1. Water and Atmospheric Moisture

3. Hydrologic Cycle

5. Moisture, Clouds, and Precipitation
Humidity
Global Precipitation
Lifting Mechanisms
Precipitation Processes
Big Question: What Causes Air to Precipitate?

6. Global Precipitation

7. U.S. Current Relative Humidity [click on map]

8. ABSOLUTE HUMIDITY: RELATIVE HUMIDITY:
Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount (weight) of water vapor per kilogram, the higher the absolute humidity.
RELATIVE HUMIDITY:
Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air.
In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature.
WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR, so with the same amount of absolute/specific humidity, cooler air will have a HIGHER relative humidity, and warmer air a LOWER relative humidity.

9. Humidity Capacity of air is primarily a function of temperature
Relative Humidity (RH) =
(actual water vapor content) x (max. water vapor capacity of the air)
Heated air becomes lower in RH because denominator gets larger
Cooled air becomes higher in RH

10. Saturation vs Air Temperature
The actual amount of
Water air can hold changes
With air temperature
Air at 104 F can hold 3 times
As much water as 68 F air !
(47 grams vs only 15 grams)
Air at 68 F can hold 4 times
As much water as air at 0 F
(15 grams vs only 4 grams)
104 F
47 grams
15 grams
68 F
32 F
4 grams

11. Saturation and Dew Point
Saturated v. unsaturated air
Dew-point temperature
temperature to which air must be cooled to reach saturation (100% RH)
water on outside of drinking glass
ice on your car window
dew and fog

12. Adiabatic Cooling: Clouds and Lifting Condensation Level (LCL)
LCL / Cloud base = dew point altitude

13. Relative Humidity and Temp.
RH fluctuates over a day or season.

14. Measuring Relative Humidity
Sling psychrometer
Hair hygrometer

15. After Saturation Occurs the Air Must Release Extra Water as Fluid
Water forms on the outside of a cold glass as the cold
Air surrounding the glass chills the air to the Dew
Point Temperature
The resulting water
is not from the glass,
the water is from
condensation of
moisture in the air
around the glass

16. In Nature Extra Moisture is Transformed to Water Droplets
Cold air next to the rain-soaked cliff is chilled
To The Dew Point Temperature & creates a
Misty Cloud
along a Rocky
Mountain slope
Air near the
Slope is 100%
Saturated

18. Fog by the Golden Gate

19. Temperature Inversions
Common Summer Inversion in Los Angeles

21. Which of the four types is this?
Clarence River, Australia

22. Fog: A Cloud on the Ground
This April fog occurred in the San Fernando Valley after a clear, cold April night. It evaporated by noon.
Fog in Glendale

23. Temperature Inversions
When warmer air overlies cooler air, pollutants and fog are trapped beneath the inversion.
Common Winter Radiation Inversion in Valleys

24. Which of the four types is this?

25. Which of the four types is this?
Appalachian
Mountains

26. An often very dense type of valley fog called Tule Fog in the Central
Valley of
California
Pacific
Ocean

27. Atmospheric Lifting Mechanisms
Air Lifting processes create clouds & precipitation
Are the only means of precipitation on Earth
Four types of lifting are recognized:
Convectional Lifting
Convergence
Orographic Lifting
Frontal Lifting

28. Global Precipitation Patterns

29. Convergence Convergence occurs
when large air masses meet & are forced to rise vertically by crowding of molecules.
This process is best seen at the ITCZ where the Trades Winds meet & rise to form towering clouds & heavy precipitation

30. Convectional Lifting
Anywhere air is warmer than its surrounding air, it will rise.
In this example an island heats more than the surrounding water and causes a massive cumulus cloud to form.

31. Convectional Lifting Over Florida
Warmer temperatures over the peninsula of
Florida, which is land, cause air to rise
compared to the cooler oceans nearby
Rising air in this
Shuttle Picture is
Shown by a
Cloud pattern
which generally
follows the shape
of the southern
Florida peninsula

32. Convectional Lifting in the Desert
Extremely high afternoon temperatures in late summer often leads to thunderstorms throughout the world’s arid regions.
Mojave Desert
The Grand Canyon in August

33. Orographic Lifting of Air
When air moving
Horizontally
Encounters a
Mountain it must
Rise over the crest
As it rises, it cools
To create clouds,
And most often
precipitation
Moisture
Lost
Dry
Air
Moist
Air
Run off NO Run off

34. Frontal Lifting of Air
Although not a mountain range, masses of moving air
Create the same effect – Unlike mountains air masses
Can provide lifting in many different locations
Fronts can lift air
Which is stable,
Creating clouds
& large amounts
Of precipitation
As rain, snow,
Sleet or hail

35. Precipitation Types / Properties

36. Snowflakes and Temperature
Snow crystal images from an electron microscope

37. Seasonal and Global Variation in Lifting Mechanisms and Precipitation
Convergence – increases when subsolar point (ITCZ) is in the region.
Convection – maximized when insolation and temperature are most intense and when marine air moves over warm land masses; common also in deserts, with their intense summer heating
Orographic – requires forced upslope rising of air (mountains)
Frontal – midlatitudes only, where cold and warm air meet and collide

38. Summary Humidity Precipitation (Rain, Snow, Sleet) Lifting Mechanisms
Relative Humidity
Relationship to Temperature
Dew Point
LCL/Cloud Base
Precipitation (Rain, Snow, Sleet)
When air is substantially cooled below the dew point, large droplets or ice crystals form and may fall if large enough.
Lifting Mechanisms
Convective, Orographic, Frontal, Convergence