Relative humidity

Relative air humidity is a term used to describe the amount of water vapour in the air. Relative air humidity is the ratio of the water vapour’s partial pressure to the saturated vapour pressure at a given temperature. Relative air humidity is expressed as a percentage.

Relative air humidity is calculated as follows: 

           Pwater vapour
RH =      Pwater         x 100%

 pwater vapour is the partial pressure of the water vapour in the air, while
 pwater is the saturated vapour pressure of water at the same temperature as the air.

The air humidity in a given range is used when designing the capacity of ventilation and cooling systems. This ensures not only that air is adequately dehumidified, but also avoids the formation of condensate on the cooling surfaces.The following h,x diagram shows humid air at 1,013 mbar. It can be used to determine the absolute water content in humid air (x), relative air humidity (%), temperature (oC) and enthalpy (h).