Calculate the heat index — how hot it actually feels — from air temperature and relative humidity.
This uses the Rothfusz regression, the formula behind the US National Weather Service's heat index tables, with the standard low- and high-humidity adjustments applied at the edges of its valid range. For combinations where the simplified heat-index estimate stays below 80°F, that simpler estimate is used instead, matching NWS practice.
Sweat cools your body by evaporating. High humidity slows evaporation, so your body can't shed heat as effectively — the same air temperature feels hotter as humidity rises.
90–103°F is "extreme caution" territory (heat cramps/exhaustion possible), and above 103°F moves into "danger," where heat stroke becomes a real risk with continued exposure.
Heat index is the warm-weather counterpart to our Wind Chill Calculator — both estimate "feels like" temperature, but heat index factors in humidity while wind chill factors in wind speed.
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