Absolute Humidity, Specific Humidity & Relative Humidity YouTube Lecture Handouts

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Absolute Humidity, Relative Humidity, Specific Humidity & Mixing Ratio
  • Humidity is the amount of moisture (water vapor) in the air.
  • Saturation: When gas holds maximum water vapor at a given temperature (holding capacity increases with rising temperature)

Absolute Humidity

  • Mass of water vapor divided by a unit volume of air (grams of water/cm3 of air)
  • It does not take temperature into consideration
  • Absolute humidity in the atmosphere ranges from near zero to roughly 30 gm/m3 when the air is saturated at 30 °C

Relative Humidity

  • Ratio of the partial pressure of water vapor (H2O) in the mixture to the saturated vapor pressure of water at a given temperature
  • Function of both water content and temperature.
  • Relative humidity is the amount of water vapor present in the air divided by the maximum amount that the air could contain at that temperature.
  • Relative humidity is expressed as a percentage.
  • RH is 100 % if the air is saturated with water vapor and 0 % if no water vapor is present in the air at all.

Specific Humidity

  • Specific humidity (or moisture content) is the ratio of water vapor mass to the air parcel՚s total (i.e.. , including dry) mass
  • Also known as humidity ratio
  • Does not change with expansion or compression of air parcel
  • It is grams of water vapor per kilogram of air.

Mixing Ratio

  • Specific humidity is approximately equal to the “mixing ratio”
  • Ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel.

Vapor Pressure

  • Measures water vapor content of air using partial pressure of the water vapor in the air
Vapor Pressure

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