The Physiologal Equivalent Temperature (PET) is a thermal comfort index that is based on a prognostic model of the human energy balance that computes the skin temperature, the body core temperature, the sweat rate and, as an auxiliary variable, the clothing temperature. It is generally based on the 2-node model proposed by Gagge et al. (1971) and was compiled and extended by Höppe (1984) to the Munich Energy Balance Model for Individuals (MEMI). The core model can be used both in an instationary and a stationary approach, but for PET only the stationary solution of the body parameters is used.

Despite its popularity, PET lacks a proper and comprehensive documentation. Basically, one needs to read the 2 papers Gagge et al. (1971) and Höppe (1984) -in german- plus the computer source code provided by VDI 3787.
There are some unlogical assumtions and errors in the original set of equations. In the ENVI-met implementation, these errors have been corrected. As a consequence, PET values calculated by ENVI-met may differ from values calculated by other programs, but we see no sense in copying wrong code or unlogical assumptions. Fundamental changes from the original are marked in the description.

• Gagge, A., Stolwijk, J., and Nishi, Y. (1971): An effective temperature scale based on a simple model of human physiological regulatory response. ASHRAE Trans., 77(1):247–262.
• Höppe, P. (1984). Die Energiebilanz des Menschen: In: Münchener Universitätsschriften- Fachbereich Physik, Wissenschaftliche Mitteilungen Nr. 49. Meteorologisches Institut, Universität München, München.
• Höppe, P. (1999): The physiological equivalent temperature – a universal index for the biometeorological assessment of the thermal environment. - International Journal of Biometeorology 43, 71-75.
• VDI (2008): VDI 3787. Environmental meteorology. Methods for the human biometeorological evaluation of climate and air quality for urban and regional planning at regional level. Part I: Climate, Blatt 2/ Part 2