Relative humidity

Relative humidity is the ratio of the current vapor pressure of the water vapor in air or gas to the vapor pressure at which the air will become saturated for its current temperature, usually expressed as a percentage. This is equal to the ratio of the mass of water per volume of air of the current water vapor pressure to the mass per volume for saturated air (the point at which dew or fog will form).

Relative humidity is often mentioned in weather reports, as it is an indicator of the likeliness of precipitation, dew, or fog, and in hot weather increases the apparent temperature to humans (and other animals) as described by the heat index.

See also: absolute humidity