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.