How heat index calculator Works
The formula
The Rothfusz regression — a nine-term polynomial fitted to the full heat-balance model — computes the apparent temperature for T ≥ 80°F and RH ≥ 40%. The mechanism: sweat cools by evaporating; humid air is already wet, so evaporation slows, and your biological cooling system throttles exactly when demand peaks.
The danger bands
- 80–90°F HI: caution — fatigue possible with prolonged exposure.
- 90–105°F: extreme caution — sunstroke, heat cramps, exhaustion possible.
- 105–130°F: danger — heat exhaustion likely, heatstroke probable with continued exposure.
- 130°F+: extreme — heatstroke imminent.
The fine print that matters
- The index assumes shade — direct sun adds up to 15°F of perceived heat.
- It assumes light wind — strong dry wind helps; hot wind can hurt.
- Wet-bulb is the deeper number: above ~88°F wet-bulb, the body cannot cool at all — the emerging heat-safety metric for extreme events.