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

4 am : T = 5 o C actual = 7g/kg capacity = 7 g/kg RH = 7 / 7 x 100 = 100% 10 am : T = 16 o C actual= 7 g/kg capacity = 14 g/kg 7 / 14 x 100 = 50% 3 pm : T = 32 o C 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 Dry bulb temperature is 35 Wet bulb depression = 35-31= 4 Wet bulb depression = 35-31= 4 RH = ? RH = ?

Dew Point temperature at which air becomes saturated during cooling. temperature at which air becomes saturated during cooling. When air temperature equals dew point, air is saturated and Condensation (or deposition) will occur 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 Rising parcel will cool due to expansion Descending parcel will warm due to compression Descending parcel will warm due to compression RATE depends on whether parcel is dry or wet (saturated) * differs from environmental temperature lapse rate