The nights are getting hotter in Philadelphia, and it has nothing to do with nightlife.
Overnight temperatures have been creeping upward for the last 25 years, more robustly than daytime highs, and health experts say that is a cause for concern.
An analysis of daily minimum temperatures showed that between 1874 and 1991, lows failed to get below 70 an average of 29 times during meteorological summers, the June 1-Aug. 31 period.
But in the last 25 years, temperatures failed to get below 70 an average of 43 times - a 50 percent increase - and 47 times already this summer.
"That's a dangerous situation," said Laurence Kalkstein, the University of Miami professor who helped Philadelphia develop its heat-alert system, held up by the Centers for Disease Control and Prevention as a national role model.
The latest stretch of steamy nights stands at six, with more to come.
While 100-plus afternoon heat indexes are attention-getting, Kalkstein and other experts say that under-the-radar overnight heating is more insidious. Without nighttime cool-downs, brick rowhouses can quickly become brick ovens after sunrise, a life-threatening hazard to the elderly and vulnerable who don't have air-conditioning.
Kalkstein has said that his computer models consistently have indicated that nights have a greater impact on heat-related mortality than daytime heating.
So far, the city Department of Public Health has reported no heat-related deaths from the latest hot spell, but Kalkstein said that could change in coming days after medical examiners complete their investigations. Plus, he said, heat-related mortality almost always is undercounted.
Wednesday marked the sixth consecutive day of 90 or better heat, and Thursday almost certainly will be Day 7.
But the recent heat wave has not rivaled those of 1988, when the temperature hit 90 or better on 18 straight days and was 100 three different times earlier in the summer, or 1993, when 118 heat-related deaths were reported in the city, or those in the steamy summers of 1995 and 1999.
Daryl Hadden, for one, was not overly impressed.
The Fayetteville, N.C., native moved to Philadelphia a few years ago and said this week's heat blast was par for the course down south.
"This is about average for North Carolina," Hadden said. "The humidity is worse there."
Still, he was not complaining that others found it sweltering, as he was selling bottled water from a cooler at Sixth and Market Streets.
Hadden said he had sold 24 bottles by the middle of Wednesday afternoon, at $1 each. On Tuesday, Hadden said, he sold 45 bottles. He did not hit the street until 4 p.m., but that coincided with the peak of the heat on a day the temperature made it to 93.
That represented a cooldown from Monday, the hottest day of the summer, when the high of 97 set a record for the date. The heat index reached 108, symptomatic of the soup of humidity covering the region, and that moisture that was wrung out by strong storms that would set a rainfall record for the date.
The city imposed its heat warning system until Monday evening, and deployed outreach teams to check on the homeless, while the Philadelphia Corp. for Aging set up its toll-free heatline. Teams of medics were stationed near the Democratic National Convention protest site in FDR Park.
The National Weather Service has posted a flood watch for the entire region from 6 p.m. Thursday until 7 p.m. Friday as another round of showers is expected. But even with the rains, the overnight lows are expected to stay at 70 or better well into next week.
Along with background worldwide warming, urban warming evidently has been a factor in the increase in warmer nights.
Warmer air can accommodate more water vapor, and that vapor inhibits cooling by preventing heat from radiating into space after dark. That phenomenon would be exacerbated in the city, where buildings and pavements spend the day soaking up punitive solar energy and are reluctant to give it up after dark.
Computer modeling has suggested that with increasing global temperatures, nighttime warming would accelerate. A trend in Philadelphia is evident. In 1991, the National Weather Service "normal" low for July was 67; today, it is 70.
By contrast, the normal daytime high has increased just one degree, from 86 to 87.