The so-called “bomb cyclone” in early January was a freak of nature that brought a lot of things — coastal flooding, damaging winds, and double digit snowfall. But there was one thing most people overlooked — the thundersnow.

Thundersnow is a dramatic weather phenomenon which, as its name implies, is simply snow accompanied by thunder (and lightning). It only occurs when specific weather ingredients come together — usually in big and intense storms. The bomb cyclone strengthened as fast as about any East Coast winter storm on record and was a prolific thundersnow producer.

But here’s the unexpected part: its thundersnow may well have been mostly artificial — not a direct product of the storm but due to manmade structures in the storm’s path. Academic research has proposed the idea that some thundersnow may be human-induced and here I present compelling evidence that it indeed was during the bomb cyclone.

Some background on thundersnow

Thundersnow is rare, and when it occurs, it typically only produces flashes within clouds. That’s why the National Weather Service relies on public reports to track it. Moreover, snow acts as an acoustic suppressor. It muffles the sound of thunder, so only those in the immediate vicinity of the flash will hear the thunder.

I’ve been informally keeping tabs on thundersnow events in the Northeast for about 10 years. This storm will go down in my books as the second most widely-heard thundersnow producer of the decade, producing over six dozen cloud-to-ground strikes. That number is second only to a storm on Feb. 9, 2017 in which over 150 bolts struck the ground, one of which sparked a fire at a Providence home. Another split a tree in Warwick, R.I., and blasted it through a house.

Because thundersnow is so uncommon and, frankly, bizarre, few efforts have been made to forecast it. However, the early January episode gave some valuable insight into the dynamics of thundersnow and what caused it.

What makes thundersnow?

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 in a frictional process called “triboelectrification.” This takes place when the air is vertically unstable.

But thundersnow storms are far from ordinary. They form in the most unusual of places–the “comma head” of cold air that wraps around on the backside of intense coastal storms. In this region, a different type of instability gives rise to thunder. It’s called “conditional symmetric instability.” It’s a balancing mechanism between large differences in temperature and pressure over short distances that nudges air aloft in slantwise paths.

During the bomb cyclone, this kind of instability aided the development of thunder and lightning in several bands of heavy snow.

But why did thundersnow occur precisely at the locations it did? I was able to trace its occurrence to the presence of high manmade towers, soaring over 1,000 feet into the sky.

Lessons from the storm

Shortly after daybreak on Thursday, Jan. 4, the first bolts of lightning came crashing down in an otherwise quiet winter scene in Montville, Conn. A flurry of more than 30 bolts hit the ground on the northwest side of Lake Konomoc in a relatively small area. Radar returns didn’t show anything particularly impressive about conditions over that location as compared to neighboring locales. So why did it get struck?

A visit to the Federal Communications Commission (FCC) website allowed me to search records of television-radio transmission towers in the area. Some digging revealed two towers owned by SpectraSite Communications, LLC., that matched the locations of the lightning strikes in Montville and neighboring Oakdale. The towers were reported to soar some 316.1 and 367.3 meters above the neighboring landscape.

The hunt for answers didn’t stop there. The towers are located in a somewhat rural part of Connecticut, so the strikes weren’t widely reported. Fortunately, limousine company Liberty Limited has an office that abuts the property on which the towers stand. Angela Ried, who works in the office of the Oakdale business, says she heard the bolts loud and clear.

“It got struck at least four or five times,” she said. “It was pretty loud.” She recognized it immediately as lightning, but was surprised to hear it in the wintertime. “I’ve worked here since ’93, and this is the first time I’ve ever seen thunder and lightning during a snowstorm.”

A similar story unfolded in Needham, Mass., near the WCVB-TV Channel 5 towers. These structures, licensed to American Towers, LLC., poke up about 1,300 feet above ground. They too sparked off about a dozen lightning strikes.

In nearby Boston, only one building got struck — the Prudential Tower, a 52-floor skyscraper with a rooftop spire topping 900 feet. The mast broadcasts signals for multiple radio stations as well. “I heard it just once,” said Owen Anastas, of Boston. He heard this particular strike. “It happened around 11:30 a.m. during an incredible snow band.”

Noticing a pattern? It’s no coincidence! Of the roughly six-dozen cloud-to-ground discharges I examined, about 90 percent terminated on human-built towers. Indeed, the thundersnow during the bomb cyclone seemed to be  largely human-induced.

These results, while anecdotal, are supported by a 2001 experiment over Chicago that investigated lightning strikes in thundersnow. The resulting study found greater than 93 percent of the lightning sampled in a snow band was probably associated with “a variety of tall, and some not so tall, structures.”

So why do these structures help generate thundersnow? Here’s an analogy: Consider a person walking on a rug that touches a doorknob. The rubbing of unlike materials such as shoes and a carpet separates charge just like ice and water droplets when they rub up against each other inside of a cloud. The structures in the sky are like a doorknob. When they get close enough, a discharge results.

So why is this all important? If a tower is struck, it increases the risk of nearby places on the ground being hit by a return stroke. Moreover, Mother Nature’s attempt to hit a tower can sometimes be ill-fated, as an errant side-strike might miss the tower altogether and hit something else.

Further study into thundersnow and structures could help us better predict it and provide better warnings.