Have you ever driven through a snowy field and noticed cylindrical rolls of fresh, white snow looking a bit like a giant frozen doughnut? What is going on there? Is some industrious farmer diligently rolling large strips of snow as though it were sod or a bale of hay? Or could there be another explanation?
What you’re looking at is called a “snow roller,” and it’s less like a carefully rolled hay bale and more like a frosty tumbleweed.
How Do Snow Rollers Form?
A snow roller is very close to what its name suggests. It’s a strip of snow that is rolled over itself several times, forming a layered cylindrical shape. Picture a hay bale, a paper towel roll, a roll of sod or a flaky doughnut. Now picture it made out of snow. That’s a snow roller.
If you haven’t seen a snow roller before, you’re not alone. They require several essential factors to form, including a snowy climate, the right terrain and the right amount of wind. Many American snow roller sightings occur in the Midwest or in the inland Northwest. Yet from time to time, even the East Coast can get in on the fun.
Snow rollers are rare and that’s because they only form under a specific set of conditions with respect to terrain, snow quality and wind conditions:
- Terrain. The ground should be flat with no major inclines or obstacles. This aligns with the terrain of many midwestern states, as well as non-mountainous regions in northwestern states like Idaho and Washington. A very slight incline is OK, as this can help the snow roll, but if the ground gets too steep, the snow roller will move too fast and break apart.
- Snow quality. Snow rollers form from wet sticky snow. However this snow must also be loose, such that it can be picked up by the wind. In many cases, wet snow packs together, and this will obstruct any snow roller formation.
- Additional materials. Most snow rollers contain small bits of ice, rock and dirt. This helps them hold their shape as they form. The snow must be loose enough to pick up this debris as it rolls along.
- Wind. Most snow rollers are formed by a firm, steady wind coming from a consistent direction. Light breezes won’t be enough to stir up the snow, but gale-force winds will cause chaos and break up any rollers.
How Big Are Snow Rollers?
According to the National Weather Service, the average diameter of a snow roller is 10 to 12 inches (25 to 30 centimeters). Some are quite small — roughly the size of a golf ball. Extra-large ones can have a diameter of around 18 inches (46 centimeters).
You almost never see individual snow rollers. Either you’ll see none at all, or you’ll see a whole field dotted with them. That’s because the conditions that create a snow roller do not affect one narrow strip of land. They affect large areas all at once. If one part of a field has loose, wet snow and a steady 20 miles per hour (32 kilometers per hour) wind, it’s likely that other parts of that field will experience the same conditions.
Snow rollers can form any time there is loose, wet snow on the ground. In order for snow to be wet, it has to pass through a pocket of non-freezing air on its way to the ground. (Dry snow stays frozen solid throughout its entire journey.) At the same time, a stiff wind must blow while the snow is on the ground. In the western U.S., this combination tends to occur in February and March. In the eastern half of the U.S., those conditions can occur almost any time in the winter months.
Still, your chances of seeing a snow roller are rare. All elements must align for these whimsical snowy formations to emerge, so consider yourself lucky if you get to see some.