How this calculator works
The NWS method first computes a simple apparent-temperature estimate and averages it with air temperature. If that estimate reaches 80 F, the Rothfusz regression replaces it, with adjustments for very low or high humidity in specified temperature bands. All regression arithmetic uses Fahrenheit and percentage RH before the result is converted to the chosen temperature unit. The index assumes shade and light wind. It does not include radiant heat, clothing, workload or individual susceptibility.
Formula
S = [0.5(T+61+1.2(T-68)+0.094 RH)+T]/2. For S>=80 F, HI = -42.379+2.04901523T+10.14333127RH-0.22475541T RH-0.00683783T^2-0.05481717RH^2+0.00122874T^2 RH+0.00085282T RH^2-0.00000199T^2 RH^2, followed by the NWS low/high humidity adjustments.
Worked example
At 90 F and 70% RH, the regression gives about 105.922 F, or 41.0678 C. At 70 F and 50% RH, the averaged simple result is 69.525 F.
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Frequently asked questions
Does sunshine change the heat index?
Direct sun adds radiant heating that this shaded index does not include.
Why use a different formula in mild weather?
The full regression is not intended for conditions with an apparent temperature below about 80 F.
Can dew point replace RH?
Yes. The tool derives RH from the dew point and air temperature using the liquid-water vapour-pressure fit.