Snow donuts

Stanford said snow rollers form when there is a hard layer on the snow, covered by several more inches of dense snow. “Then you add a steep slope and a trigger such as a clump of snow falling out of a tree or off of a rock face.”

As gravity pulls a clump down, it usually rolls down the hill and collapses, creating what the WSDOT calls a pinwheel. Or it will not roll at all, and come down in an avalanche of snow. But if the snow is the perfect density and temperature, it rolls down leaving a hole in the center, Stanford said.

Strong, gusty winds also can be a factor, according to NOAA’s National Weather Service office in central Illinois, where snow rollers have occurred.

From http://seattletimes.nwsource.com/html/localnews/2003622603_donut17m.html?1

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