Atmospheric Humidity

Atmospheric Humidity

Water vapor present in the atmosphere is known as humidity.

Vapour pressure:
When water vapour mixes with other gases of atmosphere, it exerts a pressure in all directions as do the other gases.  This partial pressure exerted by vapour is known as the vapour pressure.

Saturated vapour pressure :
When air contains all the moisture that it can hold to its maximum limit, it is called saturated air and the vapour pressure exerted by this air is called saturated vapour pressure.

Relative humidity (RH) :
Relative humidity represents the amount of water vapour actually present in air compared with the maximum amount of vapour can be held b same air at a given temperature.

 RH= Actual quality of water vapour present in a given volume of air              x 100

    Maximum amount of water vapour the same volume of air can hold

Absolute humidity (AH):
Absolute humidity is defined as the ratio of actual mass of water vapour present to the total volume of moist air in which it is contained.

It is measured in grams per cubic meter of air or in terms of partial pressure of water vapour in air in mbor mm of mercury.

AH =   Weight of water vapour
              Volume of air                  

Specific humidity:
Specific humidity is defined as the ratio of mass of water vapour to the given mass of air containing the moisture.

Thus it will be measure in grams of water vapour per kilogram of air.

Mixing ration:
  It is the ration of the mass of water vapour to the mass of dry air

Maximum ration = MV/Ma

MV   = Mass of water vapour

Ma    = Mass of dry air

Psychometric: Measurement of humidity with different instruments is called psychometric.

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