Cities across the Northeast saw a second straight day of temperatures above 90 degrees.
In Vermont, temperatures soared to 97 degrees in Burlington and 77 degrees atop Mount Mansfield, the state's highest point at more than 4,300 feet.
Meanwhile, temperatures in Georgetown, Del., and John F. Kennedy Airport in New York reached 97 degrees -- records at both locations for June 21.
Boston's Logan Airport reached 96 degrees just after 3 p.m., breaking a record set more than 60 years ago.
LaGuardia Airport in New York, Bradley Airport in Hartford, Conn., and Reagan National Airport in Washington D.C., tied their June 21 records.
For the Northeast, the 30 million people who saw the heat move in Wednesday felt even warmer temps on Thursday. Friday should be another hot one, followed by cooler temperatures on Saturday.
"American Idol" hopefuls in Newark, N.J., got a bit of a break from the heat, the Associated Press reported. They were ushered inside Newark's Prudential Center on Thursday morning to register to audition.
Heat advisories are in effect across the Northeast until then, and farther inland the situation wasn't any better.
"Excessive heat warnings" were issued for the mid-Hudson Valley and southeastern Catskills in New York. It should feel like 105 to 110 degrees there on Thursday, the National Weather Service said.
The same warnings were issued for parts of Arizona "where high temperatures are expected to reach 110 to 114 degrees," the weather service stated.
Wednesday's records included: Newark, N.J. (98), Hartford, Conn. (97), Burlington, Vt. (95), Massena, N.Y. (92), Binghampton, N.Y. (90) and Houlton, Maine (90).
In Connecticut, golf officials at the Travelers Championship on Wednesday treated dozens for heat exhaustion.
"People are coming in dizzy, a little nausea, vomiting, generally poor feeling overall," emergency medical services director John Quinlavin said. "We generally have a more mature audience here, and we do see a lot of the elderly having some problems with the heat."
The Associated Press contributed to this report.