Relative Humidity (RH)

Relative humidity is defined as the ratio of partial pressure of water vapour, , in a mixture to the saturation pressure, of pure water at the temperature of the mixture

(35.4)

If water is injected into unsaturated air in a container, water will evaporate, which will increase the moisture content of the air. and will increase. This will continue till the air becomes saturated at that temperature and there will be no more evaporation of water. For saturated air, relative humidity is 100%. Assuming water vapour as an ideal gas,

(35.5)

(35.6)

And V is the volume and T the temperature of air, the subscripts W and s indicating the unsaturated and saturated states of air respectively.

(35.7)

RH= mass of water vapour in a given volume of air at temperature T / mass of water vapour when the same volume of air is saturated at temperature T

(35.8)