Snow donuts

It seems Washington was (once again) producing snow donuts today!

Or at least so said NPR this morning. And I said in reply, "What the heck is a snow donut?" Well, ask and the internet shall provide:

Snow doughnuts are a natural occurrence in nature. They form when there is a hard layer in the snow [which] is then covered by several inches of dense snow. Then you add a steep slope and a trigger, such as a clump of snow falling out of a tree... As gravity pulls the clump down, the snow rolls down the hill, and 99.9% of the time the center of the rolling snowball collapses in on itself and creates what we call a "pinwheel"... But, if the snow is the perfect density and temperature, it rolls around onto itself leaving the hole in the center, creating the doughnut-looking shape.
-WSDOT blog