During the hot, humid summer months, meteorologists often discuss the heat index instead of the actual air temperature. But how is the heat index calculated, and why is it more important than the air temperature during some heat waves?

Spending time outdoors or exercising outside can feel very different depending on humidity levels — from crisp mornings when it seems you do not even sweat, to muggy summer afternoons when the air feels like a steam sauna.

Humidity is a measure of how much moisture is in the air, which originates from the water cycle. When the sun hits the Earth's lakes and oceans, a fraction of that water evaporates into a gaseous form called water vapor. That water vapor, or humidity, then blows from place to place as air moves around the planet.

The heat index is a complicated calculation that factors humidity into the air temperature, giving us a more representative number of how it feels outside when the humidity is high.

It looks like this:

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

Ultimately, it's all about how easily sweat evaporates off of your skin.

Sweat is the body's natural air conditioning system. Evaporation is a cooling process, so when sweat evaporates off of your face, is pulls heat out of your skin and dissipates it into the surrounding air, leaving your skin slightly cooler.

But the amount of humidity in the air has a big impact on how much of your sweat evaporates, and how easily your body can cool itself.

On a crisp, low humidity day, sweat evaporates so easily off of your skin that you may not realize you are sweating at all. In extremely dry climates like Phoenix or Palm Springs, you may even feel cold after getting out of the swimming pool on a 100-degree day. That is because water is evaporating off of your skin so rapidly in the low humidity environment, it cools your skin well below the air temperature.

In a high humidity environment, like a tropical beach, the air surrounding your body is too moist for sweat to evaporate off of your skin. Your body continues to sweat in an effort to cool itself, but heat keeps building up since the sweat is not evaporating.

The heat index is a measure of how hot the temperature and humidity feel to the human body in the shade. Heat index values can be 15 degrees hotter in direct sun.