What’s the heat index? And what does it say about how hot it actually is?

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Throughout July, as temperatures have soared across the D.C. region, meteorologists have been referring to a metric known as the “heat index” to describe just how hot it has actually been.

The metric, according to 7News First Alert Meteorologist Mark Peña, refers to “your internal temperature’s reaction to the heat and humidity.”

In other words, the heat index, or feels-like temperature, is the temperature that the human body actually feels instead of just the air temperature, Peña said.

The heat index combines biology and meteorology, Peña said. When forecasters refer to the feels-like temperature, it’s not an estimate, but rather an exact number calculated using a complex formula, he said.

“What it boils down to, it’s what your body reacts to with the heat and humidity combined,” Peña said.

The National Weather Service has strict guidelines for the criteria it uses to issue weather warnings, and Peña said an Excessive Heat Warning is issued with feels-like temperatures of up to 110 degrees. A Heat Advisory is issued for feels-like temperatures up to 105 degrees, he said.

Across the D.C. region Monday, Peña said there are a mix of Heat Advisories and Excessive Heat Warnings.

“No matter how you want to look at it, it’s hot everywhere,” Peña said. “We all just want to make sure we’re staying cool.”

Human bodies regulate internal temperatures through sweat when it’s hot outside, but Peña said sweating can only cool someone down if it can evaporate from your skin. When it’s humid and there’s less moisture in the air, it makes it harder for sweat to cool people off.

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“Whenever your body is unable to cool itself off, that’s whenever heatstroke and heat exhaustion can come into play,” Peña said. “And those can get serious really fast.”

To stay safe in environments with high heat indexes, Peña recommends taking breaks in the shade, hydrating and wearing light, loose-fitting clothing.

Do people know what the heat index is?

At Tysons Corner Center on Monday afternoon, a few shoppers did know what the heat index meant.

“I assume it’s what it actually feels like, you combine the temperature in the air, the humidity, and it’s feeling a lot hotter than it should be right now,” Steven Weber said.

Weber said he became familiar with the term because “it’s been really hot for weeks, so I keep seeing the word heat index.”

Few others we talked to were similarly accurate.

David, for one, guessed it’s “the combination of the dew point and the actual temperature.”

Nathan, meanwhile, suspected it referred to “probably just how hot it is.”

Others guessed “how hot it is compared to other places” or “how much UV radiation” there is.