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Heat dome traps much of U.S.

Published 11:24am Wednesday, July 20, 2011

CHICAGO — For millions of people enduring this week’s extreme heat and humidity, it feels like they’re living in a pressure cooker. And in a sense, they are.

Much of the United States is trapped under a heat “dome” caused by a huge area of high pressure that’s compressing hot, moist air beneath it, leading to miserable temperatures in the mid-90s to low 100s and heat-index levels well above 100 degrees. The oppressive conditions extend from the northern Plains states to Texas and from Nebraska to the Ohio Valley. And they’re expanding eastward.

Fergus Falls residents woke up this morning to temperatures in the upper 70s, with the temperature expected to hit 90 before cooling off early Thursday morning.

When a high pressure system develops in the upper atmosphere, the air below it sinks and compresses because there’s more weight on top, causing temperatures in the lower atmosphere to heat up, said Eli Jacks, a meteorologist with the National Weather Service in Silver Spring, Md.

Combined with generally clear skies and the sun’s higher summertime angle, “it gets really hot,” Jacks said.

That also explains why temperatures in, say, North Dakota this week aren’t all that different from temperatures in Houston, he said. The big difference is that people in Houston are accustomed to hot weather, while those in the north are not.

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