humidity (air)

The water vapor content of air. Also known as atmospheric humidity, or simply humidity. It is one of the main meteorological variables and can be expressed in absolute, specific, or relative terms. Outside a meteorological context, the concept of humidity is not always applied correctly; it is common to consider "humid" to be synonymous with "wet." We often refer to the humidity of ceilings or walls when, in reality, those surfaces are actually wet. To measure air humidity, hygrometers, hygrographs, thermohygrographs, and psychrometers are used. The amount of water vapor that air can hold depends on temperature, being considerably greater in warm air than in cold air.