Humidity is the amount of water vapour present in air. Absolute Humidity is the weight of actual amount of water vapour present in unit volume of air.Relative Humidity is the Ratio of water vapour in air at a particular temp. to total amount of water vapour required to saturate the same air at same temperature. It is expressed in %Specific Humidity is the Weight of water vapour present in per unit weight of air. It is not affected by change in pressure or temperature.