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Ice circle
An ice disc, ice circle, or ice pan is a natural phenomenon that occurs in slow moving water in cold climates.
Ice circles are thin and circular slabs of ice that rotate slowly in the water. It is believed that they form in eddy currents. Ice discs have most frequently been observed in Scandinavia and North America, but they are occasionally recorded as far south as England and Wales. An ice disc was observed in Wales in December 2008 and another was reported in England in January 2009. An ice disc was observed on the Sheyenne River in North Dakota in December 2013. An ice circle of approximately 50 ft. in diameter was observed and photographed in Lake Katrine, New York on the Esopus Creek around January 23, 2014. In Idaho, extreme weather led to a rare sighting of an Ice disk on the Snake River on January 22, 2014.
An unusual natural phenomenon, ice disks occur in slow moving water in cold climates and can vary in size, with circles more than 15 metres (49 ft) in diameter observed.〔Daniel Ostler (news.yahoo.com ), (recordonline.com )〕〔Blake Nicholson, (Unusual ice circle forms in North Dakota river ), November 26, 2013, Associated Press〕〔Pete Thomas, (Giant spinning ice circle discovered in North Dakota’s Sheyenne River ), November 26, 2013, GrindTV〕
==Ice discs==
Ice discs form on the outer bends in a river where the accelerating water creates a force called 'rotational shear', which breaks off a chunk of ice and twists it around. As the disc rotates, it grinds against surrounding ice — smoothing into a circle.〔 A relatively uncommon phenomenon, one of the earliest recordings is of a slowly revolving disc spotted on the Mianus River and reported in an 1895 edition of ''Scientific American''.〔Rickard, B et al: ''Unexplained Phenomena'', page 190. Rough Guides, 2000.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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