This is the time of year when the atmosphere itself seems to yield to the sultriness and becomes as lazy as the rest of us. In this case, however, that might be a dangerous tendency.

The official low temperature Friday morning was 79, a record-high minimum temperature for the date, besting the 78 of 1994.

If that forecast holds, overnight temperatures won't fall below 70 until Wednesday, and that lack of relief that could result in some heat-related deaths.

"That does increase the danger," said Al Cope, the senior scientist at the National Weather Service office.

So far, no heat-related deaths have been reported in Philadelphia, as the first four hot spells were three-and-out. "Nothing very extreme has happened this summer," Cope said.

An "excessive heat warning" was in effect Friday, when the high reached 93, but that marked only the second such warning of the season.

What is different about this heat wave is staying power, with the 90-plus conditions lapping into next week – possibly interrupted by severe storms Saturday and Sunday — as a classic and stubborn "Bermuda high" off the Atlantic coast washes the region in sultry air. In midsummer, the temperature contrasts that drive steering winds weaken, thus upper patterns tend to persist.

Typically, the days hog the media attention with their triple-digit heat indices, and those wonderful images of resourceful folks taking uncanny measures to cool themselves.

But in city neighborhoods, absent overnight cooling, the baking sun can turn those flat-roofed rowhouses into "brick ovens" in a hurry, if they don't have air-conditioning.

Laurence Kalkstein, the climatologist who helped the city develop its widely praised heat-response plan, has said that his computer models consistently have indicated that nights have a greater impact on heat-related mortality than daytime heating.

Nights in the city have become warmer generally. Our own analysis last year found that the number of ultra-warm nights – defined as lowest temperatures staying at 70 or above – jumped 50 percent in the previous 25 years.

Said Kalkstein, now at the University of Miami: "That's a dangerous situation."