HEAT INDEX combines the effects of heat and humidity. When heat and humidity combine to reduce the amount of evaporation of sweat from the body, outdoor exercise becomes dangerous even for those in good shape.

The formula to calculate Heat Index is
(1) Heat index(HI), or apparent temperature(AI)= -42.379 + 2.04901523(Tf) + 10.14333127(RH) - 0.22475541(Tf)(RH) -6.83783x10**(-3)*(Tf**(2)) - 5.481717x10**(-2)*(RH**(2)) + 1.22874x10**(-3)*(Tf**(2))*(RH) + 8.5282x10**(-4)*(Tf)*(RH**(2)) -1.99x10**(-6)*(Tf**(2))*(RH**(2))

Where Tf = air temperature in degrees Fahrenheit,
RH= relative humidity expressed as a whole number