Benjamin Franklin isn't credited with discovering the nor'easter, but he probably should get an assist.
In an oft-related account, Franklin was disappointed that a storm, packing wind winds from the northeast, ruined his chance to see the November 1743 lunar eclipse in Philadelphia.
But he was surprised to learn later that his brother, in Boston, saw it just fine, before the arrival of a nasty storm up that way, also with winds from the northeast.
Franklin made the counterintuitive leap: Surface winds don't drive storms, which tend to move southwest to northeast even if the winds are blowing northeast to southwest. The movements are governed by upper-air steering winds.
What spoiled his eclipse viewing in all likelihood was a "nor'easter," which the National Oceanic and Atmospheric Administration defines as "a strong low pressure system that affects the Midatlantic and New England States.
"A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas."
The winds howl from the northeast because winds circulate counterclockwise around storm centers.
Note that the criteria have a certain ambiguity, with no thresholds for winds or barometric pressure.
But the storm that formed off the coast and has been affecting the region had classic nor'easter characteristics earlier in the week, with the requisite winds.
And although the center was spinning in New England on Thursday, driving snows all the way to Philly, and the winds had backed around, it still was considered a nor'easter, according to Jessica L. Spaccio, at the Northeast Regional Climate Center, in Ithaca, N.Y.
Not all nor'easters end up affecting the Philadelphia region, obviously, but they form quite frequently during the cool season – October to April.
That's when the upper-air jet-stream winds blow the hardest and give developing storms spin and lift.
Based on 50 years of data, about 30 occur annually, according to a study by storm specialists Robert Dolan and Robert Davis at the University of Virginia.