LA formula è abbastanza recente, credo che il lavoro con cui è stata presentata è questo
Wet-Bulb Temperature from Relative Humidity and Air Temperature. - UBC Library Open Collections
An equation is presented for wet-bulb temperature as a function of air temperature and relative humidity atstandard sea level pressure. It was found as an empirical fit using gene-expression programming. Thisequation is valid for relative humidities between 5% and 99% and for air temperatures between 2208 and508C, except for situations having both low humidity and cold temperature. Over the valid range, errors inwet-bulb temperature range from 218 to 10.658C, with mean absolute error of less than 0.38C.
Ho trovato anche questa (il testo online manca delle formule, quello in PDF è invece completo)
Two Simple and Accurate Approximations for Wet-Bulb Temperature in Moist Conditions, with Forecasting Applications in: Bulletin of the American Meteorological Society Volume 98 Issue 9 (2017) (ametsoc.org)
Comunque, sembra che esistano diverse formule, ognuna con i suoi vantaggi e svantaggi.