What is thundersnow?

Thundersnow: It sounds like the name of a rock band, a Greek god or a movie villain, but it’s really the name of a phenomenon that causes thunder – and sometimes, even lightning – to boom and crackle during a snowstorm.

Thunder plus lightning plus snow do not equal a typical snowstorm. In fact, thundersnow is such a rarity that, according to a 1971 National Weather Service (NWS) study, fewer than 1 percent of observed snowstorms unleash thundersnow. Despite these atmospheric anomalies’ scarcity, however, recorded observations of their spectacle date all the way back to 250 BC, according to Chinese records, which were translated in 1980 by atmospheric scientist Pao-Kuan Wang of the University of Wisconsin, Madison.

Says NWS meteorologist Neil Stuart, observing thundersnow descend is “like watching a time-lapse movie of the snow building up, because it falls so quickly.” These types of storms are most common in late winter or early spring, when certain atmospheric puzzle pieces lock into place.

One of those pieces is convection, otherwise known as the upward movement of air. The sun heats the ground and thrusts masses of warm, moist air aloft, which condenses into clouds. If the warm air rises into cooler regions, the resulting turbulence causes some water molecules to lose electrons, while others gain them. This processes materializes into electrification, which is discharged as lightning, along with an abrupt heating and expansion of air.

Scientists regard thundersnow as unique because the interactions among supercooled liquid water, ice crystals and heftier ice particles can spawn lightning. Thunder then results from sound waves generated by the high-speed cooling and contraction of lightning-superheated air. This type of atmospheric instability is much more uncommon in the wintertime, which is why thundersnow happens much less frequently than summer thunderstorms.

In simple terms, thundersnow is “the same thing as a thunderstorm, except instead of rain, you’ve got snow falling,” stated NWS meteorologist Jamie Enderlen.

Chicago recently experienced thunder and lightning during a snowstorm that piled snow from 4 to 8 inches upon the city in mere hours. According to WGN meteorologist, Tom Skilling, Lake Michigan didn’t factor into the storm, which careened in from the southeast. “This is a large-scale storm being produced by a pocket of very strong winds in the jet stream,” he explained. “If you were to look down on the clouds, you’d see there’d be little pockets of taller clouds where these pockets of updrafts are occurring in the storm. It’s within those pockets that turbulence leads to lightning discharges.”

Skilling attested that thundersnow makes an appearance once or twice every few winters. “It’s a phenomenon that’s not as rare as you’d think.

Thundersnow events aren’t kept track of by weather experts, but meteorologists are in agreement that the last time Chicago was inundated by such a storm, it occurred during the blizzard of 2011. It was said to be the third largest snowstorm in the city’s history. Seventy mile-per-hour winds whipped the snow around – which was falling at a rate of 2 to 3 inches per hour, with some areas accumulating 20 to 23 inches of the white stuff – while lightning flashes blazed above buildings and streets.

However, just because lightning accompanies snowfall, it isn’t any less dangerous than the lightning of an ordinary rain storm. “It’s a vigorous form of lightning,” Skilling warned, “and you don’t want to be tangling with it if you don’t have to.”

In the US, thundersnow is more commonplace in mountainous areas such as the Rockies, which possess warm air pockets generated by sudden elevation shifts, as well as in areas of warm, large bodies of water such as the Great Lakes. Creation of snow only requires a cold environment, enough moisture to produce clouds, and rising air. Creation of thundersnow requires one additional element – thermal instability, a product of warm air.

According to Patrick Market, an associate professor of atmospheric science at the University of Missouri, temperatures must get cooler by at least 7 degrees Fahrenheit per mile of altitude to whip up the turbulence necessary for thundersnow.

Scott Steiger, an assistant meteorology professor at the State University of New York at Oswego recently determined that about six thundersnow storms happen per year in the lower Great Lakes area (Lakes Erie and Lake Ontario), generally in November and December. “These storms don’t move, so they can dump up to 7 feet of snow in one day,” Steiger says. “They are very intense snowstorms, but they are very local.”

But what about thundersnow that occurs in regions distant from mountains and lakes? In this scenario, it doesn’t derive heat found near the ground, but rather at altitudes of 10,000 feet or higher. In these more rare occurrences, a thundersnow storm has the potential to cause the most extensive damage. Stuart recalls a March, 2009 thundersnow that blanketed sections of Georgia, South Carolina and North Carolina with a snowfall of 2 to 3 inches per hour. He states that for approximately an hour around noon that day, Hartsfield-Jackson Atlanta International Airport reported visibility of only approximately 1,320 feet.

Researchers are currently interested in finding the means to predict thundersnow, thereby thwarting the damage that it can inflict. According to Market, a 30-year study of snowfall ascertained that there is an 86 percent chance that at least 6 inches of snow will fall within 70 miles of the lightning flash. These scientists are trying to discern the mixture of atmospheric conditions necessary to breed thundersnow, to aid them in more exactly predicting heavy snowfall – which they define as at least 8 inches falling at a rate of 3 to 4 inches per hour – so that warnings about hazardous weather can be issued before it hits, allowing people to take cover and get off the road. (The National Highway Traffic Safety Administration estimated that snow-related accidents were responsible for over 800 deaths in 2007.)

Where there’s smoke, there’s fire, and with more research, scientists will able to more precisely predict where there’s thunder – there’s thundersnow.