Relative humidity in an air-water mixture is the fraction of the partial pressure of vapor to the saturated vapor pressure at a specific temperature. The relative humidity of air depends on the overall pressure of the system. Relative humidity is a percentage. It is calculated by the following equation:
Φ = ew/e*w x 100%
The humidity of an air-water vapor mixture is determined through the use of psychrometirc charts if both the dry bulb temperature (T) and the wet bulb temperature (Tw) of the mixture are known. There are many several empirical relations that can be used to estimate the saturated vapor pressure of water vapor as a function of temperature. The Bucks correlation is the most commonly used equation to calculate the relative humidity. It is seen below
e*w = (1.0007 + 3.46 x 10-6P) x (6.1121)e^(17.502T/(240.97 + T))
where T is the dry bulb temperature, P is the absolute pressure, and e*w is the saturated vapor pressure.