First frost and freezing temperatures invade Philly region; 'heat island’ spares city

With conditions primed for falling temperatures overnight, frost made its first appearance of the season in the region early Saturday.

The National Weather Service Office in Mount Holly captured Emily Dickinson’s “blonde assassin" in photographs.

Temperatures dropped to 31 degrees in Pottstown, and 32 elsewhere in Montgomery County. Officially it was the coldest morning in Philadelphia since late April.

Even though temperatures didn’t reach freezing in Mount Holly, that didn’t spare the vegetation.

Air temperature readings are taken about six feet above ground, points out Sarah Johnson, a lead meteorologist at the National Weather Service Office in Mount Holly. But as early-morning runners are well-aware, it can get colder at toe level.

Skies were clear overnight, and winds calm, perfectly conducive to allow the daytime heat to escape into space.

Thanks to the “urban heat island effect” — buildings and paved surfaces are reluctant to yield their stored heat at night — Philadelphia officially didn’t get into the 30s.

» READ MORE: Climate change makes some Philadelphia neighborhoods extra steamy. The city has a plan.

In fact, in recent years, frost evidently has been showing up later in Philadelphia; the first official freezing reading is considerably later than it was in the past.

In records dating to 1874, that average first freeze date was Nov. 8. But in the 21st century, that has been pushed back to Nov. 14.

That likely is related to background worldwide warming, and also probably has something to do with the official temperature being taken at Philadelphia International Airport.

» READ MORE: Where is the temperature measured in Philly, and why?