Weather Lesson Four Moisture ATMOSPHERIC MOISTURE Gaseous water

Weather Lesson Four Moisture

ATMOSPHERIC MOISTURE • Gaseous water is called WATER VAPOR. • Major source of atmospheric water vapor is the OCEANS (70%). • Sources include ØEVAPORATION – phase change where a liquid changes to a gas. ØTRANSPIRATION – process by which plants release water vapor as part of their life functions ØRESPIRATION – 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.

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

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

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