June 22, 2009: Snow Rollers

[xoxoxoBruce]

They look a little like snow covered hay rolls, but they are not.

On the evening of March 31st, 2009, Tim Tevebaugh was driving home from work east of Craigmont in the southern Idaho Panhandle. Across the rolling hay fields, Tim saw a very unusual phenomenon. The snow rollers that he took pictures of are extremely rare because of the unique combination of snow, wind, temperature and moisture needed to create them. They form with light but sticky snow and strong (but not too strong) winds. These snow rollers formed during the day as they weren’t present in the morning on Tim’s drive to work.

The following conditions are needed for snow rollers to form:

The ground must be covered by a layer of ice to which snow will not stick.
The layer of ice must be covered by wet, loose snow with a temperature near the melting point of ice.
The wind must be strong enough to move the snow rollers, but not strong enough to blow them too fast.
Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on steep hill and begins to roll down the hill.
Because of this last condition, snow rollers are more common in hilly areas. However, the precise nature of the conditions required makes them a very rare phenomenon.

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