Heat 'dome' traps much of US in pressure cooker. 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.

"It's hot no matter what you're doing or where you are," said Tim Prader, a 50-year-old construction worker who was taking a break Tuesday at a job site in St. Louis. Although his huge Caterpillar excavator has air conditioning, he couldn't entirely escape. "When you're done for the day, you're ready to eat, drink and hit the couch."

The oppressive conditions extend from the northern Plains states to Texas and from Nebraska to the Ohio Valley. And they're expanding eastward.

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.

The dome of high pressure also pushes the jet stream and its drier, cooler air, farther north — it's now well into Canada — while hot, humid air from the Gulf of Mexico circulates clockwise around the dome, traveling farther inland than normal.

Source: yahoo