{{Short description|Large snowball formed by wind action}} {{About|the meteorological phenomenon|the snow compaction device|Snow road#History{{!}}Snow road|the movie|Sällskapsresan 2 – Snowroller}} [[File:Snow Roller in Rocky Mountain National Park.jpg|thumb|upright=1.35|Snow roller in Rocky Mountain National Park]]

{{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">{{cite web|last1=Zachos|first1=Elaina|title=Why Snow Is Forming Donut-shape Spirals in Canada|url=https://news.nationalgeographic.com/2018/02/snow-rollers-wind-phenomenon-ottawa-canada-spd/|archive-url=https://web.archive.org/web/20180214010336/https://news.nationalgeographic.com/2018/02/snow-rollers-wind-phenomenon-ottawa-canada-spd/|url-status=dead|archive-date=February 14, 2018|publisher=National Geographic|accessdate=April 20, 2018|date=13 February 2018}}</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">{{cite news|title=Snow Doughnuts Are the Real Thing|url=https://www.npr.org/templates/story/story.php?storyId=8993287|website=NPR.org|publisher=National Public Radio|accessdate=April 20, 2018|language=en}}</ref>

Because of this last condition, snow rollers are more common in hilly areas.<ref name="Wireobot">{{Cite news|url=http://wireobot.com/snow-roller/|title=Snow Donut or Snow Roller a rare interesting natural phenomenon - Wireobot|date=2016-10-27|newspaper=Wireobot|language=en-US|access-date=2016-10-27|url-status=live|archiveurl=https://web.archive.org/web/20161028020436/http://wireobot.com/snow-roller/|archivedate=2016-10-28}}</ref> However, the precise nature of the conditions required makes them a very rare phenomenon.<ref name="NPRradio" /><ref>{{Cite web|url=https://www.holzhau.de/1917-41-0-einzigartiges-naturschauspiel-zwischen-ortsteilen-clausnitz-bienenm%C3%BChle.html|title=Einzigartiges Naturschauspiel zwischen den Ortsteilen Clausnitz und Bienenmühle {{!}} Holzhau Rechenberg-Bienenmühle Clausnitz {{!}} 03.02.2019|last=Holzau|first=Tino Bellmann @ BCS Computersysteme Holzhau|website=www.holzhau.de|language=de|access-date=2019-02-03}}</ref><ref name= "BBC 2019">{{Cite news|url=https://www.bbc.co.uk/news/uk-england-wiltshire-47108382|title=Rare snow rollers spotted in field near Marlborough|date=3 February 2019|work=BBC News|accessdate=3 February 2019 }}</ref>

==Gallery==

<gallery class="center" mode=nolines widths="200" heights="200"> File:Snow Roller.jpg|Snow roller with a gear-like shape from Rocky Mountain National Park, Colorado File:Rolling snow.JPG|Snow roller in the Giant Mountains, in the Czech Republic near Poland File:Snow rollers in Venus PA, USA.jpg|Snow rollers formed overnight during high winds in Venus, Pennsylvania File:snowroller.jpg|A snow roller in Cincinnati, Ohio, United States File:Schneeringe1 Foto Kathrin Spiegler.JPG|Snow rollers leaving tracks in the snow File:Snow roller Illinois.jpg|Snow rollers at Lincoln Christian College, Illinois File:Umgeworfene Schneerige 4 Foto Olaf Parnieske.JPG|Snow rollers in Saxony-Anhalt, Germany </gallery>

== See also == *{{annotated link|Yukimarimo}}

==References== {{commons category|Snow rollers}} {{reflist}}

Roller