What one of Philly’s warmest Junes on record may mean for the rest of summer

With persistent warmth that featured nine days with readings of 90 degrees or higher and 25 days of above-normal temperatures, last month became the fourth-warmest June in Philadelphia in official records dating to 1872.

The extreme hot spell that ended last week broke before it turned deadly, and the region experienced nothing like the June tragedy of 100 years ago when Fairmount Park became an encampment site.

But with an average temperature of 77.4 degrees, last month joined a concerning 21st-century trend that tracks neatly with the warming of the planet. Of the 15 warmest Junes in Philly, nine have occurred since 2001.

Giving the June average a boost was a sequence of steamy nights, said Sarah Johnson, the warning coordination meteorologist with the National Weather Service office in Mount Holly. Overnight lows failed to get below the mid-70s on three consecutive mornings during the heat wave, about 10 degrees above normal.

In the short term, July will be getting off to a refreshing start — and does it get much better than Monday around here? — continued dry with temperatures comfortably in the 80s for the next two days.

However, the triple-digit heat indexes may return “just in time” for the Fourth of July festivities, said Johnson.

And the longer-term outlooks — supported by history — suggest that this week’s gentle run could be fool’s cool.

In summer, the atmosphere is prone to exhibit a certain torpor — not unlike a lot us — and what happens tends to keep happening.

Attesting to that persistence tendency, nine of the 10 hottest Junes on record in Philly became parts of summers that ranked in the top 25% of the warmest, according to weather service data.

The summer of 2010, No. 1 on the June list with an average temperature of 78.2, was the warmest June 1-to-Aug. 31 period — the meteorological summer — on record. The June runner-up, 1994, was part of the fifth-warmest summer.

June of 1925 is third on the list for temperature, but may well be No. 1 in historical significance.

Following a May cool spell, that month started with a blistering seven-day heat wave that included consecutive days of 100-degree temperatures. Philadelphia hasn’t experienced a 100-degree reading in 12 years.

Parts of Fairmount Park and the Delaware Riverfront “were used as overnight sleeping camps,” The Inquirer reported.

According to The Inquirer, an analysis of day-by-day mortality indicated that hundreds had died of heat-related causes, 70 of those on one day.

The season’s first heat wave can be especially dangerous to unacclimated bodies, said Laurence Kalkstein, a climatologist and heat-mortality expert.

And, of course, no one had air-conditioning, plus the air these days is a whole lot cleaner than it was in the ‘20s.

Despite the recent run of hot summers, Philadelphia has reported only about 40 heat-related deaths in the last 10 years — none so far this year.

And it’s not just the proliferation of air-conditioning, Johnson believes: “There’s certainly a lot more awareness now of how dangerous heat is.”

Evidently, the region in the days to come will be having more encounters with the heat and that invisible atmospheric moisture that frustrates the body’s cooling system.

» READ MORE: Philly summers are getting — and staying — hotter

“It looks like it’s not really going to significantly break anytime soon,” Johnson said.

The government’s Climate Prediction Center has the odds strongly favoring above-normal temperatures here and in much of the nation for the next two weeks, and above-average temperatures through the summer.

But the most recent heat wave may have upside, said Johnson. “As we get further into the summer, people are getting a little more used to the heat.”

That may be a lifesaver.