The relative humidity {displaystyle (RH}{displaystyle (RH} or {displaystyle phi )}{displaystyle phi )} of an air-water mixture is defined as the ratio of the partial pressure of water vapor {displaystyle (p_{H_{2}O})}{displaystyle (p_{H_{2}O})} in the mixture to the equilibrium vapor pressure of water {displaystyle (p_{H_{2}O}^{*})}{displaystyle (p_{H_{2}O}^{*})} over a flat surface of pure water at a given temperature:
{displaystyle phi ={p_{H_{2}O} over p_{H_{2}O}^{*}}}{displaystyle phi ={p_{H_{2}O} over p_{H_{2}O}^{*}}}
In other words, relative humidity is the ratio of how much water vapour is in the air and how much water vapour the air could potentially contain at a given temperature. It varies with the temperature of the air: colder air can hold less vapour. So changing the temperature of air can change the relative humidity, even when the absolute humidity remains constant.
Chilling air increases the relative humidity, and can cause the water vapour to condense (if the relative humidity rises over 100%, the dew point). Likewise, warming air decreases the relative humidity. Warming some air containing a fog may cause that fog to evaporate, as the air between the water droplets becomes more able to hold water vapour.
Relative humidity only considers the invisible water vapour. Mists, clouds, fogs and aerosols of water do not count towards the measure of relative humidity of the air, although their presence is an indication that a body of air may be close to the dew point.
Relative humidity is normally expressed as a percentage; a higher percentage means that the air–water mixture is more humid. At 100% relative humidity, the air is saturated and is at its dew point. In the absence of a foreign body on which droplets or crystals can nucleate, the relative humidity can exceed 100%, in which case the air is said to be supersaturated. Introduction of some particles or a surface to a body of air above 100% relative humidity will allow condensation or ice to form on those nuclei, thereby removing some of the vapour and lowering the humidity.
Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the heat index, a relative humidity of 75% at air temperature of 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F (28.7 °C ±0.7 °C).