Humidity refers to the amount of water vapour 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 vapour per unit volume of air and is expressed in units of grams of water vapour 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 vapour per unit weight of air. This unit of measurement, expressed as grams of water vapour 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 per cent.
Vapour pressure measures the partial pressure that water vapour creates. It is expressed as millibars, much like atmospheric pressure. Volumetric expansion and temperature don't affect vapour pressure. This measure is closely related to saturation vapour pressure, which refers to the amount of pressure that water vapour in saturated air creates.