National Geographic News
William Dyer sits in a beach chair in Sabbathday Lake in New Gloucester, Maine.
William Dyer, Jr., cools off in his beach chair in Maine's Sabbathday Lake on Thursday.

Photograph by Robert F. Bukaty, AP

Willie Drye

for National Geographic News

Published July 22, 2011

A stubborn high-pressure system is the culprit behind the dangerously high heat wave that's been baking much of the U.S., experts say.

The high-pressure system—a large area of dense air—is being held in place by upper-level winds known as the jet stream. Within the system, dense air sinks and becomes warmer, and since warm air can hold more moisture than cooler air, there's also very high humidity. (Learn more about Earth's atmosphere.)

Stationary high-pressure systems aren't unusual during the summer, according to Eli Jacks, a meteorologist at the National Weather Service headquarters in Silver Spring, Maryland.

But what sets this system apart is its size and strength.

"It's exceptionally strong and very wide, covering thousands of miles from border to border and from the Rocky Mountains to the East Coast," Jacks said.

(Related: "Heat Wave: 2010 to Be One of Hottest Years on Record.")

Heat Wave Part of Warming Trend?

Experts differ about whether global warming is influencing the extended period of very high heat, but Jacks emphasized it's impossible to make any correlations from a single event.

"Climate change occurs over years and decades," he said. "It's not possible to draw conclusions just because it's hot these few days."

But Kerry Emanuel, a meteorologist at the Massachusetts Institute of Technology, said the current extreme heat is "happening in the context of climate warming in general."

"Events like this will become more frequent."

(Related: "Global Warming 'Marches On'; Past Decade Hottest Known.")

While much of the U.S. is suffering, the high-pressure system that's holding the heat in place also is keeping conditions cooler than usual in the Pacific Northwest.

Rick Dittmann, meteorologist in charge of the National Weather Service office in Pocatello, Idaho, said his region has had a cooler-than-average spring and early summer. These conditions created a deeper-than-usual snowpack in the northern Rocky Mountains, he said.

Heat, Humidity a Dangerous Combo

In addition to the heat, the high humidity can be dangerous to human health, noted Maryland-based meteorologist Jacks.

During periods of unusually high humidity, sweat doesn't act as a natural cooling agent, Jacks said.

"The body can't evaporate moisture—it can't cool itself off," he said. "The body temperature actually starts rising."

(See a human-body interactive.)

The heat has already been blamed for about two dozen deaths across the U.S. this week. Unfortunately, sweltering temperatures are predicted to continue their grip on the Midwest, the Northeast, and the South.

For instance, Calvin Meadows, a meteorological technician at the National Weather Service office in Sterling, Virginia, said the high for Washington, D.C., is expected to reach 101 degrees Fahrenheit (38 degrees Celsius) today and 97 degrees (36 degrees Celsius) Saturday.

The agency has issued an excessive heat warning from noon until 8 p.m. ET, Meadows said.

Jacks, the Maryland-based meteorologist, added that the above-average temperatures would continue in much of the nation into August.

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