Water and Atmospheric Moisture

Hydrologic Cycle

Moisture, Clouds, and Precipitation

• Humidity

• Global Precipitation

• Lifting Mechanisms

• Precipitation Processes

Big Question: What Causes Air to Precipitate?

Global Precipitation

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

ABSOLUTE 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.

Humidity• Capacity of air is primarily a function of

temperature

• Relative Humidity (RH) = (actual water vapor content) x 100

(max. water vapor capacity of the air)

• Heated air becomes lower in RH because denominator gets larger

• Cooled air becomes higher in RH

Saturation vsAir Temperature

The actual amount ofWater air can hold changesWith air temperature

Air at 104 F can hold 3 timesAs much water as 68 F air !(47 grams vs only 15 grams)

Air at 68 F can hold 4 timesAs much water as air at 0 F(15 grams vs only 4 grams)

32 F

68 F

104 F

4 grams

15 grams

47 grams

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

Adiabatic Cooling: Clouds and Lifting Condensation Level (LCL)

• LCL / Cloud base = dew point altitude

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

Measuring Relative Humidity

Sling psychrometer

Hair hygrometer

After Saturation Occurs the AirMust Release Extra Water as Fluid

Water forms on the outside of a cold glass as the coldAir 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

In Nature Extra Moisture isTransformed to Water Droplets

Cold air next to the rain-soaked cliff is chilled To The Dew Point Temperature & creates a Misty Cloud along a RockyMountain slope

Air near theSlope is 100%Saturated

Fog by the Golden Gate

Temperature Inversions

Common Summer Inversion in Los Angeles

Which of the four types is this?

Clarence River, Australia

This April fog occurred in the San Fernando Valley after a clear, cold April night. It evaporated by noon.

Fog: A Cloud on the Ground

Fog in Glendale

Temperature Inversions

When warmer air overlies cooler air, pollutants and fog are trapped beneath the inversion.

Common Winter Radiation Inversion in Valleys

Which of the four types is this?

Appalachian Mountains

Which of the four types is this?

An oftenvery dense type ofvalley fogcalled TuleFog in theCentralValley ofCalifornia

PacificOcean

Atmospheric Lifting Mechanisms

Air Lifting processes create clouds & precipitationAre the only means of precipitation on EarthFour types of lifting are recognized:

1. Convectional Lifting2. Convergence3. Orographic Lifting4. Frontal Lifting

Global Precipitation Patterns

ConvergenceConvergence 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

Convectional LiftingAnywhere 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.

Convectional Lifting Over FloridaWarmer temperatures over the peninsula of

Florida, which is land, cause air to rise compared to the cooler oceans nearby

Rising air in thisShuttle Picture isShown by a Cloud patternwhich generallyfollows the shape of the southern Florida peninsula

Convectional Lifting in the DesertExtremely high afternoon temperatures in late summer often leads to thunderstorms throughout the world’s arid regions.

The Grand Canyon in August

Mojave Desert

Orographic Lifting of AirWhen air movingHorizontally Encounters aMountain it mustRise over the crest

As it rises, it coolsTo create clouds,And most oftenprecipitation

MoistAir

MoistureLost

DryAir

Run off NO Run off

Frontal Lifting of AirAlthough not a mountain range, masses of moving airCreate the same effect – Unlike mountains air massesCan provide lifting in many different locations

Fronts can lift air Which is stable,Creating clouds& large amountsOf precipitationAs rain, snow,Sleet or hail

Precipitation Types / Properties

Snowflakes and Temperature

Snow crystal images from an electron microscope

• 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

Seasonal and Global Variation in Lifting Mechanisms and

Precipitation

• Humidity– 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

Summary