Estimate apparent temperature from air temperature and relative humidity using the NWS method.
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0 – 100 %
Heat index
94.6 °F
Air temperature90 °F
Relative humidity50%
How this was calculated
Rothfusz regression with NWS adjustments
Simple estimate = ½ × [T + 61 + (T − 68) × 1.2 + RH × 0.094], averaged with TPut in your numbers: ½ × [90 + 61 + (90 − 68) × 1.2 + 50 × 0.094] = 91.05; (91.05 + 90) ÷ 2Answer: 90.53 °F80 °F or more, so the Rothfusz regression is used
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²Put in your numbers: T = 90 °F, RH = 50%Answer: 94.6 °F
No NWS adjustment applies (it is used only when RH < 13% at 80–112 °F, or RH > 85% at 80–87 °F).
Results are shown to one decimal place; NWS reports heat index and wind chill in whole degrees.
This is a shaded, light-wind estimate. It does not measure sun exposure, exertion or individual heat risk.
Method
The NWS method first computes a simple Steadman estimate and averages it with air temperature. At 80°F or above, it uses the Rothfusz polynomial with low- or high-humidity adjustments when their conditions apply.
Worked example
At 90°F and 50% relative humidity, the Rothfusz equation gives approximately 94.6°F. At lower apparent temperatures this tool identifies the simpler estimate instead.
Assumptions and limits
The index assumes shade and light wind. It is not a wet-bulb globe temperature, personal risk prediction or substitute for weather alerts. The tool accepts 40–130°F and 0–100% humidity; unusual extreme combinations can exceed the regression’s physiological applicability.