Thundersnow! The mere mention of the Washington area’s Wednesday night weather phenomenon conjures Nordic gods and Bjork lyrics — but in fact it’s not nearly so strange. “It’s basically a light flash that occurs during a snowstorm,” said Walt Petersen, a physical scientist at NASA Marshall Space Flight Center. “It’s relatively rare in the sense of observing it … but it’s not Armageddon.”

Of about 10 million cloud-to-ground lightning flashes each year in the United States, fewer than a tenth of a percent of them are associated with thundersnow, Petersen said. Still, that’s up to 1,000 flashes.