"Thundersnow" Facts: Mysterious Storms Explained

Christine Dell'Amore
National Geographic News
March 3, 2009

The late-winter snowstorm that blanketed much of the eastern U.S. on Sunday and Monday packed some serious sound and fury—emphasis on sound.

Along with the snow clouds, a rare and little-known phenomenon known as thundersnow rumbled over parts of Georgia and South Carolina.

Thundersnow—when thunder and lighting occur during a snowstorm—most often appears in late winter or early spring, experts say.

That's because the ingredients for thundersnow—a mass of cold air on top of warm, plus moist air closer to the ground—often come together during that time.

What Causes Thundersnow

Thundersnow starts out like a summer thunderstorm, Market said. The sun heats the ground and pushes masses of warm, moist air upward, creating unstable air columns.

As it rises, the moisture condenses to form clouds, which are jostled by internal turbulence.

The "tricky part" for making thundersnow, Market said, is creating that atmospheric instability in the wintertime.

For thundersnow to occur, the air layer closer to the ground has to be warmer than the layers above, but still cold enough to create snow—a very precise circumstance.

In the recent southern U.S. thundersnow storms, for instance, the atmosphere became unstable enough that thunderstorms with rain developed. Those storms then moved north where the air was below freezing, said Howard Silverman, a National Weather Service senior forecaster in Sterling, Virginia.

The thundersnow events were also coupled with "pretty decent snowfall rates," at the rapid clip of more than two inches (five centimeters) an hour, Silverman said.

Heavier snowfall is usually linked to thundersnow, both experts agreed.

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