From: Andy Soos, ENN
Published February 8, 2011 04:10 PM

Thunder Snow!

Thundersnow, also known as a winter thunderstorm or a thunder snowstorm, is a relatively rare kind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone, where the precipitation consists of ice pellets rather than snow. Snowstorms that trigger lightning are rare. Of the roughly 10,000,000 cloud-to-ground lightning flashes observed over the continental United States each year, about 0.1 percent to 0.01 percent are associated with snow, says Walter Petersen, atmospheric physicist with NASA's Marshall Space Flight Center in Huntsville, Alabama. This thundersnow happened this winter in northern Alabama and was observed first hand by sophisticated lightning mapping station in Huntsville


There are usually four forms of thundersnow:

1. A normal thunderstorm on the leading edge of a cold front or warm front that can either form in a winter environment or one that runs into cool air and where the precipitation takes the form of snow.

2. A heavy snowstorm in the comma head of an extratropical cyclone that sustains strong vertical mixing which allows for favorable conditions for lightning and thunder to occur.

3. A lake effect or ocean effect thunderstorm which is produced by cold air passing over relatively warm water; this effect commonly produces snow squalls over the Great Lakes.

4. A cold front containing extremely cold air aloft, steepening lapse rates and causing strong vertical movement which allows for favorable conditions for lightning and thunder to occur

One unique aspect of thundersnow is that the snowfall acts as an acoustic suppressor of the thunder. The thunder from a typical thunderstorm can be heard many miles away, while the thunder from thundersnow can usually only be heard within a two to three mile radius from the lightning.

Petersen oversees a NASA-backed team that deploys to various locations around the world to collect data about precipitation, information that will be used to interpret observations from a new network of Earth-monitoring satellites. The Huntsville snowstorm was a chance to add to that information bank, without tapping the travel budget.

Early results from the impromptu campaign show that the lightning traveled for 30 to 50 miles along layers in low clouds before touching ground, a finding that has implications for safeguarding power grids, among other concerns.

Scientists also found that more than 50 percent of the lightning flashes initiated off of a radio tower just east of Huntsville. They are working to flesh out the contributing roles of snow, wind and supercold water on the development and spread of the lightning.

"We've the got the ability to look at how the snowflakes originate, grow and fall, and relate that to where the lighting flashes tend to propagate through the clouds that produce the snow. Not many events are caught like that," Petersen said.

Atmospheric physicist Kevin Knupp, with the University of Alabama in Huntsville, suspects that gravity waves, which are up and down ripples in the atmosphere somewhat like waves on a beach, are the invisible hands behind thundersnow, interacting with supercold water so that electric charges can build up, leading to lightning.

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