First Warning Weather University: What is a heat index?


AUSTIN (KXAN) –  “It’s not the heat, it’s the humidity.” We’ve all heard the phrase, right? Well as it turns out, it’s both. It’s called the heat index.

It all has to do with sweat. When the body gets too hot, we sweat to cool ourselves off. Beads of sweat form, then evaporate off your skin, which then takes the heat with it.

When its’ more humid out, sweat has a harder time evaporating.  If the sweat can’t evaporate, the body can’t regulate its temperature. Which means you can’t stay cool. Because of this, the temperature of the air might feel warmer to you due the humidity.

This is the heat index. The more humid it is outside, the hotter it feels.

The equation to calculate heat index is complicated:

HEAT INDEX = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 X 10-3T2 – 5.481717 X 10-2R2 + 1.22874 X 10-3T2R + 8.5282 X 10-4TR2 – 1.99 X 10-6T2R2

But the concept is pretty simple.

Keep in mind, the heat index is what it feels like in the shade. So if you’re in direct sunlight, you can expect it to be 15 degrees hotter.

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