Relative humidity

Relative humidity: ϕ, RH [%]

Relative humidity is a percentage that describes how strongly the air is saturated with water vapor and how close it is to the saturation line.
In the h-x diagram, these are isolines from 0% to 100% – ie the state of saturation.

Relative humidity ϕ is a derived experimentally measurable quantity.

It equals to the ratio of the partial pressure of water vapor in the air pwv to the pressure of saturated water vapor at the same temperature pswv:

   pwv = 611.2 * exp(17.502 * Twb / (240.97 + Twb)) – 66.8745 * (1 + 0.00115 * Twb) * P / 100 * (Tdb-Twb)
   pswv = 6.112 * exp(17.502 * Tdb / (240.97 + Tdb))
   ϕ = pwv  /  pwv
   RH = ϕ * 100 [%]

It is also the ratio of the absolute humidity of unsaturated air and the absolute humidity of saturated air at the same temperature.

   RH= x1 / (x1 + x2) * 100.



The RH variable was introduced, for example, because many substances and organic materials have defined their equilibrium state in % RH, and which is minimally affected by temperature.

The value itself does not determine the specific amount of water in the air. For example, the term “Absolute humidity x [g/kg]” is used for this purpose.