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This calculator computes the heat index based on temperature and relative humidity using a formula developed by the National Weather Service.

The program is operated by entering a temperature in degrees Fahrenheit and a relative humidity in percent, and then pressing the Calculate button. All entries are cleared by pressing the Clear button. If either entry is out of range, a popup warning message will be displayed

Temperature : F
Relative Humidity : %
Heat Index : F


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:

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 [%]

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 130F or Higher Heat or Sunstroke highly likely with continued exposure
II Very Hot 105F - 130F Sunstroke, heat cramps, or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity
III Hot 90F - 105F Sunstroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity
IV Very Warm 80F - 90F Fatigue possible with prolonged exposure and/or physical activity

Copyright 2004, Stephen R. Schmitt