Looking across the open, snow-covered cornfield, they appear, on first glance, as if someone started to build snowmen or a snow fort then left the rolled balls of snow randomly across the field. And, although they each have a distinct "roll-up" track, close inspection show no footprints in the surrounding snow. Are these the winter version of crop circles, the work of winter spirits, or a curious product of nature?
Snowrollers in Colorado cornfield. Photo courtesy Patrick C. Kennedy, CSU-CHILL National Radar Facility
Such naturally formed snowballs are called snowrollers, cylindrical rolls of snow produced by the wind and shaped like a muff, rolled carpet, or jellyroll. Molded by strong, gusty surface winds, they are often hollow. Snowrollers range from golf-ball-size to as large as a 30-gallon drum, but typically are reported as 25-30 centimetres (10-12 inches) in diameter and a foot wide.
Small Snowrollers (arrows indicate two) in Colorado cornfield. Photo courtesy Patrick C. Kennedy, CSU-CHILL National Radar Facility
Once considered rare and unusual occurrences because their formation was often treated only as
a local curiosity, we now know that snowrollers form frequently across snow-covered regions of North America. In fact, a single snow-covered field may sport hundreds of individual snowrollers. (The smaller rollers are likely quite common but do not attract the attention of observers and thus giving the impression that snowrollers are rare.) Snowrollers are
distinguished from lumps of snow in a field built up on a rock, clod of soil, or other irregularity by the visible path leading to the snowroller over which it has been pushed.
Snowrollers appear in open fields under specific weather conditions, often forming in the wake of a strong winter storm when snow is new and winds strong. Snow rollers appear when several weather conditions are combined just right: snow cover deep and moist enough, air temperatures
withing a narrow range around the freezing mark, and winds strong enough to push the rollers.
Here is how they form. First, the ground surface must have an icy, crusty snow, on which new falling snow cannot stick. On top of this, a couple centimetres (about an inch) of loose, wet snow that sticky kind that makes good snowballs must have accumulated. The optimum air temperature appears to be around the freezing mark, from -2 to 2 oC (28 to 34 oF). Finally, a strong and gusty wind, usually blowing at 40 km/h (25 mph) or higher, is needed to build the snowroller.
Snowroller formation begins when the wind scoops chunks of snow out of the snowfield or pushes giant snowflakes downwind. These "seeds" roll, bounce and tumble, like snowy tumbleweeds, over the surface under the herding of wind. Additional snow then adheres to the seed, and the snowroller grows until it finally becomes too heavy for the wind to push.
Snowrollers leave behind a characteristic track linking the snowroller's origin to its final resting spot.
The snowroller's path can be straight ahead or erratic depending on how the gusty wind pushes the roller along. Sloped terrain may also assist in the formation and movement of the snowroller seed as the force of gravity assists the wind force pushing on the rolling snow body.
Once the snowroller becomes stationary, it enters its next stage of existence. Further snowfall may bury or surround the roller and hide its track, thus making it indistinguishable from snow
covering rocks, vegetation or bumps in the surface. If the snowroller remains exposed for any length of time, various processes will work on the snow transforming the snowroller into a rather unspectacular lump of snow. Eventually, the snow melts, thus ending the life of the snowroller.