Humidity is the amount of moisture (water vapor) in the air. It can be expressed as absolutehumidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unitvolume of air (grams of water/cm3 of air). Relative humidity is the amount of water vaporpresent in the air divided by the maximum amount that the air could contain at that temperature.Relative humidity is expressed as a percentage. The relative humidity is 100% if the air issaturated with water vapor and 0% if no water vapor is present in the air at all.