Water vapour is a gaseous constituent of air and, like any of the atmospheric gasses, exerts a pressure. Indeed, the sum of their individual pressures is the atmospheric pressure. There is a maximum amount of water vapour which air at a given temperature can hold. The vapour pressure then exerted is known as the saturation vapour pressure. The relative humidity of the air is the ratio of the actual vapour pressure to the saturation vapour pressure, the ratio being expressed as a percentage.
Vapour pressure is calculated at synoptic weather stations at each hour for which observations are made. The average annual vapour pressure over Ireland and the mean annual and mean monthly vapour pressure at Valentia Observatory are displayed below.