Temperatures in the week before Labor Day 2017 averaged a solid 5 degrees below normal in Philadelphia and never got above 81, so had the Philadelphia School District decided to start classes earlier than usual last year, some students might have been wearing jackets.

In fairness, when the School District decided that 2018-19 classes would start the week before Labor Day — with the aim of ending them on June 4 — it had no way of knowing that the region would be steamrolled by some of the most intense heat of a generally benign summer.

What resulted was a sequence of early dismissals during the first week — and yet again this week, where classes ended at noon Wednesday and will do so again Thursday — and angry complaints from sweltering teachers and students, parents, the union, and school principals.

An analysis of National Weather Service records shows that in terms of avoiding heat discomfort in aging buildings that lack air-conditioning in an urban setting, extending the school year into June is a far safer bet than starting it before Labor Day — especially in a city where about three-quarters of the schools don't have central air-conditioning.

"You send these poor kids to school, and it's this hot, they can't even concentrate," said Jason Killinger, co-chair of the HVAC department at the Pennsylvania College of Technology in Williamsport.

"We are going to come up with a new calendar committee that will come up with a calendar for next year," district spokesman Lee Whack said Wednesday evening.

Whack said that in the four school years prior to this one, heat had resulted in three early dismissals in June.

The School District has stated that late August and mid-June average temperatures are similar. While that generally is true for daytime highs, differences in overnight lows are significant.

The normal low for the Aug. 27-31 period, the School District's early-start dates, is 66; for June 4, which would be the last day of 2018-19 classes, it is 60.

Philadelphia's school buildings are in the "urban heat island," where paved surfaces and buildings absorb solar energy efficiently during the day and are reluctant to give it up when the sun goes down.

"There's a lot of concrete," Killinger said. "Once that heats up, it doesn't release the heat."

That's one reason that temperatures in suburbs can be several degrees cooler overnight.

What's more, as the summer progresses, heat accumulates in urban buildings, says Simi Hoque, a Drexel University professor and energy-efficiency specialist.

"The school has been absorbing heat over the summer," she said, "and it has to find a way to flush it out. It doesn't happen that easily at night."

Once heat builds up on a hot day, "you just can't release it by opening the windows," said Killinger.

Without overnight cooling, indoor heat can build up rapidly when the sun comes up. In fact, health experts say this phenomenon contributed at least as much to heat-related mortality in rowhouse structures as daytime heating.

An analysis of official records dating to 1874 shows that nighttime temperatures in the week before Labor Day and the days immediately after are three times more likely to remain above the 70 threshold than they are in the comparable June periods.

"That's a huge thing," Killinger said.

And not only do the June nights tend to be cooler, they don’t have to rout a summer’s worth of accumulated heat.