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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