Computes the heat index (apparent temperature) using the Rothfusz regression as used by the US National Weather Service.
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Inputs
temp_f (number, degrees F, required) min 80 max 110 : Air temperature.
relative_humidity_percent (number, %, required) min 40 max 100 : Relative humidity.
Outputs
heat_index_f (degrees F): Heat index (apparent temperature).
Steps
Rothfusz regression: HI = -42.379 + 2.04901523*T + 10.14333127*R - 0.22475541*T*R - 0.00683783*T*T - 0.05481717*R*R + 0.00122874*T*T*R + 0.00085282*T*R*R - 0.00000199*T*T*R*R, where T is temperature in F and R is relative humidity in percent.
This card uses the base regression only, valid for approximately T from 80 to 110 F and R from 40 to 100 percent; NWS applies small additional adjustments outside a normal humidity band (very low or very high RH) that this card does not include.
Round to the nearest whole degree.
Constants
None.
Sources
The Heat Index Equation (Rothfusz regression) (National Weather Service Weather Prediction Center (NOAA), 2026-09-30T19:53:00Z) - Official NWS statement of the Rothfusz regression for heat index.