A new empirical model of the temperature-humidity index

A simplified scale of apparent temperature, considering only dry-bulb temperature and humidity, has become known as the temperature-humidity index (THI). The index was empirically constructed and was presented in the form of a table. It is often useful to have a formula instead for use in interpolation or for programming calculators or computers. The National Weather Service uses a polynomial multiple regression formula, but it is in some ways unsatisfactory. A new model of the THI is presented that is much simpler-having only 3 parameters as compared with 16 for the NWS model. The new model also more closely fits the tabulated values and has the advantage that it allows extrapolation outside of the temperature range of the table. Temperature-humidity pairs above the effective range of the NWS model are occasionally encountered, and the ability to extrapolate into colder temperature ranges allows the new model to be more effectively contained as part of a more general apparent temperature index.