The relative humidity ( R H {\displaystyle (RH} or ϕ ) {\displaystyle \phi )} of an air-water mixture is defined as the ratio of the partial pressure of water vapor ( p H 2 O ) {\displaystyle (p_{H_{2}O})} in the mixture to the equilibrium vapor pressure of water ( p H 2 O ∗ ) {\displaystyle (p_{H_{2}O}^{*})} over a flat surface of pure water[10] at a given temperature:[11][12][4]