Humidity

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Humidity
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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

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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

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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

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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

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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

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Relative humidity
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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

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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

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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

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Other methods

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

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Digital methods

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