Relative Humidity

Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. It requires less water vapor to attain high relative humidity at low temperatures; more water vapour is required to attain high relative humidity in warm or hot air.

The relative humidity ${\displaystyle (RH}$ or ${\displaystyle \phi )}$ of an air–water mixture is defined as the ratio of the partial pressure of water vapor ${\displaystyle (p_{H_{2}O})}$ in the mixture to the equilibrium vapor pressure of water ${\displaystyle (p_{H_{2}O}^{*})}$ over a flat surface of pure water at a given temperature:

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