A snow roller is a rare meteorological phenomenon in which large snowballs are formed naturally as chunks of snow are blown along the ground by wind, picking up material along the way, in much the same way that the large snowballs used in snowmen are made. Often, snow rollers can be seen in open fields. As the snow and ice roll, snow is accumulated on the outside layer.
Optimal conditions for snow roller formation include having a snow with an icy covering on top of it. You can think of this layer as the stiff base on which the snowroller will start forming without sinking or running into many obstructions. In addition, the crusty and icy layer of snow allows the new accumulation not to stick. In a sense, it is a slippery location for snow chunks to roll like a bowling ball on an alley.
Next, snow rollers need a minor amount of snow accumulated on top of the icy layer. Strong winds are needed to pull out chunks of the snow and ice and move them along the snow. These winds also need to move in one basic direction. As the snow moves, layers of snow are built up on the outside. Since the top layer of snow is light, the snow in the middle of the roll can get blown out making the shape more tubular and hollow in the middle.
A perfect ‘snow roller environment’ could happen when, after a 2-3 inch accumulation of snow, temperatures rise throughout the day. In addition, melting of the top layer of snow by the sun could cause this icy layer. The temperatures then lower again to re-freeze the top layer of snow into an icy or crusty layer. If it snows later on that day, or overnight, there is a good chance a snow roller could form.