Lightning during a snowstorm.

Ask Tom Why
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Dear Tom,
Why does lightning seem to occur here only during rainstorms? We were in Utah on a ski trip and there was lightning during a snowstorm. I have never seen that here in Chicago.
— Karen Lambert
Dear Karen,
While not common in Chicago, thunderstorms do occur when it snows. The event is called thunder snow, and it usually occurs during powerful winter storms. Snowfall can be intense when accompanied by lightning and thunder, falling at blinding intensities, with accumulations as high as 4 inches an hour. Several episodes of thunder and lightning occurred during Chicago’s 2011 Groundhog Day blizzard. On Dec. 15, 1987, the city was buried by 8 inches of snow in just four hours during a major thunder snow event. Thunder snow also can occur with lake-effect snow, a product of the extreme instability caused by arctic air passing over the warmer water of the Great Lakes.


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