Cold mornings, but on Philly 'heat island’ temperatures have been 8-10 degrees warmer than outskirts

Temperatures tumbled to 17 degrees Thursday morning in Millville, N.J., as low as 13 in Ocean County, and into the teens all over the region, with two notable exceptions — Center City and the official measuring station at Philadelphia International Airport.

And on Friday, they got well into the 20s outside the city, including 23 in Pottstown, but no lower than 31 at the airport.

On Thursday, the temperature bottomed out at 27 at the Franklin Institute in Center City, a full 10 degrees higher than the morning low in Millville, and 26 at PHL.

That’s mighty cold for mid-November when the normal lows are around 40, but well above the records for the dates, and also well above readings elsewhere in the region.

Once again, the “urban heat island” effect was in evidence. Daytime heat radiates into space after dark, but those masses of building materials and paved surfaces are stingy about giving it up.

Temperatures in the city often are several degrees higher than they are in surrounding areas, a potentially dangerous difference during heat waves, particularly at night. Various studies have documented that global warming has resulted in rising overnight minimums.

» READ MORE: Weather warning: These Philadelphia neighborhoods get the hottest in a heat wave

But in winter, the warming can have benign effects on urban heating bills, and snow melts a lot faster in Philadelphia than it does in say, Pottstown. (Unless you happen to live on a sunless street in South Philly.)

One perhaps surprising byproduct of the heat island is that average winds in the city are an estimated 25% less strong than they are elsewhere. Anyone who was walking around Center City Wednesday might find that hard to believe.

Buildings tend to block the air, which behaves like a fluid, but it eventually shoots through the urban canyons and gushes along the streets.

“It funnels,” said Lee Robertson, a meteorologist at the National Weather Service, “like when you put your finger over a fire hose.” Not that we would recommend such an exercise.

The urban heat island has been a semi-hot topic for more than 200 years.

According to Helmut Landsberg, a pioneer in documenting the phenomenon, a British researcher found that temperatures in London averaged about 2.2 degrees higher than surrounding areas during November in the 10-year period that ended in 1816. London has become about eight times denser since those days.

The temperature differences between cities and their surroundings are most pronounced when winds are light or calm, and skies are clear; that allows the heat to rise unobstructed.

Early Thursday, city temperatures might have been affected by some high clouds that limited the heat escape. Lows early Friday, expected near freezing, likely will be more uniform throughout the region as skies are forecast to be mostly cloudy.

In any event, mornings the next several days should be less punitive for the region’s motorists.

AAA Mid-Atlantic responded to nearly 600 dead batteries during the historically cold Wednesday, said spokesperson Jana Tidwell, a 65% jump over the previous Wednesday.

Temperatures are due to crack 50 Friday before a cool-down during the weekend.