Relative humidity

Relative humidity is the ratio of the current vapor pressure of the water vapor in the air (or another gas), to the vapor pressure at which the air will become saturated at its current temperature, usually expressed as a percentage. This is equal to the ratio of the current mass of water per volume of air, to the mass per volume for saturated air (the point at which dew or fog will form). Once fog forms, the droplets are considered liquid and no longer part of the gaseous air, which prevents the relative humidity from ever being above 100%.

For a given dewpoint or 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. In the winter, this principle is also why the indoor humidity can be so low that it causes dry skin and itchy eyes, and large amounts of static electricity. Even if the outdoor humidity is at 100% (complete saturation), the air must be so drastically heated (especially in colder climates) that its moisture capacity, and therefore evaporation rate, increases greatly.

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.)