Weather Lesson Four Moisture

ATMOSPHERIC MOISTURE Gaseous water is called WATER VAPOR. Major source of atmospheric water vapor is the OCEANS (70%). Sources include  EVAPORATION – phase change where a liquid changes to a gas.  TRANSPIRATION – process by which plants release water vapor as part of their life functions  RESPIRATION – water vapor given off when organisms exhale (breathe out).

FACTORS AFFECTING EVAPORATION/TRANSPIRATION 1) TEMPERATURE As temperature INCREASES, evaporation _______________. INCREASES Hot air can hold more water vapor.

FACTORS AFFECTING EVAPORATION/TRANSPIRATION 1) TEMPERATURE 2) MOISTURE As moisture in the air INCREASES, evaporation _______________. DECREASES

FACTORS AFFECTING EVAPORATION/TRANSPIRATION 1) TEMPERATURE 2) MOISTURE 3) SURFACE AREA As surface area INCREASES, evaporation _______________. INCREASES

From which container will the water evaporate the fastest?

RELATIVE HUMIDITY Ratio of the water vapor in the air to the maximum amount of water vapor it can hold at a specific temperature. The percent (%) saturation.

Air Temperature Water Vapor Air Is Holding Water Vapor Air Can Hold Relative Humidity DETERMINING RELATIVE HUMIDITY 20°C 15°C 10°C 40°C 1 mb 4 mb 2 mb 10 mb ¼ or 25% ½ or 50% 1 or 100% 10%

As temperature INCREASES, (keeping the water vapor content in the air constant), relative humidity ___________________. As temperature DECREASES, (keeping the water vapor content in the air constant), relative humidity ___________________. INCREASES DECREASES

Relative Humidity Water Vapor Air Can Hold Water Vapor Air Is Holding Air Temperature 20°C 15°C 10°C 1 mb 4 mb 2 mb ¼ or 25% ½ or 50% 1 or 100% Temperature to which air must be cooled to get 100% relative humidity. This is called the...

DEWPOINT TEMPERATURE The temperature at which the air would be completely saturated with water. As air cools to the dew point, relative humidity approaches 100%. Any further drop in temperature will cause condensation.

PSYCHROMETER A device used to indirectly measure dew point and relative humidity. Contains a dry-bulb thermometer (regular) and a wet-bulb thermometer (a thermometer with a wick wrapped around its bulb and dipped into water). Both are spun in the air and as water evaporates from the wet-bulb, it cools.

Determining Dew Point and Relative Humidity The difference between the dry-bulb reading and the wet-bulb reading is the Wet Bulb Depression. Using this information and a chart [Reference Tables], one can find both the dew point and relative humidity.