Zero: Tough target
Philadelphia hasn’t hit zero in over 20 years, and here’s one reason.
In the period of record dating to the 1870s, the official temperature in Philadelphia has slipped to zero or below 45 times.
But it hasn't happened once since Jan. 19, 1994.
Granted, generally this has been a warmer period in the region, with slight increases in "normal" high and low temperatures on the official National Weather Service lists.
However, the lack of zeroes also has something to do with the location of the official thermometer at Philadelphia International Airport.
The automated observing system is in a meadow near the Delaware County border, not far from the mass of paved surfaces that constitute the airport, and near the Delaware River and a swamp.
That certainly is not the best place to take an official temperature, but that's where it is taken. It could be worse, and arguably has been. Until 1940, the official readings were taken downtown.
One thing that could make a difference Friday morning is snow cover. Chances are a few inches will be on the ground beneath the thermometer, and that would chill the overlying air.
Another factor would be the winds. The calmer they are, the better chance that any heat has of escaping into space.
If it doesn't happen Friday morning, it could happen Saturday.
But it's entirely possible the wait for zero will go on, and this hasn't been the longest wait.
That distinction belongs to the period between Jan. 23, 1936, when it went to -2, and Jan. 21, 1961, -4.