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The heat index, or apparent temperature, is how hot it really feels when humidity is combined with the air temperature, and this heat index calculator computes it from those two inputs. The body cools itself by evaporating sweat, but humid air is already close to saturated so sweat evaporates slowly, the cooling falters, and it feels hotter than the thermometer says. The tool uses the U.S. National Weather Service Rothfusz regression, a multi-term fit of temperature and relative humidity, together with its standard low- and high-humidity corrections.

As a worked example, 90 °F air at 60% relative humidity carries a heat index of about 100 °F — ten degrees hotter than the reading — and pushing the humidity to 80% drives it past 110 °F. The calculator returns the feels-like temperature, how much hotter that is than the air, and a humidity table so you can see how quickly the danger rises as the air grows muggier at a fixed temperature.

The regression is most accurate around 80 °F (27 °C) and above with meaningful humidity; below about 80 °F it reports no heat stress because humidity has little effect on comfort in cool air. It also grades the reading into the NWS danger bands — caution, extreme caution, danger and extreme danger — so a heat index above 103 °F signals a real risk of heat cramps or exhaustion, and above 125 °F a high risk of heat stroke, which is vital for planning outdoor work, sport and travel in hot weather.