Snow rollers are a rare winter weather phenomenon that occur when the conditions are just right. They are cylindrical snow balls, usually hollow, formed with the help of strong winds (30mph) and temperatures in the 37-39 degree range. A layer of wet, loose snow, preferably with an ice layer underneath is pushed by the wind across fields or down slopes. Gravity certainly helps! Snow rollers can be small or quite large (car size). They are also known as Snow donuts.