They are known as Snow Rollers, Snow Bales or Snow Doughnuts and they are just what these names suggest: Self-rolling large snowballs formed naturally as chunks of snow are blown along the ground by wind, picking up material along the way, in much the same way that the large snowballs used in snowmen are made. Unlike snowballs made by people, snow rollers are typically cylindrical in shape, because they roll just in one direction (while men create snowmen rolling snowballs in many different directions) and are often hollow since the inner layers, which are the first layers to form, are weak and thin compared to the outer layers and can easily be blown away (giving them the donut look):
This is one of the rarest meteorological phenomena because a lot of unusual conditions should occur simultaneously. Therefore people that spotted snow-bales could consider themselves lucky. The ones you can see in the first five pictures were seen and photographed by firefighter Tim Tevebaugh while on his way home from work in the evening. He spotted a field full of them near Craigmont, Idaho. They were two feet high that is more or less the maximum size thay normally reach.
The conditions for the Snow-Rollers to form are these:
- The ground must be covered by a layer of ice or crusty snow 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.
The following images are pictures of other sightings.Cincinnati (you can see perfectly the central hole):
North Cascades (formed by gravity and not by wind):