Philadelphia hasn’t set a daily high-temperature record for July in four years, but it has been setting new standards after dark.

It tied a daily record for the highest minimum temperature when the thermometer didn’t get below 81 at Philadelphia International Airport on July 20, and it broke one on Tuesday, when it stopped the drop at 79.

This is not a new phenomenon. In fact, averaged over the 10-year period, low temperatures in July have been far and away higher than they were in any other decade. The overall average low, 71.7, is almost 2 degrees Fahrenheit warmer than its nearest competitor, the 1991-2000 period. By contrast, the average July low in the 1960s was 66.1.

July 2020, with an average low of 73 through Tuesday, still has a shot at the No. 1 spot, now held by 2013, sweating nervously in the clubhouse at 73.1.

This month has had an impressive 19 days of 90-plus highs, with three more expected. But relative to “normal,” the nights have outdone the days for warmth, a big reason why this is going to be one of the warmest Julys on record in the region.

In Philadelphia, it has a shot at the No. 3 spot and almost certainly will make the top five. For state average temperatures, this July in New Jersey is sitting at No. 5, said Dave Robinson, the Rutgers University professor who is the state climatologist, but it could move all the way to No. 1 by day’s end Friday, he said.

Known for going all out with their holiday decorations in December, Alyse (left) and Perry Katsapis decided to have Christmas in July to brighten everyone’s spirits during the COVID-19 pandemic by decorating their Wallingford, Delaware County home. They set up blinking candy canes Tuesday.
CHARLES FOX / Staff Photographer
Known for going all out with their holiday decorations in December, Alyse (left) and Perry Katsapis decided to have Christmas in July to brighten everyone’s spirits during the COVID-19 pandemic by decorating their Wallingford, Delaware County home. They set up blinking candy canes Tuesday.

Why are the nights so warm?

With the planet’s temperature rising, climate researchers have been saying, water-vapor content also is rising. Warm air can hold more moisture.

Nationally, over 60% of the country reported “much above” normal overnight lows in 2018 during the meteorological summer, June 1 through Aug. 31; 0% reported “much below,” according to the National Centers for Environmental Information. The last time a significant portion of the nation fell into the latter category was in 2004.

Water vapor has an inhibiting effect on cooling after the sun goes down because it traps some of the daytime heating that otherwise would rise into space.

Philadelphia and other urbanized areas have an added anti-cooling agent — the so-called urban heat island effect. Buildings and paved-over surfaces sponge up solar energy during the day and give it up reluctantly at night.

The Atlantic and Delaware Bay are on fire in this image of sea-surface temperatures posted. Warm oceans have been helping supply our humidity.
Rutgers Coastal Ocean Observation Laboratory
The Atlantic and Delaware Bay are on fire in this image of sea-surface temperatures posted. Warm oceans have been helping supply our humidity.

And this summer, the steaminess has been getting extra juice from the ultra-warm sea surface temperatures in the Atlantic and other bodies of water that are generous suppliers of humidity.

Why should we care?

Health experts say that hot nights can be life-threatening for the heat-wave vulnerable, particularly those in brick rowhouses. The lack of overnight cooling allows heat to flare in the houses after the sun rises.

Fortunately, despite the relentless steamy heat, no deaths have been reported in Philadelphia this summer, according to the Public Health Department.

For people without air-conditioning, hot nights can be a nightmare. One large National Institutes of Health study found that increases in nighttime temperatures led to increases in complaints about lack of sleep, a potentially serious long-term health problem.

The authors found “the largest effects during the summer and among both lower-income and elderly respondents.”

Does this end?

It appears that the kettle is still on the stove for at least a few more days.

Temperatures Wednesday are forecast to rise into the low 90s — and that’s after a cold front slips through.

They are to report back toward the mid-90s on Thursday, but top out in the upper 80s on Friday and through the weekend.

But they are not forecast to get below 70 in the city.

The last time that happened was July 7.