Two quantities are commonly used to characterize the  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense: specific humidity w and relative humidity epsilon or RH, more commonly used by the general public.

The  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense w is the ratio of the  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense of moist mixture to the  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense in this same volume.

The  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense epsilon is defined as the ratio of the partial pressure of water to its saturated vapor pressure.

Relative humidity is 1 (or 100%) when the  humidity of a moist mixturemass of dry gas containedmass of water contained in a given volumerelative humidityspecific or absolute humidityvapor begins to condense. Otherwise, it is less than 1.