Hot temperatures and choking humidity levels will make for a dangerous couple of days across a good portion of the United States this weekend as summer reminds us it’s far from over. Widespread highs in the upper 90s and heat indices up to 110°F are likely from the Plains to the coast.

Heat is a silent killer—definitely not as attractive as a tornado or hurricane, but it claims many thousands of lives every year without much of a peep in the news. It’s easy for us to brush off heat as “just a summer thing” that shouldn’t even warrant a mention, but for the untold millions of people out there without access to air conditioning or fans or adequate supplies of water, even a quick blast of heat and humidity like this can cause a significant number of heat-related illnesses.

A strengthening upper-level ridge of high pressure is setting up shop over the southern United States, and it’s the culprit behind our atmosphere turning into an oven just in time for the weekend. The National Weather Service has issued heat advisories for much of the central United States in advance of this blast of heat, and it’s likely that the advisories will be extended farther east tonight and tomorrow.

The worst of the heat and humidity will unfold in the states that border the Mississippi River, with temperatures nearing 100°F and dew point values in the mid- to upper- 70s—driving heat indices close to 110°F in most spots—late in the afternoon on Saturday and Sunday. Toasty heat indices aren’t confined to the central parts of the country—just about everyone along and near the coast from Brownsville, Texas, through New York City will experience heat indices near or above 100°F for a few hours each afternoon this weekend.

Why is the heat index so important? The heat index is a value that relates to how your body handles extreme heat and humidity. When it’s hot, you sweat (who knew?), and your sweat cools you off through a process known as evaporative cooling—as it evaporates from your skin, the surface of your skin cools, cooling you off in the process. This is also a huge reason why you can cool off hot food by blowing on it before you shove it in your mouth.

Water has an easier time evaporating into the air when moisture levels are low, but as humidity levels climb, the air has a harder time accepting more water vapor. When it’s extremely hot out, you start sweating more, but if the humidity is high as well, the sweat can’t efficiently evaporate from your skin. This can cause you to overheat, quickly leading to serious illness or even death.

Scientists arrived at the heat index by studying how our bodies handle different humidity levels with regard to heat. When it’s 100°F outside and the dew point is 72°F, they’ve found that it has the same effect on your body as an air temperature of 111°F.

If you live in the areas affected by the high levels of heat this weekend, remember not to overdo it—it’s easy to push yourself in the heat without realizing you’ve gone too far—and please check on elderly or ill neighbors who might not do so well in the heat. Buildings can retain heat pretty well, and since it doesn’t cool off much at night during a heat wave, people stuck indoors without adequate cooling methods can succumb to the heat in short order.

[Images: WeatherBELL, University at Albany-SUNY, author]

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