Overnight the temperature sank all the way to 42 in Pottstown and Millville, and 44 in Mount Holly and Doylestown.
But at Philadelphia International Airport, it didn’t get below 52.
The overnight conditions provided an almost ideal environment for show-casing how humans can affect the overlying atmosphere.
The winds were calm, and the moonlit skies brilliant clear.
Those conditions are perfect for maximum radiational cooling. When the air is almost still and the skies clear, the days heat rapidly escapes from the Earth’s surface into space.
The escape isn’t nearly as clean in the cities. Buildings and paved surfaces are reluctant to give up their heat, warming the surrounding environment.
The effects would be most pronounced in Center City, but Philadelphia International Airport has its share of heat-absorbing structures and surfaces.
Cities can affect the environment in other ways, of course. We all know about how they intensify heat waves, and how wind gusts can funnel through urban building corridors.
Various studies have shown that cities can affect precipitation patterns. Convection is caused by the rising of warm air, and cities are low-grade hot plates.
These days, however, urban heat island research is compounded by sprawl. Mini heat islands have sprouted all over the place with development.
We can say with certainty that whatever else happens, the first frosts will coat the lawns in Philadelphia’s neighboring counties before it coats the windshields of Center City.