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