Relative humidity (RH) is a measure of how much water vapor is in the air compared to the maximum amount possible at a given temperature, usually expressed as a percentage. It is a ratio of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. RH is a function of both moisture content and temperature. The higher the RH, the more humid the air is.

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 an air temperature of 80.0 °F (26.7 °C) would feel like 83.6 ± 1.3 °F (28.7 ± 0.7 °C).

It is important to note that relative humidity is not the same as absolute humidity, which is a measure of the actual amount of water vapor in the air, regardless of the airs temperature. Specific humidity is another measure of the weight (amount) of water vapor contained in a unit weight (amount) of air.