Your typical thunderstorm strikes in summer, when the atmosphere is full of warm, moist air. So when lightning strikes in the middle of a winter blizzard, there is something strange going on. Thundersnow involves an entirely different type of lightning, and our skyscrapers are a key part of it.
On Groundhog Day 2011, Chicago was caught in the middle of yet another blizzard, but this one was different. Lightning seemed to shoot out of the city's iconic skyscrapers. The publication Eos recently highlighted a paper that explains what was up with this "thundersnow."
Lightning, as you know, usually strikes from the sky, travelling from cloud-to-ground. Occasionally, though, lightning will travel upward: Tall structures like buildings or wind turbines can shoot out leaders, or channels of ionised air that connect with clouds and "lead" lightning down to the ground. In most cases, the structures will only initiate leaders after lightning has already struck nearby.
In the 2011 Chicago storm, however, tall buildings were shooting out leaders on their own, without any triggering cloud-to-ground lightning. What happened? It turns out there was something else unusual going on, too. When a tall structure starts to build up a charge, as it might when charged clouds surround it, a corona of ionised air forms around the structure to buffer its electric field. But in this blizzard, high winds blew away the corona, allowing the skyscrapers to accumulate charges more easily. Hence, thundersnow.
Top image: Chicago skyline. marchello74/shutterstock