The relative humidity (RH or \phi) of an air-water mixture is defined as the ratio of the partial pressure of water vapor (p_{H_2O}) in the mixture to the equilibrium vapor pressure of water (p^*_{H_2O}) over a flat surface of pure water[REF] at a given temperature:[REF][REF] .
Relative humidity is normally expressed as a percentage; a higher percentage means that the air-water mixture is more humid.