Snow roller
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Template:Weather A snow roller is a rare meteorological phenomenon in which cylindrical snowballs are formed naturally as chunks of snow roll down hill or are blown along the ground by wind, picking up further snow along the way, in much the same way that the large snowballs used in snowmen are made. They can be as small as a tennis ball, but they can also be bigger than a car.<ref name="Wireobot" /> Most snow rollers are a few inches or centimeters wide.<ref name="NatGeo" />
Alternative names for snow rollers include: snow bales,<ref name="BBC 2019"/> snow donuts, snownuts and wind snowballs.<ref name="NatGeo" /> Unlike snowballs made by people, snow rollers are typically cylindrical in shape, and are often hollow since the inner layers,<ref name="NatGeo">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> which are the first layers to form, are weak and thin compared to the outer layers. The inner sections can easily be blown away, leaving what looks like a doughnut or Swiss roll.
Several conditions are needed for snow rollers to form:
- There must be a relatively thin surface layer of wet, loose snow, with a temperature near the melting point of ice.<ref name="NatGeo" />
- 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.<ref name="NatGeo" />
- The wind must be strong enough to move the snow rollers,<ref name="NatGeo" /> 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.<ref name="NPRradio">Template:Cite news</ref>
Because of this last condition, snow rollers are more common in hilly areas.<ref name="Wireobot">Template:Cite news</ref> However, the precise nature of the conditions required makes them a very rare phenomenon.<ref name="NPRradio" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name= "BBC 2019">Template:Cite news</ref>
GalleryEdit
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Snow roller with a gear-like shape from Rocky Mountain National Park, Colorado
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Snow roller in the Giant Mountains, in the Czech Republic near Poland
- Snow rollers in Venus PA, USA.jpg
Snow rollers formed overnight during high winds in Venus, Pennsylvania
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A snow roller in Cincinnati, Ohio, United States
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Snow rollers leaving tracks in the snow
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Snow rollers at Lincoln Christian College, Illinois
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Snow rollers in Saxony-Anhalt, Germany