent example of convergence

44. Convergence

46. Localized Convective Lifting • Unequal heating causes some places to be warmed more than other places, i.e., parking lot versus wooded area • Parcel of air that is heated will rise – these parcels are often called thermals • Birds and hang gliders ride on thermals • This is called localized convective lifting

47. Convection

48. As long as air in a balloon is hotter than the surrounding air, it will rise

49. Atmospheric Stability • If an air parcel is cooler than the surrounding environment, it tends to sink, and does not rise – called stable air • If an air parcel is warmer than the surrounding environment, it tends to rise – called unstable air / like a hot air balloon • How high does the parcel rise? Until its temperature is the same as the surrounding environment

50. Environmental Lapse Rate • The actual temperature of the atmosphere at any height in the atmosphere, based on observations • Adiabatic changes are based on a parcel of air moving vertically in the atmosphere – does not include horizontal movement or mixing • Air that rises, that is cooler than the surrounding environment, will sink if allowed to