Air

Absolute and relative humidity

Absolute humidity is measured weighing the water vapour (in grams) contained in 1 m3 of air. This, however, is not a very useful parameter in meteorology: it is more important to know how much water it is potentially possible to obtain in the form of rain from a given quantity of air. For this, another parameter, relative humidity, is utilized. At a certain temperature and pressure, air can contain a fixed quantity of water vapour: when this amount is reached, the air becomes saturated with vapour and any small variation of pressure or temperature or any addition of vapour make the air over-saturated: the excess water vapour condenses in the form of small drops of liquid water. For a determined amount of water vapour contained in air at a fixed pressure, the temperature at which condensation takes place is called condensation temperature or dew point temperature. Relative humidity is the percentage ratio between the quantity of water vapour present in the air and the quantity of vapour required to make the air saturated with moisture, at the same temperature. A relative humidity of 100% indicates that the air is saturated with vapour and about to condense the water vapour in the form of drops of water: from a meteorological point of view, it is a condition that is potentially favourable to bringing about precipitations. On the contrary, a low relative humidity indicates dry air that is not favourable to precipitations.