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Humidity

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If condensation and evaporation are equal, then the air is at saturation ... The difference between the saturation vapor pressure and the actual vapor pressure ... – PowerPoint PPT presentation

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Title: Humidity


1
Humidity
2
Definition
  • Humidity is a moderate degree of wetness
    especially of the atmosphere
  • For our purposes, humidity is the amount of water
    vapor in the atmosphere

3
Mixing ratio
  • The ratio of the mass of water vapor in a parcel
    to the mass of the remaining dry air
  • g kg-1
  • i.e. 5 g water vapor per kg of dry air

4
Specific humidity
  • The ratio of the mass of water vapor in a parcel
    to the mass of the total air
  • g kg-1
  • i.e. 5 g water vapor per kg of dry air
  • very similar to mixing ratio, but uses the mass
    of total air, not just dry air

5
Absolute humidity
  • Mass of water vapor per unit volume of dry air
  • g m-3
  • A measure of density
  • Not commonly used due to frequent changes in the
    volume of air

6
Saturation vapor pressure
  • Water vapor is constantly evaporating and
    condensing
  • If evaporation is greater than condensation, the
    air is unsaturated.
  • If condensation and evaporation are equal, then
    the air is at saturation
  • This saturation pressure varies by temperature

7
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8
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9
Relative humidity
10
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11
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12
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13
Dew point
  • The temperature to which air must be cooled in
    order for condensation to occur
  • Latent heat of condensation
  • Related to 100 relative humidity and the
    saturation vapor pressure curve

14
Vapor pressure deficit
  • es - e
  • The difference between the saturation vapor
    pressure and the actual vapor pressure
  • Given in units of pressure, usually Pa
  • The most common measurement for plant sciences

15
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16
Measuring humidity
  • Sling psychrometer
  • Two thermometers one wet, one dry
  • As you swing, the wet bulb cools
  • Cools more in drier air
  • The difference in wet and dry temperatures is
    related to relative humidity
  • Lookup in a table

17
Other methods
  • Hair hygrometer
  • Measures the expansion and contractor of hair as
    a function of relative humidity

18
Digital methods
  • Dewpoint hygrometer
  • Take a sample of air
  • Cool it and watch for condensation on a mirror
  • Gives dew point
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