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‘Feels like’ temperature in heat and humidity.
The heat index, or apparent temperature, is how hot it actually feels to the human body when relative humidity is combined with the air temperature. On humid days, sweat evaporates more slowly, so your body sheds less heat and the air feels warmer than the thermometer reads.
This calculator uses the U.S. National Weather Service (Rothfusz) regression to turn a temperature and humidity reading into a feels-like value and a heat-stress risk category, helping you judge when outdoor work, sport, or travel becomes dangerous.
The regression is defined for T ≥ 80 °F (27 °C) and works best at R ≥ 40%. The NWS applies small corrections when humidity is very low (< 13%) or very high (> 85%). Below 80 °F the heat index is undefined, so the feels-like value is taken as the air temperature.
The NWS flags 'Danger' from a heat index of 103 °F (39 °C) — where heat exhaustion is likely and heat stroke is possible — and 'Extreme danger' at 125 °F (52 °C) and above, where heat stroke is highly likely with continued exposure.
Your body cools itself mainly by evaporating sweat. Humid air is already close to saturated, so sweat evaporates slowly and carries away less heat, leaving you feeling hotter than the air temperature alone suggests.
No. The standard heat index assumes you are in the shade. Direct sun exposure can make it feel up to about 15 °F (8 °C) hotter than the calculated value.
The Rothfusz regression is only valid at roughly 80 °F (27 °C) and above. Below that, humidity has little warming effect, so the feels-like temperature is treated as the same as the actual air temperature.
Dew point is the temperature at which air becomes saturated and is a direct measure of moisture in the air. Heat index combines that moisture (as relative humidity) with the air temperature to estimate how hot it feels to a person.