The National Weather Service devised the "Heat Index" (HI) which is the "apparent temperature" for a hot humid day. It is given in degrees Fahrenheit and is an accurate measure of how hot it really feels. Since HI values were devised for shady, light wind conditions, exposure to full sunshine can increase HI values by up to 15F.

The HI temperature is calculated from the following formula:
THI =
16.923 + 1.8521210-1T + 5.37941RH - 1.0025410-1TRH + 9.4169510-3T2 + 7.2889810-3RH2 + 3.4537210-4T2RH - 8.1497110-4TRH2 + 1.0210210-5T2RH2 - 3.864610-5T3 + 2.9158310-5RH3 + 1.4272110-6T3RH + 1.9748310-7TRH3 - 2.1842910-8T3RH2 + 8.4329610-10T2RH3 - 4.8197510-11T3RH3
In which,
T = temperature in degrees Fahrenheit [F]
RH = relative humidity in percent [%]