Ice Circle ... how does it form ?
Can anyone explain how it has been formed ?
An ice circle is a rare phenomenon that occurs in slow moving water in cold climates. It consists of large discs of ice that rotate slowly in the water. It is believed that they form in eddy currents. Ice circles have most frequently been observed in Scandinavia and North America, but one was recorded in Britain in January 2009. A Met Office spokesman was described as "kind of a bit stumped to be honest". Additionally the photo shows a collection of small ice circles (the largest about 12" (30cm) in diameter) seen on the River Llugwy at Betws-y-coed, North Wales, in December 2008. A fortnight of no rain had resulted in low water levels, and there had been sub-zero temperatures for a week.
Ice circles occur at bends in the 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 perfect circle.
Another kind of ice circle are actually ice pans, or surface slabs of ice that form in the center of a lake or creek, instead of along the water's edge. They can be explained by quick shifts in temperature. As water cools, it releases heat that turns the water into frazil ice — a collection of loose, needle-shaped ice particles that can cluster together in an ice pan. If a lake accumulates enough frazil ice and the current is slow, over time, the pan can become a hanging dam: a dense, heavy piece of ice with high ridges and a low centre.