Just how rare is thundersnow?

copyright the Chronicle February 9, 2011

by Tena Starr

It’s hard to say exactly how rare Saturday’s night’s winter thunderstorm was, says Chris Bouchard, a meteorologist at the Fairbanks Museum in St. Johnsbury.

“I don’t have any numbers on its frequency,” he said.  “But lightning as frequent as Saturday’s is something I’ve never experienced with snow falling.”

The official term for a thunderstorm with snow is “thundersnow.”

Thunder and lightning might occur once or twice a winter in the state, Mr. Bouchard said.  But generally it’s very localized.

“There might be one flash over one town. This last event was pretty unusual because there were hundreds of lightning strikes on Saturday night, in lots of towns.  I’ve seen snowstorms with a flash here and there, but nothing with frequent lightning like that.”

One reason thunderstorms don’t often occur in winter is because warm air is usually behind their development, and there just isn’t much warm air around in winter.

Thunderstorms are caused by rapidly rising air currents, which form very tall clouds, sometimes billowing up over 40,000 feet in height.

Inside the thunderstorm, charge separations occur.  “No one is exactly sure how that happens,” Mr. Bouchard said.  “A leading theory is that the different types of precipitation particles found within thunderstorms force a charge separation when they collide.”

Every thunderstorm produces both snow and graupel (also known as soft hail), even during the summer months.  Normally, warm air near the surface forces these to melt into rain before they reach the ground.  Updraft speeds vary from the inner core to the outer edges of the storm.  That means that in some parts of the cloud, snow rises at the same time that heavier graupel is falling past it.  That leads to a lot of mini-collisions.

“We know the snowflakes are traveling upward with a positive charge,” Mr. Bouchard said.  “Snowflakes go up because they’re light and fluffy.”

The lower portion takes on a negative charge as graupel falls through it.  Once the charge difference builds to a high enough level, it can overcome the resistance of the air, and you get a big static discharge, Mr. Bouchard said.  “That’s lightning.

“The best way to get updraft speeds sufficient to produce lightning is with warm air, and we don’t often have warm air around in the winter,” he said.

Saturday night there was warm air rising into thunderstorm updrafts however, as air originating over the Atlantic moved in.

“There was a lot of rising motion, and that led to the charge separation that caused the thunder and lightning.

“It’s pretty unusual to see snow with thunderstorms in Vermont.  It usually happens with Nor’easters.  But typically with Nor’easters lightning is very sporadic and unpredictable.”

contact Tena Starr at tenas@bartonchronicle.com

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