By Mary Wisniewski
CHICAGO (Reuters) - In Minnesota, a state legendary for its cold winters, the heat and humidity is so high this week that highway pavement was buckling in the Minneapolis-St. Paul area.
Temperatures in the mid- to upper-90s across the country's mid-section Tuesday caused cities like St. Louis and Detroit to open cooling centers for overheated residents. Cincinnati's fire department sent out extra personnel on calls, so commanders could rotate out exhausted teams.
And in Chicago, some public school students struggled to concentrate on finals in un-air-conditioned classrooms as the outside temperature approached a record 97 degrees.
"It's perspiratory and sweltering," said AccuWeather.com meteorologist Elliot Abrams. "You take a shower and step outside and the sweat beads form almost immediately."
Abrams warned that people who work outside have to watch out for possible over-exertion and heat stroke.
The high temperatures in the Midwest started Monday and are expected to last through Wednesday, spreading to the east on Thursday, according to Jim Allsopp, meteorologist for the National Weather Service. He said temperatures in Illinois are running "well above normal," by about 15 degrees.
"In the Midwest, it's pretty early to be getting this hot," Allsopp said. He said "very warm air" is coming into the area from the southwest.
In the Minneapolis-St. Paul area Monday, 28 pavement failures caused by the heat and humidity popped up on state and U.S. highways in mid-afternoon, causing traffic snarls, said T. K. Kramascz, spokesman for the Minnesota Department of Transportation.
"I don't remember having seen as many as this in one afternoon," Kramascz said. He expects the area could get more of the same Tuesday.
Chicago school principals have been told to keep students hydrated and move classes if necessary from rooms exposed to sunlight, according to guidelines distributed to schools. Principals may relax dress codes and allow students to wear shorts, the guidelines said.
Relief is coming, according to Allsopp, who said the weather should start cooling off Thursday and Friday in the Midwest, though the south will stay hot.
(Writing by Mary Wisniewski; Additional reporting by James B. Kelleher and Bruce Olson; Editing by Jerry Norton)