Once in a great while nature provides us with a spectacular display of snowballs littering the surface of flat, snowy fields or woodlands. These rounded structures, called snow rollers or snow donuts (there is often a hole in the center of them), require a precise balance of air temperature, ice, snow, moisture and wind in order to form.
To begin with, the ground surface (typically quite level) must have an icy, crusty snow, on which new falling snow cannot stick. On top of this, there needs to be about an inch of loose, wet, sticky snow. The air temperature needs to be around 32 degrees F. Last, but not least, there must be a strong, gusty wind blowing 25 miles per hour or more. Snow rollers begin to form when the wind scoops chunks out of the top inch or so of snow and these chunks roll, bounce and tumble just like tumbleweeds, downwind. They gather additional snow as they roll and become larger and larger until they are too large for the wind to push. Snow rollers can be as small as a tennis ball or as much as two feet in diameter, depending on how strong the wind is and how smooth the surface of the snow is. There can be hundreds of them in a field or patch of woods. When a snow roller starts to form, there is no hole in the center of it. As it picks up speed and snow, the thin center often crumbles, forming snow “donuts.” (Thanks to Judy Howland for photo op.)