That warning remains in effect Thursday, since it should be three or four degrees toastier and more humid. And, as has been the case in recent summers, the nighttime warmth has been at least as impressive as the daytime heat.
In fact, there's a chance that by the time you read this, Philadelphia will have broken a record for highest minimum temperature for June 24. The forecast low was 76, a degree higher than the record.
No deaths have been reported in this heat wave, which began Sunday, but health officials say the lack of nighttime cooling is a serious concern. The temperature has not dipped below 70 since Saturday.
"We really worry about the nights as much as the days because of the cumulative impact," Szatkowski said. Overnight heat is a menace to the vulnerable elderly in rowhouse neighborhoods. Without cooling when the sun goes down, those homes heat up in a hurry after daybreak.
Szatkowski added that the sun is beating on rooftops with maximum feasible ferocity because the summer solstice was just days ago.
Factors inhibiting cooling after sunset include urban buildings and paved surfaces, which decrease cooling after sunset, and perhaps an increase in water vapor in the air as a result of worldwide warming. Vapor retards heat from escaping into space.
Whatever the reasons, an Inquirer analysis shows that in the last 30 years, the nights have warmed twice as rapidly as the days compared with the previous 30 years. The highs increased from 84.1 to 85.2, and lows from 64.8 to 67.3.
It didn't get below 73 Wednesday morning, but the region caught a major break in the afternoon with a strong west wind that knocked out some of the water vapor, Szatkowski said.
That, combined with a substantial cloud cover, kept temperatures and heat indexes in check. The effect evidently was quite localized, he said, as areas such as Harrisburg and Dover, Del., were far more uncomfortable.
Regardless, it was plenty hot at Camden's Northgate Park for Paulszewski, one of 200 volunteers at the Camden Clean Campaign event.
She came with the best of intentions, but in deference to the heat, Paulszweski, 60, thought it wiser to become a spectator.
The heat was having no such effect on Carlos Santiago, 12, a fifth grader at Raphael Cordero Molina School in North Camden. The students were getting a heat-day perk.
"If it's over 90 degrees, we are allowed to go in the sprinklers," he said. "So I'm happy it's so hot. My mom might even fill up the pool today."
On the 2300 block of Nicholas Street in North Philadelphia, cousins Navon Cooper, 8; Mahogony Weeks, 4; Quadir Rayfield, 7; and Tony Weeks, 2, were cooling off as their street was turned into a water playground.
Under Play Street, a Department of Parks and Recreation program, neighborhoods get permits that allow them to close off streets between 10 a.m. and 4 p.m. The program also provides children with food as an extension of the school lunch program.
The heat is a far more serious concern for the elderly, especially in Philadelphia.
A heat warning sets off several actions. The city alerts its 7,000 block captains, and urges relatives, neighbors and volunteer groups to check on the elderly. It beefs up its emergency medical staff, and the Philadelphia Corp. for Aging operates a Heatline, 215-765-9040 (press 4).
Between 11 a.m., when the line officially started, and 2:30 p.m., 56 calls were received from people between 46 and 80, said Heidi Gambino, Heatline supervisor. But only 10 of those calls raised enough concern that they were referred to nurses from the Department of Health for further screening.
The heat is also an issue in smaller cities and boroughs, such as Norristown and West Chester.
It was a busy day at the Safe Harbor of Greater West Chester shelter, said director Glenn L. Fricke. More than 65 people came in at lunchtime Wednesday, many with children.
"I would assume a lot of these families don't have any air conditioning," Fricke said. "I'm assuming it's because of the weather, because they all come in and say, 'Thank God, we're out of that heat.' "
Contact staff writer Julia Terruso at 610-313-8110 or firstname.lastname@example.org.
Inquirer staff writers Melissa Dribben and Anthony R. Wood contributed to this article.