Relative Humidity indicates how moist the air is. Actual vapor pressure is a measurement of the amount of water vapor in a volume of air and increases as the amount of water vapor increases. Air that attains its saturation vapor pressure has established an equilibrium with a flat surface of water. That means, an equal number of water molecules are evaporating from the surface of the water into the air as are condensing from the air back into the water.

Humidity is a measure of the amount of water vapor in the air. The air’s capacity to hold vapor is limited but increases dramatically as the air warms, roughly doubling for each temperature increase of 10° C (18° F). There are several different measures of humidity. The specific humidity is the fraction of the mass of air that consists of water vapor, usually given as parts per thousand. Even the warmest, most humid air seldom has a specific humidity greater than 20 parts per thousand. The most common measure of humidity is the relative humidity, or the amount of vapor in the air divided by the air’s vapor-holding capacity at that temperature. If the amount of water vapor in the air remains the same, the relative humidity decreases as the air is heated and increases as the air is cooled. As a result, relative humidity is usually highest around dawn, when the temperature is lowest, and lowest in mid-afternoon, when the temperature is highest.

Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent:

Relative Humidity (RH) = (Actual Vapor Density) / (Saturation Vapor Density) × 100

Relative humidity is also approximately the ratio of the actual to the saturation vapor pressure.

Relative Humidity (RH) = (Actual Vapor Pressure) / (Saturation Vapor Pressure)



Cited "Weather," Microsoft® Encarta® Online Encyclopedia 2004 © 1997-2004 Microsoft Corporation. All Rights Reserved.
"Relative Humidity," University of Illinois WW2010 Project © 1997 by the University of Illinois Board of Trustees. All Rights Reserved.