Just before daybreak Wednesday, the official reading at Philadelphia International Airport dipped to 31, ending the city's autumnal freeze drought.

Windshield scraping already has been part of morning rituals in areas away from the city heat islands – even Atlantic City saw 29 back on Oct. 19.

But at the thermally challenged airport station, in the proximity of the river and paved surfaces and buildings, this is the first freezing reading of the season.

In the context of recent years, a first official freeze on Nov. 23 isn't all that late.

On average in the last decade it has occurred on Nov. 13, compared with the 143-year average of Nov. 8.

It could well be that mid-November freezes will be the norm in a warming world. But as we have observed, we see a rather remarkable decadal variability in that first-freeze date.

For example, the average date was Nov. 16 in the 1930s, and Oct. 24 in the '60s.