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:
THI =
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:
| 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 |
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