Humidity is a term for water vapor in the air, and can refer to any one of several measurements of humidity.
Absolute humidity is an amount of water vapor, usually discussed per unit volume. The mass of water vapor, , per unit volume of total moist air, , can be expressed as follows:
Absolute humidity in air ranges from zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C.
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of a flat sheet of pure water at those conditions. The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.
Relative humidity is normally expressed as a percentage and is calculated by using the following equation, it is defined as the ratio of thepartial pressure of water vapor (H2O) in the mixture to the saturated vapor pressure of water at a prescribed temperature.
Relative humidity is often used instead of absolute humidity in situations where the rate of water evaporation is important, as it takes into account the variation in saturated vapor pressure.
Specific humidity is the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio. Specific humidity ratio is expressed as a ratio of mass of water vapor, , per unit mass of dry air .
That ratio is defined as: