Humidity – Absolute humidity, Specific humidity, Relative humidity (RH)

[Understanding Weather & Climate, Books a la Carte Edition (6th Edition) #ISBN-13: 978-0321773227 #ISBN-10: 0321773225 CH 5 Atmospheric Moisture]

Absolute humidity

  • Absolute humidity – is simply the amount of moisture in a volume of air, expressed as the number of grams of water vapor contained in a cubic meter of air (g of water / cm3 of air )
  • Absolute humidity will be whenever air expands or contracts – depends on volume (so temperate dependent)


Specific humidity

  • Specific humidity – is the mass of water vapor existing in a given mass of air, expressed as the number of grams of water vapor per kilogram of air (g of water / kg of air)
  • Advantage of Specific humidity is that:
    • Temperature independent
    • Volume independent
  • Limitation of Specific humidity is that:
    • there is a maximum amount of water vapor that can exist at a particular temperature – Saturation Specific Humidity (= Maximum Specific Humidity)


Relative humidity (RH)

  • Relative humidity (RH) – the amount of water vapor in the air to the maximum possible at the current temperature.

RH= (specific humidity/ Saturation Specific Humidity) * 100%

  • Drawback of RH – depends on Temperature
    Even though the amount of water vapor remained constant! The RH decrease if temperature is increasing – RH inverse proportion to Temp  ( Temperature ↑ , RH ↓ )

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