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A Study in Opposites: Thundersnow

© Keith C. Heidorn

As a wet snow falls, I watch the flakes shifting down from a deep-grey sky. Then, to my left, a throaty rumble following a muted flash catches my attention. Lightning and thunder? During the winter season? For most of us, thunder and snow in the same sentence, let alone the same word thundersnow, seems a contradiction, the symbols of summer and winter weather coexisting.

For most North Americans, the combination, known as thundersnow, is a very unusual event. Even where they are most common, thundersnow occurs no more than a few times per year, usually when winter storms with substantial warm and humid air sectors spawn thunderstorms along their fronts, or when air crosses a large lake or rises over a mountain range.

The central American states, mostly those west of the Mississippi River (Missouri, Kansas, Nebraska, Iowa, Colorado, Minnesota, Oklahoma and into northern Texas), define a loose arc in which thundersnows occur a few times each winter. For residents around the Great Lakes, the Great Salt Lake, or in the continent's mountainous terrain, however, thundersnows can be more common, though often many are unreported. Thundersnows have also been reported around the Sea of Japan and the North Sea. Mountaineers scaling Mount Everest, and other high peaks, have reported thundersnows during their ascents as some of their scariest moments.

Observations of thunder/lightning with snowstorms have been found in records and commentaries since the 19th century in the Western world and in China for nearly a millennium. However, only in the past decade have meteorologists delved into the phenomenon of thundersnow, likely due to the uncommonness of the events. In the United States, only 1.3% of cool-season (October-May) thunderstorms reported coincide with snowfall, and only 0.07% of snowstorms have associated thunder. (A five-year study of thundersnows in the central US by University of Missouri meteorologist, Patrick Market, began in 2003.)

Reports of thunder/lightning with snow are also hindered by several factors. Thunderstorms dropping snow to the surface generally produce fewer lightning strokes than rain-dropping thunderstorms. The snowflakes absorb and diffuse the light of lightning and sound of thunder more readily than does falling raindrops. (While thunder during rain is generally heard 6.5-8 km (4-5 miles) away, the range for snow-related thunder is no more than 1.6 km (1 mile). The colder weather in which thundersnow occur may also contribute to the relative rarity of the event as fewer observers are out of doors.

Causes of Thundersnow

What causes thunder and lightning during some snowstorms? Those who study the formation and life cycle of thunderstorms forming during the warm season understand that a strong, warm and moist updraft is a necessary condition. In the warm season, these updrafts generally arise from the surface, but they need not. It appears that only those thundersnows developing over bodies of warm water arise from warm surface air. Most others, particularly those observed in the US central states, develop when the necessary convective process starts at higher altitudes than the surface air layer.

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