Weather Break: Snow Rollers

The following is a transcript of an episode of the Weather Break radio show, written by Dr. Jon Schrage.

Recent heavy snowfalls in the Rocky Mountains have caused a number of recent avalanches, which have been in the news quite a bit lately. Avalanches can happen under a number of conditions, but they generally happen when a heavy, sticky layer of snow falls on a weaker, older, crusty layer. If the snow is heavy enough and the slope is steep enough, the older layer of the snow cannot bear the weight and the top layer of snow begins to slide down the hill. Other avalanches happen when an overhanging ledge of snow, known as a cornice, breaks free and tumble down a mountainside.

All types of avalanches, of course, involve steep terrain—the snow won’t move if the land is flat, right? Or will it? While avalanches certainly are potentially destructive and dangerous, under the right weather conditions snow WILL move across flat surfaces in the form of what meteorologists call “snow rollers”.

Snow rollers are like snowballs, but they are barrel-shaped. They are generally quite rare, but they can happen whenever the snow is light but sticky and the winds are quite strong. When the wind starts rolling a small quantity of the snow across the surface, the tiny snowball begins to pick up more snow. Picture the way most people make a snowman; they start with a fairly small snowball and roll it across the yard. The snowball grows and grows until it forms the base of the snowman. Now, when you are making a snowman, you want the big snowball to be ROUND, so if the snowball becomes lop-sided or barrel-shaped or whatever, you push the snowball a different direction for a while. In the case of snow rollers, however, the wind just keeps pushing them in the same direction, so instead of forming round snowballs the snow roller become shaped like a barrel or a log.

Snow rollers have been reported that were as much as 3 feet in diameter, although most are much smaller than that. Generally, they occur in groups; you don’t usually see just ONE snow roller, but rather hundreds or even thousands of them may form over an open, snow-covered surface like a field or a frozen lake.

Snow rollers are fascinating, but they aren’t dangerous or severe. Here at Weather Break, we weren’t able to find any documentation about how fast these snow rollers form and tumble across the surface, but it certainly isn’t fast enough to cause damage or injury to anyone. Maybe we’ll eventually find some video online of snow rollers forming.

One of the biggest snow roller events in recent years happened on February 11 and 12, 2003, when gusty winds produced large, impressive snow rollers over much of Illinois. We have links to lots of pictures of snow rollers from that event and other similar storms on our website. You find that and much more at weatherbreak.creighton.edu.