Trapped in a pressure cooker
For the 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 and Canada are 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 from 35 C to 40 C and humidex levels well above 40 degrees. The oppressive conditions extend from the northern United States to Texas and from the west coast 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, travelling farther inland than normal.
Combined with generally clear skies and the sun’s higher summertime angle, “it gets really hot,” Jacks said.
Thunderstorms can develop around the perimeter of the dome — called the “ring of fire” — bringing temporary relief to some areas, said Kevin Birk, a National Weather Service meteorologist in Illinois. But this dome is so large that the heat rebuilds quickly, Birk said.
While heat domes aren’t uncommon, this one is unusual because of its size and duration. It began three days ago and may last seven to 10 days in some locations.
But relief is on the way. Cooler air should begin moving into the central U.S. this weekend, as a strong pool of air from the jet stream begins to push hot air out of the way before making its way east. By Monday, temperatures will drop to about 29 C in the north, while they still could be sweltering in the east of the country, he said.
“This is really an exceptional event, I think it’s fair to say . . . in terms of scope and duration,” Jacks said.
The Associated Press