Humidity refers to the amount of water vapor that is present in the air. Factors such as air temperature and controls over evaporation affect the humidity of a parcel of air. Several ways exist to express the level of air humidity, each providing a different piece of information.
Absolute humidity measures the weight of water vapor per unit volume of air and is expressed in units of grams of water vapor per cubic meter of air (g/m3). Air temperature and atmospheric pressure affects absolute humidity, resulting in information that is not very useful. For that reason, absolute humidity is not often used as a unit of measurement .
Specific humidity measures the weight of water vapor per unit weight of air. This unit of measurement, expressed as grams of water vapor per kilogram of air (g/kg), does not change depending on temperature or atmospheric pressure. Specific humidity as a unit of measurement is more useful than absolute humidity.
Relative humidity refers to the ratio of the amount of moisture in the air at a certain temperature to the maximum amount of moisture that the air can retain at the same temperature. In other words, relative humidity measures how much of the moisture capacity of the air is used. Relative humidity is expressed as a percentage and is highest during rain, usually reaching 100 percent.
Vapor pressure measures the partial pressure that water vapor creates. It is expressed as millibars, much like atmospheric pressure. Volumetric expansion and temperature don't affect vapor pressure. This measure is closely related to saturation vapor pressure, which refers to the amount of pressure that water vapor in saturated air creates.