Atmospheric humidity
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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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Atmospheric humidity

Atmospheric Humidity


Global water cycle precipitation evaporation

Global water cycle:precipitation = evaporation


Phase changes of water latent heat transfer

Phase changes of water (latent heat transfer)


Humidity

Humidity

  • Amount of water vapor in atmosphere

  • Expressions of humidity:

    • Specific humidity

    • Relative humidity

    • Dew Point


Specific humidity

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

Line is maximum (capacity) specific humidity


Relative humidity rh

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%)


Atmospheric humidity

  • 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


Atmospheric humidity

  • 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%


Calculating rh from specific humidity

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

sling psychrometer

  • Psychrometric tables

  • dry bulb and wet bulb

  • wet bulb depression =

    dry bulb temp - wet bulb temp


Atmospheric humidity

  • 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

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


Atmospheric humidity

  • Dew formation: condensation in place

  • Frost formation: deposition in place


Atmospheric humidity

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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