Derivation and analysis of the outdoor Wet Bulb Globe Temperature index (WBGT) with a human thermal engineering approach ? Part 2. Properties of the WBGT formula for outdoor conditions with solar radiation

The authors present a theoretical derivation of the WBGT formula for outdoor conditions that was originally developed from the results of experiments on human subjects, based on a heat balance equation between the human body and its outdoor environment. The three coefficients of wet bulb temperature Tw, globe temperature Tg, and air temperature Ta were expressed in almost the same way as in the indoor WBGT formula, but they contain a new element characterizing solar radiation. In addition, we calculated the coefficients in the theoretically derived formula, changing the amounts of metabolic activity, clothing worn, wind velocity, and solar radiation. We obtained the new formula WBGT = 0.84Tw + 0.30Tg 0.08Ta, characterized by a negative coefficient of air temperature Ta, as an alternative to the original outdoor formula WBGT = 0.7Tw + 0.2Tg + 0.1Ta. Finally, we indicated the characteristics of the WBGT as an index as well as instructions for use.