A. The weather conditions that lead to thunderstorms can occur any time of year. When updrafts draw warm, moist air a mile or more above the ground, the moisture condenses and falls as precipitation. Whether that precipitation falls as rain or snow depends on the air temperature nearer the surface.

"It's more likely that you're going to get the conditions that lead to a thunderstorm — whether it's snow or rain — when the air is warmer and, therefore, more moisture-laden," said Jonathan Martin, chairman of the department of atmospheric and oceanic sciences at UW-Madison. "Those conditions can occur in the wintertime, but they're less frequent."

A winter thunderstorm is called "thundersnow" and may punctuate periods of lighter snow showers. "When we do get thundersnow, it's very intense snowfall," he said. "It snows very heavily for a short period of time, exactly analogous to the very intense rain you get under a thunderhead in the summertime."

The effects can be quite striking, Martin says. Snow muffles the sound of thunder but reflects lightning, making it appear very vivid.

— Produced in cooperation with University Communications

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