Snow Rollers: Snow balls formed naturally by strong winds blowing across a flat, snow covered field. Three conditions have to occur for formation of snow rollers: The ground must be icy or be covered with a snow crust. Winds must be strong and gusty. A wet snowfall of at least a couple of inches. Once a snow roller is formed the wind pushes it to roll, collecting snow as it moves, in some cases even creating tracks of bare ground. (snow doughnuts).