]> 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. In a system of moist air, the (dimensionless) ratio of the mass of water vapor to the total mass of the system. The liquid water present within a sample, usually expressed in percent by weight. Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers.