The mystery behind thundersnow, a rare winter phenomenon

When Patrick Market began researching thundersnow more than two decades ago, he received two very different kinds of responses.

“One type was, ‘Thank you for doing this, I knew that I’d seen this and nobody believed me,’” recalls Market, who is now the director of the University of Missouri’s School of Natural Resources. “And then the other type was, ‘There’s no such thing as thundersnow, this never happens in wintertime storms, and you’re wasting our money.”

But as he and other scientists have shown, thundersnow—which is when thunder and lightning occur during a snowstorm—is a very real winter weather phenomenon with serious implications.

(What are winter storm watches, advisories, and warnings?)

If you witness those phenomena at the same time, “somebody nearby you is getting at least a half a foot of snow,” Market says. In fact, his 2006 study showed that 86 percent of thundersnow events were associated with storms causing more than six inches of snowfall in a 24-hour period—more than enough to cause havoc on our roads and to our homes.

Though thunder and lightning in a snowstorm is a predictor of heavy snowfall, researchers have recently learned heavy snowfall is not a predictor of thunder and lightning—and are still uncovering why.

Until the last 20 years, researchers couldn’t even reliably identify thundersnow, let alone study its inner workings. With new technology monitoring weather from space, we’re learning more about thundersnow than ever. This research will have real-life impacts on public safety, from creating warning systems to even changing guidance for when to fly planes and launch rockets.

What causes thundersnow?

Scientists believe that thundersnow is caused by the same conditions as a summer thunderstorm: Turbulence in the atmosphere causes moist and relatively warm air near Earth’s surface to rise, where it condenses to form clouds filled with supercooled liquid water, tiny ice crystals, and a form of soft hail called graupel. The more that mixture crashes around inside the cloud, it can create an electrical charge and result in lightning and thunder.

But Sebastian Harkema, a Ph.D. candidate at the University of Alabama in Huntsville who studies thundersnow, says we don’t really know for sure how or why this process occurs in the wintertime.

During a winter storm, the “warm” air near the ground is still very cold, or below freezing. When that air rises into the even cooler atmosphere, it produces less of the supercooled liquid water that is believed to be important for producing summer thunderstorms.

“What I’m trying to understand is to what extent [that liquid] is important for this specific wintertime scenario,” he says. Preliminary results from Harkema’s research, which is funded by NASA’s Future Investigators in NASA Earth and Space and Technology program, suggests that the supercooled liquid water and graupel aren’t as important for generating lightning in the winter as they are in the summer. But it’s not yet clear why.

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How rare is thundersnow?

Thundersnow has been a challenge for researchers to truly understand in part because it’s a relatively rare phenomenon. In the decades before Market began researching thundersnow, we mostly only knew about it through occasional accounts from people who saw lightning or heard thunder during a snowstorm.

But those telltale signs tend to be masked during a snowstorm. Although you can hear thunder rumbling well into the distance during the summer, snow is very good at absorbing sound waves—meaning you’ll only hear thunder if you’re within a few miles of it. (And Market points out the muffled sound can also easily be mistaken for a snowplow). It’s also harder to distinguish a flash of lightning against the backdrop of a bright white flurry-filled sky.

(Here's what you need to know about lightning.)

Yet reports of thundersnow have increased in the decades since Market began his research. “It seems like every two weeks in the wintertime somebody posts a video to their Instagram,” he says. “Is it really happening more or are we observing it more?”

Both things might be true. Harkema’s work has shown that tall radio towers that conduct electricity may be responsible for generating more thundersnow in urban areas. Additionally, the ubiquity of everyday technology like doorbell cameras has also made it easier to capture images of a storm from the safety of your own home.

Most importantly, scientists have now found ways to detect thundersnow without hearing a sound. Not only have our sensors for detecting lightning improved over the years, but satellite imagery has also now made it possible to see where lightning is occurring from space.

Where does thundersnow occur?

These new technologies are helping scientists pinpoint exactly where thundersnow is happening at any given time. The satellite-borne geostationary lightning mapper can detect lightning flashes across an entire hemisphere. Wherever those flashes overlap with snowfall, that’s thundersnow, Harkema says.

He adds that thundersnow is particularly common in the Colorado Front Range, upper Great Plains, and the Great Lakes where wind turbines generate electricity. But thundersnow can happen anywhere—even in the relatively warm Huntsville, Alabama. “When we do get snow, there’s a pretty good chance there’s thundersnow.”

More than just identifying where thundersnow is taking place, the GLM has given researchers a glimpse into what these lightning flashes look like from some 22,000 miles above Earth. Even though there seem to be fewer lightning flashes in thundersnow events than in summer storms, Harkema says the evidence is mounting that thundersnow flashes are also larger than those of summer thunderstorms—and could pose more of a threat to people on the ground.

(How to stay safe in a lightning storm.)

Research like Harkema’s is crucial to getting to the point where we can better predict when thundersnow will happen—which will help develop better warning systems for when snowfall is likely to be dangerously heavy and for people to seek shelter from larger flashes of lightning. Implications of this research go even further: findings could be useful in storms year-round—potentially making it safer to fly planes and launch rockets in the years to come.