]> The pressure of a vapor in equilibrium with its condensed phase (liquid or solid). 1. Generally, some measure of the water vapor content of air. The multiplicity of humidity measures is partly due to different methods of measurement and partly because the conservative measures (mixing ratio, specific humidity) cover an extremely wide dynamic range, as a result of the rapid variation of saturation vapor pressure with temperature. 2. Popularly, same as relative humidity. The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels. The amount by which the water vapor in the air must be increased to achieve saturation without changing the environmental temperature and pressure. A thermodynamic function of state; the value of the mixing ratio of saturated air at the given temperature and pressure. The vapor pressure of a system, at a given temperature, for which the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase; that is, the vapor pressure of a system that has attained saturation but not supersaturation. In a system of moist air, the (dimensionless) ratio of the mass of water vapor to the total mass of the system. The pressure exerted by the molecules of a given vapor. The liquid water present within a sample, usually expressed in percent by weight.