What is the heat Index?

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by Evan Stewart

WVEC.com

Posted on March 10, 2010 at 4:21 PM

Like the wind chill, the heat index (or apparent temperature, "feels-like" temperature, or humiture) is another number to tell you how hot it feels on a summer day.

When you perspire, your sweat is evaporated from your skin, making you feel cooler. When the humidity is high; however, your sweat doesn't evaporate as fast, making you feel hotter.

The VERY complex formula for Heat Index:

Heat Index = (16.923 + 0.185212*T + 5.37941*RH - 0.100254*T*RH + (0.941695e-2)*T^2 + (0.728898e-2)*RH^2 + (0.345372e-3)*T^2*RH - (0.81497e-3)*T*RH^2 + (0.102102e-4)*T^2*RH^2 - (0.38646e-4)*T^3 + (0.291583e-4)*RH^3 + (0.142721e-5)*T^3*RH + (0.197483e-6)*T*RH^3 - (0.218429e-7)*T^3*RH^2 + (0.843296e-9)*T^2*RH^3 - (0.481975e-10)*T^3*RH^3)

Where RH is the relative humidity in whole numbers (74, not 74% or 0.74) and T is the temperature in °F.

Got all that?

We usually use a chart:

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