Heat Index calculation.
The formula below (from Wikipedia) approximates the heat index in degrees Fahrenheit, to within ±1.3 °F (0.7 °C). It is the result of a multivariate fit (temperature equal to or greater than 80 °F (27 °C) and relative humidity equal to or greater than 40%) to a model of the human body. This equation reproduces the NOAA National Weather Service table below (except the values at 90 °F (32 °C) and 45%/70% relative humidity vary unrounded by less than ±1, respectively).
$${\displaystyle \mathrm {HI} =c_{1}+c_{2}T+c_{3}R+c_{4}TR+c_{5}T^{2}+c_{6}R^{2}+c_{7}T^{2}R+c_{8}TR^{2}+c_{9}T^{2}R^{2}}$$
where $\mathrm {HI}$= heat index, $\mathrm {T}$= ambient dry-bulb temperature (in degrees Fahrenheit), $\mathrm {R}$ = relative humidity (percentage value between 0 and 100), and the parameters $c_i$, $i = 1,\cdots, 9$.
$${\displaystyle {\begin{aligned}c_{1}&=-42.379,&c_{2}&=2.049\,015\,23,&c_{3}&=10.143\,331\,27,\\c_{4}&=-0.224\,755\,41,&c_{5}&=-6.837\,83\times 10^{-3},&c_{6}&=-5.481\,717\times 10^{-2},\\c_{7}&=1.228\,74\times 10^{-3},&c_{8}&=8.5282\times 10^{-4},&c_{9}&=-1.99\times 10^{-6}.\end{aligned}}}$$
Put in current temperature (in Fahrenheit) and humidity (%) to calculate the Heat index.
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