What the Heat Index Is
The heat index — the "feels like" temperature — is how hot the air actually feels to your body once humidity is taken into account. When humidity is high, sweat evaporates more slowly, your body cools less efficiently, and the same air temperature feels hotter. This calculator uses the Rothfusz regression, the same equation the US National Weather Service (NWS) uses for its official heat-index charts.
For example, 32 °C (90 °F) at 80% relative humidity feels like about 45 °C (113 °F) — well into the "Danger" range. That gap between the thermometer and what your body feels is why heat-illness warnings are based on the heat index, not the raw temperature.
Questions people ask
What is the heat index?
The apparent temperature: how hot it feels when air temperature and relative humidity are combined. High humidity slows sweat evaporation, so the air feels hotter than the thermometer reads.
How is the heat index calculated?
With the NWS Rothfusz regression, a formula in air temperature and relative humidity. Below about 27 °C (80 °F) a simpler NWS formula is used instead, because the regression is not valid there.
What heat index is dangerous?
NWS thresholds: Caution 27–32 °C (80–90 °F), Extreme Caution 32–39 °C (90–103 °F), Danger 39–51 °C (103–124 °F) and Extreme Danger above 52 °C (125 °F).
Is the heat index the same as "feels like"?
In hot weather, yes — "feels like" in summer is the heat index. In cold weather, "feels like" instead uses wind chill, which combines temperature and wind.
Educational estimate based on the NWS heat-index equation; individual risk varies with sun exposure, activity, hydration and health. In a heat emergency, follow local health-authority guidance.