Water vapor present in the atmosphere is known as humidity.
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 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.
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.