A vehement nor’easter whipped across the American East Coast on Wednesday, inundating residents with wet, heavy snow and tempestuous winds. In some areas, the violently blustery scenario was punctuated suddenly with a thunderous boom and a crack of lightning, prompting people to ask: Was that … thundersnow?

The word may seem unfamiliar but is actually a meteorological term, albeit a clunky one (“snunder” might have been more elegant). According to a tweet from the National Weather Service posted Wednesday, around the time the first booms were felt throughout the region, thundersnow is a phenomenon caused by the vertical mixing of unstable regions of air.

“Shallow layers of unstable air lead to enhanced upward motion, increasing snow growth and causing enough electric charge separation for lightning!” read the infographic accompanying the tweet.

Fortunately, it’s a pretty rare event.

According to a report published by the Royal Meteorological Society in 2009, thundersnow occurs in only 0.07 percent of snowfalls. “In addition, thundersnow storms generally produce fewer lightning strokes than thunderstorms with rain during the warm season,” the authors wrote, as if doing so would assuage readers of the storm’s terror.

The “recipe” for thundersnow is a simple one, but its key ingredient is rarely available at the same time as the others. Thunderstorms in general require only three ingredients.

First, there must be enough moisture in the air to form clouds that can precipitate. That moisture can come from either local sources (like water rising off a nearby lake or ocean) or it can blow in from long distances.

Secondly, there needs to be a force to lift the moisture high enough to form the clouds. This often takes the form of air rising up the side of a mountain range or warm air moving in from a front.

The last ingredient for a thunderstorm is an “unstable temperature profile,” which is essentially a rapid upward shift from warm to cold that ensures the moisture gets sucked up quickly and vigorously. “Such an unstable temperature profile favours strong, upward motion to lift moist air, ensuring an adequate supply of moisture into the clouds and allowing the process of electrification to occur,” the authors write.

Together, those three ingredients will make a thunderstorm. But you need a fourth, far less common ingredient to make thundersnow.

Thundersnow cannot occur without cold air, which makes sense. If the air is too warm, the precipitation will just fall to the ground as rain.

Ultimately, the only difference between a thunderstorm and a thundersnowstorm is that one has frozen precipitation. In both instances, the way that thunder and lightning is is pretty much the same. Electrification is a result of atmospheric mixing — that’s the rapid rise of hot air, which propels cold air downward — which is thought to form electrical charges in tiny ice particles.

“The separation of these charges eventually may produce an electric field that becomes so large that dielectric breakdown happens, producing an electrical spark – the lightning stroke,” write the authors.

Here’s an image, snapped by NOAA satellites on March 6, of these ingredients coming together to form the nor’easter that produced thundersnow.

thundersnow nor'easter
 The low-pressure system that formed the nor'easter over the Atlantic on March 6.

Fortunately, since thundersnow is so rare, it hasn’t been associated with too many injuries, though Wednesday’s events did at least cause one New Jersey woman to require medical assistance after she was struck by lightning while holding an umbrella.

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