1 Atmospheric Humidity

2 Global water cycle: precipitation = evaporation

3 Phase changes of water (latent heat transfer)

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

5 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

6 Line is maximum (capacity) specific humidity

7 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%)

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

9 4 am : 10 am : 3 pm : T = 5oC actual = 7g/kg capacity = 7 g/kgRH = 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%

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11 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%

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

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14 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 = ?

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16 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

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18 Dew formation: condensation in placeFrost formation: deposition in place

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