A psychrometric chart is a complex graph of the thermodynamic parameters of (moist) air at a constant pressure. The key properties represented are wet bulb temperature (the lowest temperature at which any fluid can be cooled); dry bulb temperature (the typical measure of air temperature recorded by a thermometer exposed to the air, but shielded from radiation and moisture); and absolute and relative humidity and enthalpy (the energy content of air). The chart can therefore be used in relation to a specific site location in order to assess local requirements for climate control in buildings. The yellow box outlined on the chart indicates the area that humans perceive to be ‘comfortable’; however, this box (or comfort envelope) can be expanded in various ways by applying certain types of local climate control. For example, rather than heating or cooling the air inside a building for it to be within the normally accepted temperature range, air temperature can stray beyond this range if local shading protects inhabitants from direct sunlight or, in a humid climate, if mechanical fans perceivably ‘cool’ the air by increasing its velocity. John Farrell of XCO2 Energy describes this low-energy alternative to air-conditioning as ‘extending the comfort envelope’. It is important to note that atmospheric pressure varies with altitude; however, for altitudes lower than 600 m (1,969 ft) the sea-level psychrometric chart is considered adequate.