WASHINGTON — Sweltering temperatures across half the country have people doing what they can to stay cool, and some of them will need to keep doing it for the rest of the week.

Some schools in the Northeast planned to close early for a second day today so students would not have to suffer in buildings with no air conditioning. Others canceled classes altogether.

“A lot of people were complaining,” said Stephanie Poff, 12, a sixth-grader at an elementary school about 70 miles north of Philadelphia that sent students home early Wednesday. “It is hard to study when it’s hot out because all you’re thinking about is, ‘I wish I could be in air conditioning.'”

The six-to-10-day outlook from the federal Climate Prediction Center calls for continued above-average readings centered on the mid-South, including Louisiana, Mississippi and Alabama, and extending as far as the Great Lakes and New York and New Jersey.

“I’m staying in my house. I’m going to watch TV and have a cold beer,” said 84-year-old Harvey Milliman of Manchester, N.J. “You got a better idea than that, I’d love to hear it.”

Cooling centers opened in Chicago, Memphis, Tenn., and Newark, N.J., as a refuge for those without air conditioning.

In the Northeast, the scorching heat was forecast to subside by Friday as a cold front sweeps in with cooler, drier air. High temperatures across the region are expected to stay in the low 80s through the weekend.

“It will be a very dramatic change,” said Charlie Foley, a meteorologist with the National Weather Service in Taunton, Mass. “Even though the temperature will go down, the big difference that people will appreciate will be the lower humidity.”

Authorities say hot weather was so intense in southwestern Michigan that it buckled pavement on an interstate, forcing the roadway to close for a few hours Wednesday, according to the Battle Creek Enquirer.

If scientists are right, we’d better get used to the heat. A new study from Stanford University predicts that global climate change will lead permanently to unusually hot summers by the middle of the century.

Temperatures in the 90s were recorded across much of the South, the East and the Midwest on Wednesday. Baltimore and Washington hit 99 degrees, breaking high-temperature records for the date that were set in 1999, according to the National Weather Service. The normal high for the date is about 82.

Philadelphia hit 97 degrees, breaking a 2008 record of 95, and Atlantic City, N.J., tied a record of 98 set in 1999. Chicago reached 94 by midafternoon Wednesday, though highs were only forecast to be in the low 60s by today.

Forecasters said it felt even hotter because of the high humidity, yet several southern states were also experiencing drought. The ridge of high pressure that brought the broiling weather is expected to remain parked over the region through today.

In Oklahoma, where temperatures have reached 104 four times so far this month, the Salvation Army said more people are seeking help with high utility bills earlier in the season, and paramedics responded to more heat-related illnesses.

Authorities blamed the heat for deaths of five elderly people in Tennessee, Maryland and Wisconsin in recent days. Two men died in Wisconsin after they jumped into a river to cool off and got pulled underwater. Officials in Long Island, N.Y., reminded people to check on their relatives and friends, especially the elderly.

That is likely to continue in the coming month, with the hot weather extending west into New Mexico and Arizona. The three-month outlook shows excessive heat focused on Arizona and extending east along the Gulf Coast. Cooler-than-normal readings are forecast from Tennessee into the Great Lakes states.

At Stanford, Noah S. Diffenbaugh and Martin Scherer analyzed global climate computer models and concluded that by midcentury, large areas of the world could face unprecedented heat. They said the coolest summers will be hotter than the hottest ones of the 1900s.

Global warming in recent years has been blamed on increasing concentrations of gases such as carbon dioxide in the atmosphere. The permanent shift to extreme heat would occur first in the tropics and reach North America, South America and Eurasia by 2060, the scientist report in a paper that will be published in the journal Climatic Change Letters.

It’s hard to stay cool at a baseball game, but Reds and Cubs fans were trying at Great American Ball Park in Cincinnati, which had issued a heat emergency.

Kathryn Burke, of Pikeville, Ky., wore a straw hat, brought two bottles of frozen water, and a portable mister.

“And I brought the knowledge to leave when I’ve had enough of the heat,” she said.

One Cubs fan wasn’t so concerned.

“Sunblock, water, and shades, then enjoy the game,” said Brad Daniels of his heat defenses. “Hey, it’s baseball. We’re here to see the boys of summer.”

Among those sweltering in the Newark, N.J., heat was Alejandra Perez, who was barbecuing chicken, ribs and shish-kebabs over an outdoor grill at Manny’s BBQ Restaurant & Deli in the city’s downtown. Newark reached 99 degrees, breaking a record of 97 set in 1999.

“I’m from El Salvador, and it’s hot there, but the heat is much worse here,” she said in Spanish.