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 15°F. The HI temperature is calculated from the following formula:

T_HI =
16.923 + 1.85212·10^-1·T + 5.37941·RH - 1.00254·10^-1·T·RH + 9.41695·10^-3·T² + 7.28898·10^-3·RH² + 3.45372·10^-4·T²·RH - 8.14971·10^-4·T·RH² + 1.02102·10^-5·T²·RH² - 3.8646·10^-5·T³ + 2.91583·10^-5·RH³ + 1.42721·10^-6·T³·RH + 1.97483·10^-7·T·RH³ - 2.18429·10^-8·T³·RH² + 8.43296·10^-10·T²·RH³ - 4.81975·10^-11·T³·RH³

In which,

T = temperature in degrees Fahrenheit [°F]
RH = relative humidity in percent [%]

The effects of high heat index values are given in the following table provided by the NWS:

Category	Classification	HI [°F]	General Affect on People at High Risk
I	Extremely Hot	130°F or Higher	Heat or Sunstroke highly likely with continued exposure
II	Very Hot	105°F - 130°F	Sunstroke, heat cramps, or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity
III	Hot	90°F - 105°F	Sunstroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity
IV	Very Warm	80°F - 90°F	Fatigue possible with prolonged exposure and/or physical activity

Copyright © 2004, Stephen R. Schmitt