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.

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