water vapour pressure

The atmospheric pressure which is exerted by water vapour (water in its gaseous state). It is one way of measuring the humidity of the air. At a given temperature, an increase of water vapour in the air corresponds to an increase in the humidity of the air. Water vapour is supplied to the atmosphere by evaporation of water from oceans, lakes, wet land surfaces or from vegetation (transpiration). Water vapour absorbs the Sun's radiation. As a result, the sunlight received at the earth's surface will be more intense in a dryer atmosphere.