1. Atmospheric Moisture

2. Moisture Exists in solid, liquid, and gas in atmosphere
Water enters the atmosphere by:
Evaporation – liquid changes into gas
Transpiration – plants release water vapor
Evapotranspiration – evaporation + transpiration together
Remember it’s the Sun that provides the energy for these to occur

3. Factors affecting Evaporation:
amount of energy available (more = faster evaporation)
surface area of the water (spread out = faster evaporation)
degree of saturation (more saturated = slower evaporation)
wind speed greater wind speed = greater evaporation)

4. Humidity
Absolute Humidity - water vapor in each unit volume of air (or moisture capacity)
- hotter air can hold more water vapor than cold air
Relative humidity – ratio of the amount of water vapor in the air to maximum amount it can hold
The one you see in forecasts

5. Which is heavier? H2O O2 N2 N2 N2 O2 O2 H2O
Which is heavier dry air or wet air?
H2O O2 N2 N2
vs. N2 O2 O2
H2O
Would a baseball travel further on a night where the air is saturated or dry?

6. Dew Point
Dew point – temperature at which air is saturated (filled with water)
If air temp = dew point relative humidity = 100%
If air temp drops below dew point: water vapor condenses to liquid water

7. Measuring Moisture
Sling Psychrometer – instrument used to measure relative humidity and dew point

8. Cloud Formation and Types

9. Why Do Clouds Form? Clouds need three things to form: Cooling
Condensation Nuclei
Saturated Air

10. What Causes Cooling?
There are Four Processes that lead to the cooling of air-
#1 Convective Cooling
Rising air expands because pressure decreases
Expansion causes the air to become cooler
Adiabatic Temperature Changes are changes in temperature from the expansion or compression of air

11. Convective Cooling (Continued)
When the rising air reaches the dew point temperature, water vapor begins to condense
The condensation level is the height above the ground at which condensation takes place

12. #2 Forceful Lifting An event occurs that forces the air to rise
Air moving up and over a mountain

13. #3Temperature Changes Two air masses with different temperatures mix
The temperature change of the combined air masses may be cooler than the dew point

14. #4 Advective Cooling
Warm, moist air moving over cool land or water causes the air temperature to drop
Forms low clouds or fog

15. Cloud Condensation Level

16. Condensation Nuclei
The water vapor needs a surface on which to condense.
Condensation nuclei are tiny, less than mm in diameter. There MUST be solid particles in the air for the water vapor to condense onto.
Examples include:
Suspended particles of atmospheric dust
mineral particles
ash from fires
volcanic dust
microscopic organisms
vaporized meteors
salt from sea spray

17. Saturated Air Air temperature must be equal to the dew point.
Thus, 100% humidity. . . Clouds form!

18. Cloud Types

19. Cloud Types 1 Stratus Clouds Cumulus Clouds Cirrus Clouds
sheet-like, or layered.
Cumulus Clouds
puffy, like cotton balls.
Cirrus Clouds
thin and wispy.

20. Cloud Types 2 High Clouds (above 6,000 m) Mid-level Clouds
Cirrus
Cirrostratus
Mid-level Clouds
Altostratus
Altocumulus
Low-level Clouds
Stratus
Cumulus, Cumulonimbus

21. Cirrus Clouds (thin and wispy)

22. Cirrostratus Clouds

23. Altocumulus Clouds

24. Fair-Weather Cumulus Clouds

25. Cumulonimbus 1

26. Cumulonimbus 2

27. Finally, what do we call a cloud on the ground?
FOG!

28. Fronts

29. Fronts Fronts – two air masses of different characteristics meet
cold fronts push forward like a wedge, showery precipitation
warm fronts are forced upward as they overtake a cold air mass, steady precipitation
occluded fronts form when a cold front catches up to a warm front, stronger more developed storms
stationary fronts are fronts that are not moving forward

30. Cold Front – wedge shape forcing warm air upward
Rain or snow showers

31. Warm Front – warm air travels up cold air like a ramp
Steady rain or snow

32. Front Symbols

33. Development of Occluded Front
Occluded fronts form when a cold front catches up to a warm front
Stormy weather!!
Developed storms