There must be a relatively thin surface layer of wet, loose snow, with a temperature near the melting point of ice.[1] Under this thin layer of wet snow there must be a substrate to which the thin surface layer of wet snow will not stick, such as ice or powder snow.[2] The wind must be strong enough to move the snow rollers,[2] but not strong enough to blow them apart. Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on a steep hill and begins to roll down the hill.[4]