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Humidity is the condition of atmosphere with reference to its water content.

It’s measured with hygrometer or psychrometer which consists of wet and dry bulb thermometers kept in Stevenson screen.

Dry bulb thermometer is a thermometer covered with muslin bag immersed in water while dry bulb thermometer has no muslin.

How It Works

  • When air is dry there is a lot of evaporation from the muslin.
  • Evaporation cools the bulb of thermometer resulting in a low temperature reading.
  • When humidity is high there is little evaporation from the muslin.
  • The wet bulb thermometer is cooled at a slower rate and both thermometers show almost the same temperature reading.
  • The difference in readings between the two thermometers is used to determine relative humidity.

                               Interpretation of Hygrometer Readings

  • When the 2 readings are the same, relative humidity is 100% i.e. the air is saturated.
  • If the difference is small, humidity is high.
  • If the difference is big, humidity is very low.

Calculating Humidity

Absolute Humidity

– Actual amount of water vapour a given volume of air can hold. It’s expressed in g/m3.

Specific Humidity

– Mass of water vapour in a given mass of air. It’s expressed in g/km.

Relative Humidity

– Ratio between the absolute humidity and the maximum amount of water the air can hold expressed in a percentage.

R.H.=A.H/Maximum amount of water the air can hold at the same temperature.


  • If the air at 20◦c contains 10g/m3and given air can hold a maximum of 20g/m3.calculate the relative humidity.



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