Ordinary thunderstorms are relatively simple to understand. Like a bubble in a pot of boiling water, pockets of air climb upward. When they become tall enough, the top of the cloud freezes. It’s that vertical momentum into the freezing layer that causes ice crystals to become charged.

That scenario is tough to get in the wintertime; instead, thundersnow typically develops in one of three ways — all of which are rare.

In the Northeast, and in particular New England, thundersnow can crop up in strong low-pressure systems that brew winter storms known as nor’easters. Due to the spinning nature of these sprawling storms, which rotate counterclockwise, pockets of air are forced diagonally upward into the atmosphere. Picture jiggling a bowl of soup and watching the fluid slosh up the horizontal surfaces of the atmosphere. The air climbs and ascends high enough to get charge separation. At that point, BOOM — a surprise, rogue flash of lightning.

Interestingly, cloud-to-ground thundersnow events in nor’easters are oftentimes man-made. That’s because the positive charge in lower regions of the snow cloud is broad and diffuse, often too weak to spark a lightning strike. But tall man-made objects like transmission towers and skyscrapers can focus that positive charge, concentrating it until it’s strong enough to ignite a discharge.

Other times, thundersnow forms like conventional summer thunderstorms, but obviously in colder environments. This is often the case along strong arctic cold fronts. Instead of squall lines of thundery downpours, snow squalls form, and sporadic cloud to ground lightning accompanies the frigid frontal passage.

It can also form in lake-effect snowstorms, primarily only off Lakes Erie and Ontario. The pair are among the shallowest of the Great Lakes, allowing them to heat up more quickly during the summertime. Into October and November, water temperatures remain mild — currently around 50 degrees — even as air in the teens and twenties blows along it.

In the case of Lake Erie, frigid air traversing the 240-mile fetch of the lake encountered pockets of air warmed by the lake itself. Those air parcels ascend into the cold, dense atmosphere, towering to 15,000 or 20,000 feet high. The vertical convection, or vertical heat transfer, allows for charge separation, and thundersnow can result. It’s not uncommon for snowfall rates in Upstate New York bands to approach 4 inches per hour.

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