Philly summers are getting — and staying — hotter

High temperatures are constant, overnight relief is lacking, and mugginess is on the rise, an Inquirer analysis shows.

As the planet has been warming up, summers have been getting more sultry in Philly. Temperatures are consistently higher. Nighttime relief has been wanting. And it’s not unique to Philly. “It’s happening all over the country,” said Sarah Johnson, who is in charge of the warning program at the National Weather Service Office in Mount Holly. The air has become moister, and that invisible broth of water vapor is a challenge to the body’s remarkable cooling system.

On average during the last 30 years, temperatures have been about 2 degrees above 20th-century averages for the June 1-Aug. 31 meteorological summer. That’s almost identical to the global increases for the three-month period, according to the National Centers for Environmental Information.

With a heat wave due to continue at least through Thursday, this June has a chance to become the warmest in at least 10 years.

What’s more, in recent summers, the cool breaks between hot spells have become scarcer. And it’s not just the thermometer; it feels palpably worse.

Summer days and nights are more consistently muggy.

Not only are temperatures higher, summers also have been offering fewer cool interludes, with nights warming up faster than the days.

All that moisture in the air inhibits cooling after sunset, meteorologists explain, trapping the solar heat baked into the ground and all those paved surfaces and buildings, keeping it from radiating into space.

That constitutes a cumulative hazard, with cooler nights becoming scarcer.

• Take a look at how dramatically overnight temperatures have risen. Each point indicates how often a particular low temperature occurred. In the 1941 to 1970 period, the average low was about 64 degrees. That value was observed more than 150 times.

• Those 30 years saw a range of nighttime temperatures during the summer, from a handful of observations in the high 40s up to a few in the high 70s. But most frequently, temperatures were in the 60s overnight.

• But in the period from 1991 to 2020, warmer nights got more frequent. The average low rose to about 68 degrees.

• Readings dropped below 65 degrees on about half of the nights in the mid 20th-century, but only one in five in the recent decades.

• For nights above 70 degrees, that pattern was flipped. In the mid-century era, just one in five nights was that stifling. In the more recent period, almost half of them were.

When Philly’s rowhouses don’t cool down in the dark, they heat up quickly in the light. As a former staffer in the medical examiner’s office said, that can turn them into dangerous “brick ovens.”

That can be especially hazardous for elderly people who live alone and the poor and vulnerable.

Said Eric Klinenberg, sociologist at New York University and author of a seminal treatment of the 1995 deadly Chicago heat wave, “We have a tendency to treat them as expendable.”

Refreshing nights are now the exception

Dew points have also trended up, meaning the air is becoming damper and more uncomfortable. The amount of moisture in the air as measured by the dew point – the temperature at which water vapor comes out of hiding – has increased subtly.

In recent decades, only about a quarter of summer nights have been dry and “comfortable,” generally considered to be those having dew points under 60.

At 65, sweat starts becoming less effective; at 70, consider it uncomfortably muggy; and 75 and up would be oppressive.

The atmosphere over Philly, on average, is about 2.2% moister than it was in the 1950s, and meteorologists say that likely is related to the warming planet – warmer air can hold more water. It is that invisible vapor that inhibits the body’s cooling system.

Our sweat, oozing from two million glands, has a cooling effect when it evaporates – note the chill we feel stepping out of a shower as water beads vanish – but if the air is moist, sweat merely clings to our skin.

There are fewer degrees of separation

Day-to-day variability is down, meaning the warmer spells have fewer cool interludes to break the heat. In the last 30 years, the temperatures from day to day, on average, have varied by 4 degrees – compared with 4.8 degrees at the beginning of the last century.

• Let’s shift from considering low temperatures to high ones. Specifically, let’s look at the daily high temperatures in the month of July in 1926. That month, high temperatures varied widely.

• The temperature cracked 100 degrees on back-to-back days …

• … But there were also a couple days when the temperature didn’t even get above 70 degrees.

  All in all, readings that month bounced around quite a bit.

• Compare that with July 2020, typical of warmer summers in the last decade.

• Temperature ranges were narrower. In 2020, only one day stayed below 80, and it never managed to hit 100 degrees, unlike in 1926. In fact, Philly hasn’t hit triple digits since 2012, its longest 100-less stretch in over a century.

It’s hotter — and not just in summer — so why are fewer people dying?

Warmth has been accelerating in every season. The growing seasons have been growing longer, and allergists say pollen is filling the air (and noses) sooner and lingering longer.

Warmer summers mean higher utility bills for those who have air-conditioning, and potentially more danger for those who don’t.

Still, in the 10-year period that ended in 2002, Philadelphia reported about 400 heat-related deaths, compared with just 41 in the last 10 years, according to the Health Department.

The less-deadly spells could be related to there being more moisture in the air. That keeps temperatures up at night, but down in the daytime, said Johnson, because the sun has to divert energy to cook the water vapor.

For the first two decades of this century, heat-mortality experts say, heat deaths were down across the country, and they have various hypotheses, including more awareness of the dangers.

Sociologist Klinenberg has his own explanation: “Just plain luck.”