The variable

Just as air, warmed by contact with the ground, is transferred into the atmosphere by processes of diffusion, turbulence and convection, so too is the water vapour produced by evaporation. The ratio in which the net radiative energy is divided between heating the atmosphere, heating the ground and evaporating water is dependent on many factors, such as the amount of water actually available, the nature of the ground and the type of vegetation. Knowing the rate of evaporation of water is useful information in hydrology, meteorology and agriculture, but it is difficult to measure. However, the amount of water vapour in the air, i.e. the air's humidity, is easier to measure and this chapter looks at how it is done; Chapter 6 addresses the more difficult problem of how evaporation rates are measured.

Units and terminology

Hygrometry is the measurement of the water content of solids, liquids and gases; in environmental applications this usually means the water content of the atmosphere. Water vapour exerts a pressure, the vapour pressure (VP) of water, which can be measured in any of the usual units of pressure, such as millibars. Above a water surface in an air-filled container, the VP rises to a maximum level, the saturation vapour pressure (SVP), beyond which it cannot rise any further at that temperature (the rate at which water molecules are leaving the surface of the water being then the same as the rate at which they return due to molecular bombardment).