The document discusses moisture and humidity, including: 1) The different states of water and the latent heat involved in phase changes between solid, liquid, and gas. 2) Factors that determine the amount of water vapor the air can hold, such as temperature. 3) Relative humidity and how it is calculated based on temperature and mixing ratio. 4) Mechanisms that cause air to rise and condense to form clouds, such as orographic lifting and convection.

1. CH. 17 – MOISTURE

2. Changes of State of Water Heat = energy Adding heat to an object makes atoms vibrate faster Heat ≠ temperature! Temperature = average of all molecular motion

3. Change of State of Water Latent heat – energy added to or released by substance w/out changing temperature Latent = “hidden” Ex: Beaker w/ ice and water

4. Humidity Describes amount of water vapor in air - water vapor sits in spaces between dry air gases Saturation = spaces filled to capacity w/ water vapor

5. Dry Air Analysis (consistent worldwide) Nitrogen = 78% Oxygen (O 2 ) = 21% Argon = <1% Carbon Dioxide = 0.035% TOTAL 100%

6. Water Vapor Capacity Table Temperature dependent Hotter air = molecules moving farther apart = more space for water vapor

7. Relative Humidity - measures how close air is to saturation w/ water vapor - expressed as a percentage - is temperature dependent!

8. Relative Humidity Mixing ratio - actual amount water vapor by weight in one kg of air

9. Examples of humidity problems Temp ( o C) M.R. (g) R.H. 1. 25 4 ? 2. 10 3.5 ?

10. True or False? Air with a higher relative humidity has more water vapor in it than air with a lower relative humidity?

11. January Temperatures 50% RH in Siberia 30% RH in Australia

12. Relative Humidity Question: If temp. increases w/out changing the mixing ratio, what happens to relative humidity?

13. Factors that change relative humidity 1) Change in air temperature Assuming no change in mixing ratio: Increase temp. = __________ RH Decrease temp. = _________ RH

14. Rel. hum. vs. temp

15. Factors that change relative humidity 2) Change amount of water vapor a) Add water vapor through ____________ b) Remove water vapor by ____________

16. Dew Point Temperature at which air is saturated without changing the mixing ratio - air has to ________to reach dew point

17. Examples of humidity problems Temp ( o C) M.R. (g) R.H. 1. 25 4 20% 2. 10 3.5 50% What is dew point of Example 2?

18. Condensation in the atmosphere On surface = dew Above surface = fog Aloft = clouds

19. Necessary conditions for condensation 1) Saturated Air (air at dew point) 2) Condensation Nuclei Ex: dust, salt, smoke, ash “ hygroscopic particles”

20. Saturated air Two methods for cooling air to dew point: 1) Environmental Lapse Rate - air is cooler the further you are from Earth’s surface - rate changes daily

21. Cooling to Dew Point: Method #2 2) Adiabatic Temp. Change - caused by changing air’s volume * No heat (energy) is added or removed from the air!

22. Adiabatic Temperature Changes Air expands = temp. decreases Ex: aerosol spray can Air compresses = temp. increases Ex: bicycle tire pump heating up

23. Cloud Formation As air rises, it expands & cools Dry adiabatic rate = cooling of unsaturated air - constant 10 o C per km

24. Condensation Latent heat released by water vapor Ex: In summer, can “sweats” - latent heat warms can

25. Cloud Formation Wet adiabatic rate = cooling of saturated air - always < 10 o C per km Varies depending on amount of condensation in cloud (latent heat warms surrounding air)

26. Review cloud forming process: 1. Air rises 2. Air expands 3. Air cools adiabatically

27. Review cloud forming process: 4. Air reaches dew point altitude varies based on how much water vapor is in rising air 5. Condensation begins

28. Review cloud forming process: * Key to cloud formation is to get large amounts of air to rise!

29. Mechanisms forcing air to rise: 1) Orographic Lifting - physical barrier to wind Clouds form on windward side of mountain As air descends, it compresses & heats up adiabatically

30. Orographic Lifting “ Rain shadow” deserts on leeward side of mountains Ex: Basin & Range in Nevada

32. Mechanisms forcing air to rise: 2) (hot air rises) LocaConvective lifting lly, caused by intense heating on summer afternoons - forms cumulus clouds

33. Mechanisms forcing air to rise: 3) Convergence Surface air moves from high pressure to low pressure (wind)

34. Mechanisms forcing air to rise: 4) Frontal Wedging Cool air acts as a barrier forcing warmer air to rise Regional cause of clouds

35. Frontal Wedging Major factor in Midwest - warm moist air (Gulf of Mexico) - cold dry air (Canada)

36. Fog Cloud with its base at or very near the ground Ex: Radiation fog (ground fog) - fairly clear nights w/ high relative humidity

37. Radiation Fog Air above ground cools rapidly to dew point - after sunrise, fog evaporates from ground up (“lifts”)

38. Air Stability Causes difference in cloud development Ex: gray layered clouds vs. thunderheads

39. Stable Air Air resists vertical displacement - air at surface is colder than temp. of surrounding air - Force air to rise Ex: Orographic lifting

40. Unstable Air Air that does not resist vertical displacement - air at surface is warmer than surrounding air - begins and continues to rise on its own (ex: “convective lifting”)

41. Weather Conditions Stable Air: a) Characteristically sheet-like layered clouds (stratus) Ppt.- producing = nimbostratus b) light to moderate ppt. of long duration

42. Stratus clouds

43. Weather with stable air: c) Other phenomena: - poor visibility & haze - fog, sleet, glaze - inversions (smog alerts)

44. Inversions

45. Weather Conditions Unstable Air: a) Clouds w/ considerable vertical development b) Short episodes of heavy ppt. (cumulonimbus clouds)

46. Cloud of vertical development

47. c) Good visibility except during rain

48. Weather w/unstable air d) Extreme weather phenomena: - lightning & thunder - hail - tornadoes - hurricanes