Chapter 5 Rational Temperature Indices of Thermal Comfort

This chapter describes physical or physiological indices of the thermal environment that have already been proven useful or that may prove valuable in future. Basic principles are discussed in the chapter by which indices, for thermal comfort in particular, may be developed and their usefulness judged. With the balance equation for human heat exchange with the thermal environment as a starting point, the following rational temperature indices have been developed: (1) operative temperature (TO), which is a linear average of the ambient air temperature (Tα,) and mean radiant temperature (Tr), weighted by their respective linear transfer coefficients; (2) humid operative temperature (Toh) and the new effective temperature (ET*). Both Toh and ET* are indices of the total heat exchange from the skin surface, while To is a measure of the sensible heat exchange only; Toh is based on an equivalent saturated environment, while ET* on one with 50% RH. The effective transfer coefficient governing the evaporative loss from the skin surface is always inseparable from the associated skin wettedness factor. Transfer coefficients for sensible and insensible heat from the skin surface are modified by clothing insulation. When a reference environment is used with standardized clothing and air movement it is possible to describe any test environment in terms of (3) a standard operative temperature (Tso); (4) a standard ambient vapor pressure (eso); and (5) the standard effective temperature combining both sensible and insensible heat transfer.