Humidity is the concentration of water vapor in the air. Humidity can as expressed as absolute humidity, specific humidity, or relative humidity. Relative humidity is an important index in weather forecasts to predict the likelihood of precipitation, dew points or fog. Relative humidity is expressed as a ratio of actual water vapor in the air to a maximum theoretical water vapor (saturated). The capacity of air to hold water is dependent on temperature. Warmer air has more water vapor, due to changes in evaporation and thermal energy. When heat and humidity combine, the effect on the body is to slow the evaporation of sweat from the body, leading to an uncomfortable feeling.

The formula for relative humidity is:

Relative Humidity (RH) = Actual Water Vapor Density/Saturation Water Vapor Density x 100%

Vapor Density is the ratio of the weight of a given volume of one gas to the weight of an equal volume of another gas, typically hydrogen, at the same temperature and pressure. The dew point is the measure of water vapor density.