I heard part of this story on NPR the other day, and just looked it up to share:
Maintenance crews in Washington state spotted a snow doughnut last week on the North Cascades Highway. At first glance, a snow doughnut may look like a man-made creation. But Mike Stanford, who snapped this photo, assured NPR's Robert Siegel that snow doughnuts are real — but rare — natural occurences.
The snow doughnut in the photo is about 2 feet in diameter. "It's formed by a clump of snow falling off of a cliff or a tree into the snow pack. And if the conditions and temperature are just right, as gravity takes over, it pulls the snow down, and it rolls back on itself," Stanford says. "Usually the center collapses and it creates what we call a pinwheel."
Ever seen one? It looks like a big, frosty Cheerio!