Heat Index Calculator (NWS / Rothfusz)
Apparent temperature combining air temperature and relative humidity. NWS Rothfusz regression with Steadman 1979 baseline.
Result
- Air temperature90.0 °F
- Relative humidity60%
- Heat index99.7 °F
- Lift over ambient+9.7 °F
- NWS warning bandEXTREME CAUTION (90-103 °F) — heat cramps + heat exhaustion likely.
- Formula usedRothfusz polynomial regression (NWS)
Step-by-step
- Input: 90.0 °F, 60% RH.
- Simple Steadman estimate = 91.52 °F.
- Average of (simple HI, T) ≥ 80 °F → use full Rothfusz regression (9 terms + low-RH / high-RH corrections).
- Heat index = 99.68 °F = 37.60 °C.
How to use this calculator
- Enter air temperature and relative humidity from your local weather report or a digital thermometer/hygrometer.
- The calculator picks the Rothfusz regression automatically when conditions warrant it (avg of simple-HI and T ≥ 80 °F).
- Read the NWS warning band — these are the levels above which activity should be limited or moved indoors.
About this calculator
The Heat Index combines air temperature and humidity into a "feels-like" temperature that quantifies how hard it is for sweat to evaporate. At high humidity, sweat can't evaporate efficiently, so the body's primary cooling mechanism fails — making the same air temperature feel substantially hotter and more dangerous. The NWS uses Lans Rothfusz's 1990 polynomial regression of Steadman's 1979 multivariate apparent-temperature paper. The formula is calibrated for sun-shaded conditions; full sun adds another 8-15 °F. Heat exhaustion warning thresholds (NWS): 80-90 °F = caution; 90-103 °F = extreme caution; 103-124 °F = danger; ≥124 °F = extreme danger. Wet-bulb globe temperature (used by the military and sports leagues) is a more direct heat-stress metric but requires solar-radiation + wind-speed measurement; Heat Index is the consumer-friendly approximation.