The concentration of water vapour in the atmosphere. The absolute humidity is the mass of water vapour per unit volume of air, usually expressed in kg m−3. A useful measure is the relative humidity, the ratio, expressed as a percentage, of the moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure. The specific humidity is also sometimes used: this is the mass of water vapour in the atmosphere per unit mass of air. See also hygrometer.