The snow balls are called snow rollers. For snow rollers to form, the ground must be covered by a layer of ice or packed snow. That layer of ice must then become covered by wet or loose snow with a temperature near the freezing mark. The wind must be strong enough to pick up some snow. The balls are formed as the snow is blown along the ground by wind! Snow rollers are a fairly rare meteorological phenomenon. Saulnierville, Nova Scotia.