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Atmospheric Humidity

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.

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Atmospheric Humidity

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

  9. 4 am : • 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%

  10. 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%

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

  12. 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 = ?

  13. 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

  14. Dew formation: condensation in place • Frost formation: deposition in place

  15. 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

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