Here are the 3 ways thundersnow happens in the United States

The curious phenomenon, when thunder and lighting are heard and seen during a snowstorm, has been documented since at least the year 1099.

Boston, MA- 3/7/2018 - Rudy Hersh (cq) takes his dogs for their nightly walk as snow swirls around the Bunker Hill Monument during a snowstorm (Bill Greene/Globe staff
Rudy Hersh takes his dogs for their nightly walk Wednesday as snow swirls around the Bunker Hill Monument in Charlestown. –Bill Greene / The Boston Globe

In the middle of a nor’easter that disrupted travel in the Northeast and inundated areas already drenched from last week’s storm, some New Yorkers and others on the East Coast made a resounding discovery: “thundersnow.”

The curious phenomenon, when thunder and lighting are heard and seen during a snowstorm, has been documented since at least the year 1099. But its rarity — a 2009 analysis found that only 0.07 percent of snowfalls are accompanied by thunder — means that it never ceases to excite.

The captivating nature of thundersnow was documented in an article in The New York Times on Sept. 24, 1884:

“I don’t believe there has been a thunderstorm that I have witnessed for the past 30 years,” said a gentleman of this city, “which has not given me occasion to ask scores of people if they had ever seen a thunder and lightning snowstorm, and I never met but one yet, besides myself who ever had.”

There are three ways thundersnow can develop in the United States, according to John Nielsen-Gammon, the Texas state climatologist and a professor of atmospheric science at Texas A&M University.

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The first, which is most common to the central United States, mirrors the way regular thunderstorms are formed: Warmer, moist air close to the ground rises and mixes with colder, denser air higher up. As the hot air moves up and the cold air moves down, the atmospheric instability can trigger lightning and the thunder that comes with it.

In the case of thundersnow, air closer to the ground is just warm enough to rise and create atmospheric mixing, but it’s still cold enough that falling snow can stay frozen. This often happens in the Rockies, when the mountains trap pockets of warm air. “The air is cold enough down low that as the frozen precipitation falls out of it, it never encounters a warm enough layer of air to melt,” said Nielsen-Gammon. So instead of rain and lightning, there’s snow and lightning.

The second scenario tends to happen around the Great Lakes in early winter. A thunderstorm develops even though temperatures are very cold, thanks to the relatively warm moist air that comes from the lakes. It’s intertwined with the same conditions that bring the region its lake effect snow.

In the third scenario the low pressure of a powerful winter storm creates strong upward motions in the air, generating the atmospheric instability that in a typical thunderstorm would be driven by temperature changes. This is likely what led to New York City’s thundersnow.

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Even when the conditions are right, you won’t always notice the thunder. Snow is more likely to absorb the light of lightning and the sound of thunder than rain.

But thundersnow tends to be accompanied by heavier snowfall, which you may indeed notice when your commute is snarled.

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