This winter, snow is on a roll.
This week, people in Ottawa, Canada, were mystified when an unusual cold weather phenomenon began plowing through their yards. It looked like the snow was garnering a mind of its own, inexplicably balling itself into donut-shaped spirals and rolling across the icy ground. But this rare occurrence is a natural phenomenon rather than Frosty the Snowman coming to life; it has a name, and it's called "snow rollers."
Also known as "snow bales," "wind snowballs," or "snow donuts," snow rollers are the cold weather equivalent of tumbleweeds. They form when wind pushes snow across the ground, gathering it into a hollow cylinder. Although some formations appear more squashed than others, bigger snow rollers can be a few inches wide and travel a couple feet, leaving trails behind in their wakes.
Only very specific conditions can support snow rollers, since the phenomenon needs the right mixture of moisture, snow, wind, and temperature. There must be a light dusting of snow on top of an icy layer on the ground, often on a hill or other expanse with no protruding vegetation. The dusting needs to be just wet enough so that it can adhere to itself but not stick to the ground. The wind must be around 30 miles per hour to coax the snow into its cylindrical shape, and the temperature must be three to five degrees above freezing. This level of cold allows for the rolling snow to form these fun shapes, but it doesn't let the snow melt completely.
In the rare cases where snow rollers happen, it's often not in busy cities like Ottawa. More frequently, this phenomenon is triggered in the countryside. In 2014 in the U.S., people reported spotting snow rollers in the Midwest. In January, snow rollers were spotted in Scotland.