It was an afternoon eight years ago that David Schultz will never forget. Snowflakes were falling down as big chunks and bolts of light flashed through the snow-whitened sky. Then there was the sound of strangely muted thunder.
"It was a muffled rumble," remembered Schultz, who was then a graduate student in Albany, N.Y., and is now a meteorologist at NOAA's Severe Storms Laboratory in Norman, Okla. "It was brilliant."
So-called thundersnows are rare events that feature thunder, lightning and heavy snowfall. Despite their drama — but perhaps because of their infrequency — very little is known about them.
Schultz is one of the few U.S. meteorologists to have investigated the storms and now another researcher, Patrick Market of the University of Missouri in Columbia, hopes to learn more. Market recently received a $460,000 grant from the National Science Foundation to find out how often the storms occur and why.
"The thunder and lightning are the attention grabbers," Market said. "But they are really symptoms of a larger issue — sometimes these storms can generate an awful lot of snow over a small area."
Case in point: In January 1994, a huge storm deluged Louisville, Ky., in about two feet of snow. People throughout the city recalled seeing flashes of light and growls of thunder. A similar storm struck Columbia, Mo, a year later. Thundersnow storms are more common in the Great Lakes region and in the mountains, but remain rare even there.
Schultz has estimated that only 0.07 percent of recorded snowstorms are associated with thunder. And, among thunderstorms, only 1.3 percent of these storms in cool seasons feature snow. Part of the problem may lie in documenting the storms. Shultz points out that heavy snow has a way of obscuring sound and light — the telltale signs of a thundersnow.
"Where you might hear a regular thunderstorm from four to five miles away, you may not hear or see a thundersnow from a mile away," he said.
Despite the spotty record on the storms, ancient texts prove people have been witnessing them for centuries. Descriptions of the events date to at least as early as the 19th century in Western literature, says Schultz. And Chinese texts dating to 1099 A.D. reveal that Chinese warriors believed the storms were precursors to an enemy attack.
Thundersnows may have seemed mystical in earlier centuries, but now scientists understand a mix of warm air and moisture are required to brew them.
As a storm cloud gathers up warmer air (such as air over a lake or ocean surface), the air rises in the cloud's structure and creates a churning of air masses within the cloud. This turbulence mixes snow crystals and supercooled water droplets in the cloud and the turbulence knocks off electrons from the water particles, causing positive and negative charges in the cloud to separate.
The negative charges collect at the base of the cloud and are attracted to positive charges either on the ground or in nearby clouds. As the negative charges rush toward the positive charges, the positive charges rise and meet them and this creates a bolt of lightning. The lightning heats the air to about 50,000 degrees Fahrenheit, causing the air to quickly expand. The air then cools and contracts, creating a "clap" of thunder.