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Snowrollers

© Keith C. Heidorn

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

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