The heat index (also called the "apparent temperature" or "humiture") is the temperature that the human body perceives when relative humidity is factored in alongside actual air temperature. High humidity reduces the rate of sweat evaporation, which is the body's primary cooling mechanism, making it feel hotter than it actually is.
This calculator uses the NWS Rothfusz regression for conditions where the heat index exceeds 80°F (27°C), and a simpler formula below that threshold. The equation was originally developed by George Winterling in 1978 and has since been refined by the National Weather Service.
The heat index is most meaningful when the air temperature is at least 27°C and relative humidity is above 40%. Below these thresholds, the result is shown but may differ from other sources.