Relative humidity is the ratio of the current vapor pressure of water in the air (or any gas), to the vapor pressure at which the air would become saturated at its current temperature, usually expressed as a percentage. (This is equivalent to saying the current mass of water per volume of air, and the mass per volume for saturated air.)
"Saturation" means in this context the inability to absorb further water vapor; it can be observed in the form of failure of liquid water (or ice, at appropriate temperatures), exposed only to the saturated air, to lose mass (and volume) by evaporation. It also corresponds to the possibilility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of droplets of liquid. (Even though these droplets may be so small as to fall imperceptibly slowly through the mixed gas we call air, this behavior is too different from that of water vapor to reflect it in the same scale. This explains the restriction of relative-humidity discussions to 100% and below.)
(The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function.)
For a given dewpoint and its corresponding absolute humidity, the relative humidity will change inversely with the temperature. This is because moisture capacity increases with temperature – the principle behind everything from hair dryers to dehumidifiers. This explains the low levels (in the absence of measures to add moisture) of indoor humidity in winter, reflected by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% humidity) outdoors, heating of whatever outside air comes indoors raises its moisture capacity, reflected in decreased relative humidity and increased evaporation rates from moist surfaces.
Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals), by preventing the evaporation of perspiration from the skin. This effect is calculated in a table, resulting in the heat index or humidex.
A device used to measure humidity is called a hygrometer, one used to regulate it is called a humidistat, or sometimes hygrostat. (These are analogous to a thermometer and thermostat for temperature, respectively.)
See also: heat index
