Note: Exposure to full sunshine can increase heat index values by up to 15°F.
Note: This calculator follows the methods used by NOAA's Weather Prediction Center to calculate the heat index. This is a refinement of Rothfusz (1990, Technical Attachment SR 90-23).
Rothfusz (1990) is not appropriate for heat indices less then 80°F. The following equation is used for heat indices below 80°F:
If the heat index is 80°F or higher, then Rothfusz (1990) is used:
If the relative humidity is less than 13% and the temperature is between 80°F and 112°F (inclusive), then an adjustment is subtracted from the Rothfusz (1990) heat index :
If the relative humidity is greater than 85% and the temperature is between 80°F and 87°F (inclusive), then an adjustment is added to the Rothfusz (1990) heat index :
In the above equations, HI is the heat index in °F, T is the temperature in °F, and RH is the relative humidity in percent.