Wed Mar 24, 2004
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The tall whirling column danced across the forest clearing like a writhing snake,
suspending snow in its rapid rotation. Then as quickly as it appeared, it was gone. Hi,
I'm Bryan Yeaton, and this is The Weather Notebook.
The short-lived weather phenomenon I just described goes by the names of snow
devil, snow spout, and snow whirly. While cousins of the dust and sand devils, snow
devils arise mostly from the interaction of a strong wind and a feature of the topography
or landscape instead of intense surface heating.
Snow devils usually live from half a minute to perhaps several minutes. They form
when wind flowing past an obstacle develops a transitory vertical eddy that spins off
downwind. If the resulting vortex is strong enough, it can lift loose snow off the surface
and suspend it in the flow as the snow devil skitters across the landscape. The larger
the obstacle and the stronger the wind speed, the greater the chance a large snow
devil will form.
Observations of these whirlwinds suggest they range in size from 15 to 30 feet in
diameter with heights to about 35 to 45 feet and have the power to scour the track
beneath them. Under ideal conditions, they may even become monsters. A report from
the Australian Antarctic Expedition in the early years of the last century suggested one
had sufficient strength to move 1500-pound objects.
Smaller snow devils may form in the wake of buildings where conditions are conducive
to the formation of wake eddies. Often under these conditions, a series of weak snow
devils whirls off in succession, sending snow high in the air.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is supported by Subaru of America and the National Science Foundation, with special
funding from Davis Instruments of Hayward, California.