So even if the temperature was 90 degrees, it would feel like 117 degrees if the humidity was 85 percent. At 100 degrees and 65 percent humidity, the heat index would rise to 136 degrees. And that’s in the shade.

Stepping into the sun’s rays can boost the heat index by up to 15 degrees, so avoid that window seat on the bus.

But why does humidity turn hot days into hellscapes? The answer boils down to sweat.

We sweat to keep ourselves from overheating . Put simply, as the moisture evaporates, it takes some of the heat with it. But when the air is a sticky, humid mess, sweat evaporates more slowly, plastering shirts to stomachs and keeping the human body from cooling itself off.

How bad is it going to get this week?

Pretty bad. The excessive heat warning in the Midwest cautions that the heat index could rise to 110 degrees from Thursday through Sunday, all but ensuring that many outdoor weekend plans will be canceled in favor of air-conditioned naps.