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.
