Freezing everywhere but at the Airport

Hundreds of thousands of the region's motorists went bobbing for the ice-scrapers this morning after overnight temperatures fell to the wintry mid- and upper-20s.

Ghostly layers of frost -- Emily Dickinson's "blond assassin" -- covered lawns and fields.

However, the official thermometer at Philadelphia International Airport exercised its powers of freeze immunity, with a big assist from human beings.

Philadelphia is situated on an "urban heat island," where solar energy stored by buildings and hard surfaces is slow to escape at night.

The differences between the city and surrounding areas is starkest during overnights with clear skies and light or calm winds.

Those conditions are ideal for allowing heat to radiate into space. In the city, however, the heat island effect inhibits the cooling.

Evidence of the heat island effect was dramatic Monday morning.

At Pottstown, the temperature at 7 a.m. was frigid 25, and lows reached 27 at McGuire Air Force Base; 28, at Blue Bell; 29 at Atlantic City Airport.

But it didn't get below 35 at the Philadelphia airport, where getting to freezing is a particular challenge.

The automated observing system is located in a meadow near the river and the marsh in Tinicum, and is subject to all sorts of micro-climate effects. That's one reason that zero readings are so infrequent there.

We've written before about how the first official freezes of the season – when the temperature reaches 32 – have been arriving later in Philly.

That might have something to do with worldwide warming, but in reality the airport is not a great indicator.