If you have never seen or heard of a snow roller or snow donut, it's because these amazing natural formations occur very rarely, under only the most perfect conditions and, last for very brief periods. In summary, you have to be at the right place at the right time.
Snow rollers are essentially snowballs, which are formed when large amounts of snow gets blown along the ground by wind. However, unlike normal snowballs, these are cylindrical in shape and mostly hollow in the middle. That is because the inner layer, the first to form, is often very weak and thin, and prone to blowing away, leaving what resembles a swiss roll or sometimes even a donut.
For these rare amazing 'snowballs' to occur, all the following weather conditions must exist:
- The ground must be covered with ice, so that the snow is not stuck to it.
- The snow itself must be wet and at a temperature that is close to the melting point of ice.
- They must either be formed at the perfect incline, so that the gravitational pull will roll them down, or be exposed to wind, that is strong enough to move them, without destroying them.
While most of us will probably not encounter these live in our lifetime, Tim Tevebaugh an Idaho Firefighter, who took these amazing pictures, was lucky enough to see them, while driving back from work on March 31st, 2009. Tim estimates that while most of the snow rollers were 18 inches high, some were as big as 2 ft. Pretty Amazing!
Sources: nationalweatherservice-spokane, Dailymail.co.uk,wikipedia.org