Humidity and Clouds   

 

Moisture in the Air              

Evaporation

·       Moisture enters the atmosphere through the process called evaporation of water

·       Evaporation is the process by which water molecules escape into the air

·       Evaporation uses the sun’s energy to turn water into gas       water vapor!

Humidity

·       Water vapor, or moisture, in the air is called humidity

·       Amount of humidity always changes

Amount depends on the temp. of air

o     more water can evaporate into warm air then cold air

·       Relative Humidity

o     Percentage of moisture air holds relative to the amount it can hold at a particular temperature

§       Ex:  Can hold 12g of water vapor, only holding 9g

9/12= .75 

to convert to % just x by 100

100 x .75=  75% RH

§       Changes with temp. and pressure

Warmer air holds more water vapor and will also have a higher relative humidity

·       Dew Point

o     Point at which air is saturated (100% humidity)

o     Dew happens when air contains all water vapor possible and temp. cools so water vapor changes into liquid       when temp. is below freezing: frost (ice crystals) can form

Measuring Humidity

·       Psychrometer

o     Uses 2 thermometers, one with cloth over bulb; other one uncovered

o     cloth is dipped in water and the 2 thermometers are twirled

o     As air passes over thermometers, wet cloth cools to give wet bulb temp; dry bulb temp. is given to dry thermometer

o     To determine relative humidity using table

§       Calculate difference in wet and dry bulbs temp.; using difference and dry bulb temp you can find RH