Recently, two Idaho residents named Dan and Dennis Robbins stepped outside after a snowstorm. They witnessed a surreal site: dozens of icy cylinders made entirely from snow, strewn across a hill with no sign of human interference.
— NWS Boise (@NWSBoise) February 3, 2016
According to CityLab, these unusual hollow formations are called snow rollers. (They're also sometimes referred to as snow doughnuts.) They're extremely rare, and only form in very precise weather conditions. A strong wind picks up a chunk of snow on a hill, bluff, or other inclined surface and rolls it along the ground. As the snow piece moves, it picks up even more snow, growing larger and larger until it's eventually too big for the wind to push it any further. The snow on the ground has to be the right mixture of wet and loose, icy and packed, for the accumulated mass to form a perfect wheel. In other words, the perfect (snow)storm of events needs to exist for snow rollers to blow into your own backyard.
Throughout the past few years, snow rollers have been spotted everywhere from Washington State to Parkersburg, West Virginia and Norwood, New York. Last year, they popped up across Ohio, Illinois, and Pennsylvania. If you're lucky enough to see a snow roller in your own town this winter, take a picture. After all, they don't come along every day—and they won't last long.