1. Chapter 4 Moisture and Atmospheric Stability
This chapter covers:
Water
states of matter
heat capacity and latent heat
Humidity and dew point
Adiabatic temperature changes in the atmosphere
Atmospheric stability

2. The Hydrologic Cycle

3. States of Matter: Solid, Liquid and Vapor

4. Gas (Vapor) widely spaced molecules no bonding between molecules
molecules move at high speeds
very compressible

5. Liquid Closely spaced molecules Moderate bonding between molecules
molecules move at medium speeds
Slightly compressible

6. Solid (i.e., ice) closely spaced molecules
Strong, rigid bonding between molecules
No molecule movement – only vibrations
Fairly incompressible

7. Solid Water: Ice

8. Liquid Water

9. Water Vapor

10. Heat Capacity and Latent Heat of Water

11. Saturation
Condition in which the air is holding the maximum amount of water vapor possible
Amount of water vapor present at saturation depends on
Temperature; more vapor at higher temp. Very strong effect
Pressure; more vapor at higher pressure.

12. Absolute Humidity Amount of water vapor present in air
Given as grams water vapor per cubic meter of air
Value is affected by air pressure

13. Mixing Ratio
Amount of water vapor present in air, but more useful than absolute humidity
Given as grams water vapor per kilogram of air
Typically ranges from 0 to 4%
Value is not affected by air pressure

14. Saturation
Air is limited in how much water vapor it can hold without water droplets forming
Saturation is the point at which air can’t hold more water vapor
Mixing ratio at saturation depends on temperature, and somewhat on pressure

15. Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs

16. Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs

17. Saturation Mixing-Ratio: How much water vapor can be present in air at different temperatures

18. Relative Humidity the humidity we feel
Amount of water vapor in air relative to maximum possible amount (saturation mixing ratio)
Example
Temperature: 20oC
Saturation mixing ratio=14g vapor per 1 kg air
Actual vapor content = 7 g per 1 kg air
Relative humidity = 7 g / 14 g x 100% = 50%

19. Dew Point Temperature to which air must be cooled to become saturated
Assumes no change in mixing ratio
Relative humidity is 100% in air that’s at its dew point
Stating air’s dew point is essentially the same as stating its mixing ratio

20. Air’s saturation mixing ratio and relative humidity change with temperature

21. Which has larger mixing ratio?
Which has higher relative humidity?
Death Valley
Antarctica

22. Hotter:
Higher mixing ratio,
Lower relative humidity
Colder:
Lower mixing ratio
Higher relative humidity

23. Relative Humidity, Mixing Ratio and Air Temperature
Hotter air can hold much more water vapor than cold air
Hotter air can have more vapor in it than cold air, yet have lower relative humidity

24. Relative Humidity Changes with Temperature Daily

25. Air Temp, Dew Pt. & Relative Humidity in Heber

26. Dew Point Temperatures

28. Adiabatic Temperature Changes
Air cools when it expands, warms when its compressed
Rising air expands and cools
Sinking air is compressed and warms
Adiabatic refers to temperature changes w/o heat transfer
Very important!

29. Adiabatic Temperature Changes

30. Dry & Wet Adiabatic Rates
Saturated air cools less as it rises because condensation of water releases heat
Dry adiabatic rate = 10oC / 1000m = 5.5oF / 1000 feet
Wet adiabatic rate = 5 to 9oC / 1000m (2.75 to 5oF/1000ft)

31. Dry & Wet Adiabatic Rates

32. Lifting Condensation Level
As air rises, it expands and cools
Level (altitude) at which it is cooled to its dew point is the lifting condensation level
Clouds form above this level if air is rising

33. Causes of Lifting Orographic – wind blows over mountains
Frontal wedging – warm air forced over colder air
Convergence – winds blowing together
Convection – solar heating creates hot air that rises

34. Important along Wasatch Front, much of Western U.S.
Orographic Lifting
Important along Wasatch Front, much of Western U.S.

35. Frontal Wedging
“Storm Fronts”

36. Convergence

37. Convection

38. Cause of Rain Shadow Desert
Rising air cools at wet adiabatic rate
sinking air warms at dry adiabatic rate
Cause of Rain Shadow Desert

39. Atmospheric Stability
Stable Air = Air that tends to not rise
Unstable Air = Air that tends to keep rising (regardless of orographics, fronts, etc.)
Importance – rising air cools, makes clouds, precipitation, even tornados

40. What Controls Stability
Depends on adiabatic cooling rate (dry and wet) vs. Environmental Lapse Rate
Environmental Lapse Rate = the actual, existing decrease in air temperature with altitude

41. Atmospheric Stability, cont.
Three types of stability:
Absolute stability
Absolute instability
Conditional instability

42. Absolute Stability
Environmental Lapse rate is less than wet adiabatic rate
As air rises, it cools so much (even if its saturated) that it becomes cooler than surrounding air so it stops rising

43. Absolute stability

44. Absolute instability
Environmental lapse rate is greater than dry adiabatic rate
As air rises, despite cooling at dry adiabatic rate, it becomes progressively warmer than surrounding air and rises faster

45. Absolute instability
Absolute Instability

46. Conditional Instability
Environmental Lapse rate is greater than wet adiabatic rate, less than dry adiabatic rate
As air rises, if it is unsaturated it tends to not rise, but once its saturated it keeps rising

47. Conditional Stability
Conditional Instability
Conditional Stability

49. NWS Storm Prediction Center
Focuses on dangerous thunderstorms
Produces estimates of convective stability for locations across the country twice daily
Main website:
Soundings (weather balloon data which provide information on environmental lapse rate and more) with stability analysis (somewhat advanced scientifically):

50. Chapter 4
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