When the nights get that unbearable after a day of high heat, "we do see an increase in heat-related illness," said Steve Alles, a Philadelphia Health Department physician.
On Wednesday, the official temperature at Philadelphia International Airport reached 97 degrees at 3:18 p.m., just one shy of the record for the date, set in 1923 and 1931. It felt like 101, and after two relatively pleasant weeks, it was as though nature flicked on the summer switch to "high" just in time for the solstice, which occurred at 7:09 p.m.
It was hot all over the Northeast, even at the Shore, where it hit 94 in Atlantic City. The heat producer was huge high pressure, or heavier air, in the atmosphere over West Virginia, said Dave Dombek, a meteorologist at Accu-Weather Inc.
And the ocean wasn't a lot cooler. Late yesterday, the surf temperature off Cape May was 78.6. "It's been incredible," Dombek said. "I've been watching water temperatures for 30 years. I've never seen that."
Today's temperatures in Philadelphia should take aim at the June 21 record, 99, also set in 1923.
The National Weather Service is forecasting an overnight low of 77, which would beat the record for the highest minimum temperature for a June 22 by two degrees.
Fortunately, this hot spell is likely to break before exacting a significant toll in mortality. However, the trend toward warmer nights remains disturbing to heat-mortality experts.
In terms of contributing to fatalities, "the nighttime can be as big or bigger than the daytime highs," Szatkowski said.
Those nights are especially dangerous in rowhouse neighborhoods where elderly people live alone without air-conditioning. A hot night makes the job of daytime heating all the easier, and those houses can warm up in a hurry once the sun comes up.
Generally, nights have been getting warmer.
From 2002 to 2011, the average nighttime low in Philadelphia in summer was 67.5. By comparison, the average low in the 1960s was 64.9. Last summer's average overnight lows were the fourth-warmest in Philadelphia records dating to 1874.
Evidently that is a function of background worldwide warming. Warmer air can hold more moisture, and that water vapor blocks cooling by preventing heat from radiating into space after dark. That's a particular problem in the city, where buildings and pavements spend the day soaking up punitive solar energy.
Computer modeling has suggested that with increasing global temperatures, nighttime warming would accelerate.
Said Szatkowski, "I'd say that's come to pass."
Contact Anthony R. Wood at 610-313-8210 or at firstname.lastname@example.org