While the United States deals with its usual conga line of snow and ice storms that are a frequent visitor in the middle of winter, our closest ally across the pond found itself in the midst of an unusual situation: not only did parts of typically temperate Great Britain see a decent amount of snow this week, but the snow was accompanied by thunder. Parts of the United Kingdom experienced thundersnow as a potent winter storm brought heavy precipitation, blustery winds, and the threat of significant coastal flooding to Scotland and England.
A strong ripple in the jet stream allowed a low-pressure system to develop near Scotland and work its way down the eastern English coast late this week, bringing more than half a foot of snow to higher elevations of Scotland and coating areas as far south as London with a light dusting of snow. That doesn’t sound like much, but just like bigger cities in the United States, even a small amount of snow can cause mass chaos if it hits at the wrong time. The storm led to mass flight cancellations, snarled highway traffic, and even caused thousands of evacuations along the coast, where authorities worried that a storm surge—sea water pushed inland by strong winds—would inundate coastal communities.
But the adverse impacts of the snowstorm were overshadowed in news coverage by that unusual buzzword: thundersnow. Thundersnow occurs during a thunderstorm that produces snow instead of rain. The mechanisms that produce rainy thunderstorms and snowy thunderstorms are largely the same, even if the air temperature is below freezing.
A band of snow can become strong enough to produce lightning through two processes known as convection and forcing. Convection occurs when an area of warm air quickly rises through cooler air above it. Convective snow is most common during lake effect snow events like those you’d find on Lake Ontario or Lake Erie, since the process requires extreme vertical temperature gradients that can result from bitterly cold air flowing over a warm body of water.
The other process, known as forcing, is what caused the thundersnow in the United Kingdom this week. A strengthening low-pressure system involves fast, dynamic changes in the atmosphere, especially when one of these storm systems quickly gains strength. Such a fast-developing storm can cause large amounts of lift in the atmosphere, a process that forces air to swiftly rise like you’d see during convection. This creates intense bands of snow that can grow so strong that they produce thunder and lightning. This process is responsible for the thundersnow that occurs during blizzards and nor’easters, those powerful storms that regularly hit the U.S. East Coast during the winter. Thundersnow can be pretty exciting—just ask The Weather Channel’s Jim Cantore:
The name “thundersnow” can be a bit misleading. One of the most enjoyable things about a snowfall is how silent it is outside when there’s a thick blanket of snow on the ground. Snow absorbs sound waves so efficiently that you can usually only hear ambient noises immediately around where you’re standing. Snow muffles the sound of thunder for the same reason. Thunder that might be audible for many miles during a rainy thunderstorm might only be audible for a few thousand feet away from where the lightning struck. Unless the lightning strikes very close to where you are, you might only see a bright flash during thundersnow without ever hearing the thunder.
While thundersnow is a fascinating phenomenon to encounter, it does involve lightning, after all, and it’s just as dangerous as any other lightning bolt you’d see in a rainy thunderstorm. If you’re ever lucky enough to experience thundersnow, the event is best enjoyed indoors and out of harm’s way.
January 13, 2017 – 5:30pm