On the Use of Wet Bulb Temperature as an Environmental Index

Abstract

The cooling capacity of an atmospheric environment is examined with respect to a wet object of a given surface temperature. The maximum cooling capacity (MCC) is defined as the sum of the sensible and latent heat fluxes out of a unit area of the object. The MCC can be used as a quantitative measure of the upper limit of the sum of some internal energy dissipation (total internal energy production minus mechanical work) and the absorbed radiant flux in a given object in thermal equilibrium. It is found that, for a given surface temperature and wind speed, the MCC is essentially a function of the wet bulb temperature of the ambient air with a very weak dependence on the ambient air temperature and pressure. It is further shown that the ambient temperature and pressure dependence can be ignored for practical purposes. A simple equation relating the MCC to surface temperature and ambient wet bulb temperature has been derived. Thus, in a given environment the wet bulb temperature sets a quantitative upper limit for the intensity of prolonged exercise.