Atmospheric Humidity

Global water cycle: precipitation = evaporation

Phase changes of water (latent heat transfer)

Humidity Amount of water vapor in atmosphere Expressions of humidity: Specific humidity Relative humidity Dew Point

Specific humidity mass of water vapor per mass of air (g/kg) higher temperature, higher maximum specific humidity Warm air can “hold more water” than cold air Latitudinal patterns

Line is maximum (capacity) specific humidity

Relative Humidity (RH) : actual amount of moisture in air RELATIVE to the maximum capacity amount that the air can hold RH (%) = actual amount moisture max. capacity x 100 (“saturated air” means that RH = 100%)

relative humidity varies with temperature Warm air has a higher capacity than cold air to hold moisture During day, relative humidity and temp. have inverse relationship

4 am : 10 am : 3 pm : T = 5oC actual = 7g/kg capacity = 7 g/kg RH = 7 / 7 x 100 = 100% 10 am : T = 16oC actual= 7 g/kg capacity = 14 g/kg 7 / 14 x 100 = 50% 3 pm : T = 32oC actual = 7 g/kg capacity = 35 g/kg 7 / 35 x 100 = 20%

Calculating RH from specific humidity: a) If temperature = 20 °C and actual sp. hum. is 7.5 g/kg, what is RH? Solution: 1. Use graph to find capacity amount. 2. RH = (7.5 / 15) x 100 = 50% b) If temperature is 25 °C and actual sp. hum. is 5 g/kg, what is RH? RH = (5/20) x 100 = 25%

sling psychrometer Psychrometric tables dry bulb and wet bulb wet bulb depression = dry bulb temp - wet bulb temp

If air temperature is 35°C and wet bulb temperature is 31°C, what is relative humidity? Dry bulb temperature is 35 Wet bulb depression = 35-31= 4 RH = ?

Dew Point temperature at which air becomes saturated during cooling. When air temperature equals dew point, air is saturated and Condensation (or deposition) will occur Examples: glass of ice water on a hot day; window in shower room

Dew formation: condensation in place Frost formation: deposition in place

Rising parcel will cool due to expansion Descending parcel will warm due to compression RATE depends on whether parcel is dry or wet (saturated) * differs from environmental temperature lapse rate