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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. Most recently an ice circle of approximately 50 ft. in diameter was observed and photographed in Lake Katrine, New York on the Esopus Creek on January 23, 2014. Snow and freezing temperatures have battered many parts of the U.S in January, with temperatures as low as -10 degrees Centigrade/14 Fahrenheit recorded in many States. In Idaho, the extreme weather led to a rare sighting of an Ice disk on the Snare 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. Credit: Daniel Ostler http://my.news.yahoo.com/video/rare-sighting-ice-disk-idaho-104748782.html
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.
River specialist and geography professor Joe Desloges states that ice pans are "surface slabs of ice that form in the center of a lake or creek, instead of along the water’s edge. As water cools, it releases heat that turns into 'frazil ice' that can cluster together into a pan-shaped formation. If an ice pan accumulates enough frazil ice and the current remains slow, the pan may transform into a 'hanging dam', a heavy block of ice with high ridges low centre.