SPRINGFIELD, MASS. ? A third day of unseasonable heat blistered the eastern half of the country Thursday, making tornado cleanup miserable in Massachusetts and sending country music fans in Tennessee to hospitals, while the surge in demand for energy knocked out power to sections of downtown Detroit.

Relief was on the way in the Northeast, however, as an approaching cold front triggered evening thunderstorms. Tens of thousands lost power in parts of New England as the storms passed through.

The persistent heat and resulting storms has been blamed for at least eight deaths from the Plains to the East Coast, where authorities prepared emergency rooms and encouraged neighbors to check on the elderly as temperatures soared above 100 in spots.

Detroit officials intentionally cut power to city hall and a convention center Thursday to prevent the municipal power system from crashing from high energy demand — even though temperatures had tapered to the 70s after two days above 90. Equipment failures knocked out power to several other government buildings and traffic lights in parts of the downtown.

“Because there was a short window of time, we had to make a decision to take some of our customers off to prevent a blackout of the entire city,” Detroit mayoral spokeswoman Karen Dumas said.

Some Northeastern schools canceled classes or closed early for a second day Thursday so students would not have to suffer with no air conditioning. Cooling centers opened in Chicago, Memphis, Tenn., Newark, N.J., and other cities as a refuge for those without air conditioning.

In New Jersey, records of 102 degrees were recorded at the Newark and Atlantic City airports, beating their respective previous records of 99 degrees and 98 degrees set in 2008. The temperature also reached 102 degrees at Ronald Reagan National Airport near Washington, matching a record set in 1874. Philadelphia hit 99 degrees, one degree higher than a record set in 1933.

“I’d love to be indoors, but I don’t make any money that way,” said Jose Serrano, a landscape worker cutting lawns and trimming bushes in Toms River, N.J. “When it comes to working in these conditions, you just do what you have to do, you know?”

In Springfield, chain saws whirred amid high heat and humidity as workers cleared tree branches and other messes left by tornadoes that struck the area last week, killed three people and left hundreds living in shelters. The temperature hit 92 on Thursday.

As survivors sought clothing vouchers, diapers and other supplies, volunteers pressed cold water on them because many still lack electricity and thus refrigeration. Fire trucks passed out cases of water in addition to tarps for patching roofs.

Light clouds provided scant relief for the volunteers frequently seeking shade in Red Cross tents.

“The heat is certainly making it a little more impossible,” said Linda-Jo Perks, co-commanding officer of the Springfield branch of the Salvation Army.

While the Northeast began seeing some relief late Thursday as a cold front swept through with cooler, drier air, the scorching heat was to linger for days in the South. Music fans in Tennessee had that to keep in mind as three major festivals commenced.

The four-day Bonnaroo Music and Arts Festival began Thursday, with 80,000 general admission tickets sold for the event being held on a 700-acre farm about 60 miles southeast of Nashville. In Chattanooga, 600,000 people are expected over nine days at the Riverbend Festival.