AUSTIN (KXAN) – With blistering heat this summer, you’ve likely heard a few people say “it’s not the head, it’s the humidity.” Turns out both things are true: it’s called the Heat Index.

The heat index is used to calculate not the actual temperature of the air around you, but instead how hot you feel. This all comes down to sweat.

When the body gets too hot, it sweats to cool itself off. Beads of sweat form, then evaporate off of your skin. As they evaporate, they pull the heat away with them.

When it is more humid outside, sweat has a harder time evaporating. If the sweat can’t evaporate, then your body can not regulate its temperature.

This means you can’t get cool and the temperature around you feels warmer. This feeling is called the heat index.

Calculating the heat index

To calculate the heat index, meteorologists use a complicated formula:

HI = -42.379 + 2.04901523*T + 10.14333127*RH – .22475541*T*RH – .00683783*T*T – .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH – .00000199*T*T*RH*RH

There are also calculators they can turn to instead, like the heat index calculator one used by the National Oceanic and Atmospheric Administration. A meteorologist needs to know both the temperature and the relative humidity to be able to use the calculator.

While the calculation is pretty complex; the concept is pretty simple. The heat index represents what it feels like in the shade. If you’re in direct sunlight, it can feel 15-degrees hotter, but neither temperature nor heat index readings in the sunshine are considered official.

First developed in 1979, the heat index takes into account clothing and average body mass. The wind chill, which is used to determine how cold someone feels when hit with a gust of wind, uses similar calculations.

Not everyone uses the heat index. Canadians rely on the humidex, which is similar to the heat index, but it uses colder temperatures to calculate.