Philly and other places have been setting records this week before sunup

While they may not have the sizzle of triple-digit high temperatures, Philadelphia and other cities in the Northeast and Midwest have been setting under-the-radar records this week even before the sun could get out of bed.

As hot as the afternoons have been, the nighttime warming has been more impressive — and arguably of more significance — said Peter Mullinax, meteorologist with the National Weather Service’s Weather Prediction Center.

“That’s when we start to see more pronounced heat-related illnesses and fatalities,” he said. No cooling at night inhibits buildings — and bodies — from rebounding from the effects of the previous day’s extreme heat.

In addition, the night heating “puts more stress on the power grid, it puts stress on people’s pockets. It ends up costing more,” Mullinax said.

Along with hitting 100 degrees for the first time since 2012, Philadelphia set records for high daily minimum temperatures three days in a row starting Sunday, and tied a record Wednesday. The first such streak in 130 years. Tuesday marked the first time since 1952 that the temperature failed to get below 80 degrees in Philly.

And although the heat dome is about ready to give it up around here — Friday’s high may be several degrees lower than the lows this week — Thursday will feature one of the warmer June daybreaks in the period of record.

On Sunday, high temperature records were set in about 10 stations in the Midwest, said Mullinax. Record-high minimum readings were reported at 50 stations.

In the 21st century, 23 record highs have been reached at Philadelphia International Airport, and 47 record-high lows, an Inquirer analysis of weather service data of daily highs and lows showed .

“That’s not surprising,” said Alex Staarmann, climate specialist at the weather service in Mount Holly. He noted that on Tuesday, all nine official stations in the Mount Holly network tied or set high-minimum records on Tuesday.

“Multiple factors” would explain the phenomenon, he said, and two of the big players would be urbanization and Earth’s rising temperatures.

Philadelphia and other cities have created so-called “urban heat islands” where temperatures tend to be warmer than surrounding areas, especially at night.

The I-95 corridor in the Northeast constitutes a veritable archipelago of heat islands.

“We just continued to build,” said Mullinax. The buildings and paved surfaces store solar energy by day and reluctantly give up after the sun goes down.

A more pervasive factor here and all over the world is the increasing amount of water vapor in the air as the planet’s temperature rises, he said. Warmer air can hold more water.

When the air is overstocked with water vapor it inhibits the body’s natural cooling system, which relies on the evaporation of sweat.

At night it impedes cooling by blocking the cumulative daytime heating from escaping into space. Thus surfaces are primed for heating when the sun comes up.

Water vapor levels have been increasing over the last couple of decades, he said.

High temperature records fell on Monday, Tuesday, and Wednesday, when it reached 99.

However a “back door front” — that’s one that moves northeast to southwest — is about to import some cooler maritime air from the Gulf of Maine and the North Atlantic, Mullinax said.

Onshore winds from the northeast are slated to rout the heat dome.

Thursday still is expected be muggy, but highs and heat indexes are forecast to stay below 90, and refreshing showers are likely.

Friday’s forecast high in the mid-70s might feel like a cold wave.