Relative humidity

Relative Humidity


Relative humidity is a measurement of the amount of water vapor (the gas phase of water) in a mixture of air and water vapor. It is most commonly defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions. The relative humidity of air thus changes not only with respect to the absolute humidity (moisture content) but also temperature and pressure, upon which the saturated vapor pressure depends. 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.

Since warm air will hold more moisture than cold air, the percentage of relative humidity must change with changes in air temperature. The graphic below illustrates this relationship. Again we have three parcels or containers of air. The number of water vapor molecules is the same in each container. At 40° air temperature, the parcel is saturated and will hold no more molecules of water vapor. The relative humidity is 100 percent. If the temperature of that air parcel is raised by 20°, it will hold about twice as many water molecules to reach saturation. Thus, the new relative humidity is now 48 percent. If the temperature is raised another 20°, it will again double its capacity to hold water vapor molecules. The relative humidity is only 24 percent. [1]


  1. Utah State University. Atmospheric Moisture. 2008. [1]


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