Relative Humidity -- from Eric Weisstein's World of Physics

This entry contributed by Dana Romero

According to Dalton's law, the pressure of a gas mixture is equal to the sum of the partial pressures of the gases of which it is composed. In the atmosphere, water vapor always contributes a partial pressure to the total pressure, usually expressed in terms millimeters of mercury (or inches of mercury). At any given temperature the partial pressure of water can never exceed the vapor pressure for that temperature, otherwise it would condense into liquid water.

The relative humidity is the ratio of the partial pressure of water which is actually present to the vapor pressure of water at that temperature, expressed in percent. For example, if the local atmosphere has a partial pressure from water vapor of 5 mm of mercury and the temperature is 20 , the relative humidity would be

(since the vapor pressure of water at 20° is 17.5 mmHg).

Absolute Humidity, Dewpoint, Humidity, Partial Pressure

References