Heat alerts are in effect for 23 states and three major cities on Friday, affecting more than 110 million people. (Capital Weather Gang)


This week’s heat wave might not be exceptionally intense, but it’s certainly a far-reaching one...

New York City, Chicago and Los Angeles are all under heat advisories or excessive heat warnings as of Friday morning. This is the first time in at least 10 years that the three most-populous cities in the United States have been under these alerts simultaneously, reports USA Today. And it might be longer than that — we don’t really know, the alert archives only go back to 2005.

That’s a lot of people who are going to be roasting Friday and this weekend. More than 110 million people, actually.

In New York City, the heat index is expected to climb toward 100 degrees for its 8.4 million residents on Friday. Chicago faces a heat index of 109 degrees. The area around Los Angeles is expected to climb to highs between 100 and 110 degrees through Saturday. At least it’s a dry heat?

Across the Central United States, from the Upper Midwest to the Gulf Coast states, high temperatures peak in the upper 90s to near 100 degrees Friday and Saturday. With dew points in the mid-70s (read: very high humidity), the afternoons will feel more like 110 or 115 degrees.

It’s the dew point that measures how much moisture is in the air. It’s the temperature to which the air needs to cool for dew to form — at this point, we can say the air is “saturated” with moisture, and it cannot hold any more. Dew points around 60 or below tend to be comfortable (depending on your perspective), but once they hit 70 degrees, the sweat begins to drip in earnest.

The heat index is a measure of what the temperature feels like to our living bodies. If it’s 90 degrees, and the humidity is only 40 percent, it feels pretty much like 90 degrees. But if it’s 90 degrees, and the humidity is 95 percent, it feels like an oppressive 117 degrees.

There’s good news for the Midwest, at least, in that this blast of hot air is only expected to last through the weekend, and then temperatures will drop back to more seasonable, comfortable levels next week.

On the East Coast, though, the outlook is not so favorable. Highs will remain in the 90s through at least Wednesday in New York City. In D.C., peak heat might not arrive until Monday — possibly the first 100-degree high temperature since July 26, 2012 — and then the 90s stick around through the end of next week.

July 22

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