A temperature measured from how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature
a measure of the combined effect of heat and humidity on the human body. U.S. meteorologists compute the index from the temperature T (in °F) and the relative humidity H (as a fraction; that is, H = 0.65 if the relative humidity is 65%). The formula used is HI = -42.379 + 2.04901523 T + 1014.333127 H - 22.475541 TH - .00683783 T2 - 548.1717 H2 + 0.122874 T2H + 8.5282 TH2 - 0.0199 T2H2.
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.