THI = (1.8 T + 32) - (0.55 - 0.0055 RH) (1.8 T - 26), with the temperature in
degrees Celsius and the relative humidity in percent. This is the formula the
heat-stress literature calls NRC (1971); other THI variants exist and differ by a few
tenths, if a study uses another one, compute it and pass it as the covariate, the
reaction norm does not care where the axis came from.