For much of U.S., summer feels like a pressure cooker

CHICAGO — If the extreme heat and humidity lingering over much of the nation feels like a steam bath, it's because the same principles are at work in the atmosphere.

Vast amounts of warmth and moisture have become trapped under a huge "heat dome," bringing record-breaking temperatures and thick, tropical air to scores of cities from North Dakota to the Ohio Valley. Now the system is moving east to spread the misery to some of the country's most densely populated areas through the weekend.

In addition, the heat wave is putting significant stress on the nation's power grid as homeowners and businesses crank up their air conditioners.

Utilities say they're ready for high power demand, and widespread electricity shortages or outages are unlikely. Lines and equipment are not fully taxed and there is more generating and transmission capacity available than usual because of the weak economy. Also, not many major storms are in the forecast, meaning fewer downed power lines.

Nationwide, Thursday and Friday will be hotter than any time since 1950, said Travis Hartman, the Energy Weather Manager at MDA Earthstat, which provides forecasts for utilities and other weather-dependent businesses.

"It's going to mean elevated power demand for an extended period of time for a lot of people," he said.

To meet demand, utilities are firing up special power plants used only a few days a year, delaying scheduled maintenance in order to keep all equipment on line and testing heat-sensitive switches and other equipment with high-tech devices like thermographers that can gauge temperatures to one-tenth of a degree.

"These are the days everyone wants to have their ACs on, their computers going while they watch TV," says Jon Jipping, chief operating officer of ITC Holdings Corp., a transmission-grid operator that owns grids in Iowa, Michigan and four other Midwest states. "These are the days we get ready for."

Problems can arise when the grid comes under maximum strain. Equipment can't cool off, and it can't handle as much power as usual. Lines, transformers and switches are working at full capacity and can be overwhelmed by power surges that can result from a blown piece of equipment or downed power line.

Even drops in power demand can be perilous. When a thunderstorm drenches a big, hot city, there is a quick drop in power demand because suddenly millions of air conditioners don't have to work so hard. When power flow changes rapidly, either because of a surge or a sudden drop-off, devices meant to prevent equipment failures could trip, cutting power to customers.

Peak summer demand can be nearly double the demand of a typical day in a mild month like April or October. The PJM Interconnection, which operates the transmission grid in parts of 13 mid-Atlantic states, hit a record peak demand of 146,082 megawatts Tuesday. That compares to a typical April peak load of 78,000 megawatts.

The heat dome forms when a high-pressure system develops in the upper atmosphere, causing the air below it to sink and compress because there's more weight on top. That raises temperatures in the lower atmosphere, 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.

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

The formation of the dome also explains why conditions in, say, North Dakota aren't much different this week than in Houston. The big difference is that people in Houston are accustomed to hot weather. Those in the north are not.

"In places where the highest temperature you ever expect is in the 80s and you're at 102, there are big health concerns," because fewer people have air conditioning or fans, Jacks said. "Heat is the No. 1 killer out of all weather hazards."

What's more, because of the humidity, even nighttime brings little relief.

Humidity makes the weather feel far hotter because the body, which cools itself by perspiring, has to work harder when the air is already moist.

"It's harder to cool down," said Jannie Ferrell, a National Weather Service meteorologist.

This difference is expressed by the heat index — a measure of humidity combined with temperature. On Tuesday, the heat dome produced some eye-popping heat-index readings: 129 degrees in Newton, Iowa, 122 in Gwinner, N.D., and 121 in Taylorville, Ill.

Although heat domes are not rare, this one is unusually large and long-lasting. It began three days ago and may persist for seven to 10 days in some locations, meteorologists said. On Wednesday, it had begun moving out of Texas and the Dakotas, headed east and northeast. By Thursday, temperatures in Washington, D.C., were forecast to hit 100, and the heat could linger along the Atlantic seaboard.

Thunderstorms can develop around the perimeter of the dome — called the "ring of fire" — bringing temporary relief to some areas. But this dome is so large that the heat rebuilds quickly, said Kevin Birk, a National Weather Service meteorologist in Illinois.

No widespread deaths have been reported, but the heat sent dozens of people to hospitals and disrupted many routine activities.

As hot air blew over the cooler waters of Lake Michigan on Tuesday, a thick fog shrouded many of Chicago's beaches. Lifeguards had to turn away swimmers because they could not see beyond the water's edge.

Cooler air should begin moving into the Plains states this weekend, as a strong pool of air from the jet stream begins to push hot air out of the way in the Dakotas and into Minnesota before going east.

By Monday, temperatures in many places will drop into the mid-80s, although cities in the East could still be sweltering.

"This is really an exceptional event, I think it's fair to say ... in terms of scope and duration," Jacks said.