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What exactly is a heat index and how does it work?

The thermometer doesn’t always reflect what it really may feel like outside. So what does science say about that?

LITTLE ROCK, Ark. — You hear meteorologists mention heat index or “feels-like temperatures” all the time whenever we see the return of hot and humid weather. The thermometer doesn’t always reflect what it really may feel like outside.

The heat index is known as apparent temperature. It’s measured by combining both the temperature and the humidity and how it feels like to the human body.

High humidity means the air is statured with moisture. When the atmosphere becomes too saturated, it’ll stop absorbing the water vapor. 

When you sweat during high humidity conditions, it becomes more difficult for the sweat to evaporate. It’ll be harder for the body to also regulate its temperature, which is why it’ll feel hotter. 

A little bit of wind can help with the evaporation process to make you feel cooler, which is why a fan really helps.

The heat index is measured under shady conditions and light winds, so heat index values could be up to 15 degrees higher when exposed to the sun directly. 

It’s different with wind chill, which is based on how quickly exposed skin loses heat in the wind under clear, dry, and cold conditions.

The National Weather Service heat index chart has different colors that correspond to the caution level one should take based off exposure or strenuous activity.

Let’s compare a city with temperatures in the 90s that has humidity levels above 50% versus a city in the desert with triple digit heat and humidity below 15%. 

It’ll feel hotter in the city with the temperatures in the 90s vs. the one in the 100s because of the humidity. If the humidity is low enough, the heat index could be lower than the actual temperature.

We’ll begin to see these 80 to 90 degree days more often, with July and August being our hottest months here in the mid-South. 

Heat is the number one weather-related killer, so make sure to stay cool and hydrated often during the summer.