An important relationship expressed by the Psychrometric Chart is the energy content of the air. This total air energy is the sum of both the dry bulb temperature of the air (sensible heat) and the vaporised moisture content in the air (latent heat). Each gram of water vapour represents around 2400 Joules of latent heat energy, enough to raise 1 kilogram of air (just under 1m³) by around 2.4°C (2400/1012).
The sum of the latent energy and the sensible energy is called the air enthalpy. Enthalpy is expressed in units of kilojoules per kilogram (kJ/kg) of air and its associated moisture. Air at 0°C and 0% relative humidity is assumed, by convention, to have an enthalpy of 0.0 and is used as the basis for the enthalpy scale.
Graphically, the lines of constant air enthalpy follow almost exactly the line of constant wet bulb temperature. Therefore wet bulb temperature, which can be simply measured, provides a relatively accurate measurement of the heat content of the air. The higher the wet bulb temperature the greater the energy content. Generally enthalpy is used only for numerical calculations of the energy required to change the conditions of the air (e.g., heating, refrigeration and air conditioning design in engineering). For the purposes of describing human comfort, however, wet bulb temperature is usually used to represent the energy content.