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: 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.

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 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

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 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

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

Fog by the Golden Gate

Temperature Inversions Common Summer Inversion in Los Angeles

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

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

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?

Which of the four types is this? Appalachian Mountains

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

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

Global Precipitation Patterns

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

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.

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

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

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

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

Precipitation Types / Properties

Snowflakes and Temperature Snow crystal images from an electron microscope

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

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