How it works
The heat index combines air temperature and relative humidity into an apparent temperature: how hot it feels to the body. High humidity slows the evaporation of sweat, the body’s main way of shedding heat.
This calculator follows the National Weather Service procedure. It starts with Steadman’s simple formula; if the result averaged with the temperature is 80 °F or higher, it uses the Rothfusz regression with the NWS adjustments for very dry (RH below 13%) and very humid (RH above 85%) conditions.
Heat index values are for shady, light-wind conditions. Direct sunshine can raise the effective value by up to 15 °F, and strenuous work makes heat illness more likely at any level.
Formula
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²
Questions people ask
What is the heat index at 90 °F and 60% humidity?
About 100 °F, matching the NWS heat index chart. That falls in the extreme caution category.
How is heat index different from humidex?
Humidex, used in Canada, is based on dew point and uses a different formula. Values are similar in range but not interchangeable.
What heat index is dangerous?
NWS classes 103–124 °F as danger and 125 °F or higher as extreme danger. Heat advisories are typically issued around a heat index of 100–105 °F, depending on location.
Is heat index the same as WBGT?
No. Wet bulb globe temperature also accounts for sun, wind and radiant heat, and is used for work and athletic guidelines. Heat index uses only temperature and humidity.