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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Presentation Transcript
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
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%)

slide8
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
slide9
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

slide14
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
slide18
Dew formation: condensation in place
  • Frost formation: deposition in place
slide20
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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