Ice Circle forms in Wisconsin River; On-looker in Awe
A spinning ice circle was caught last weekend on the Wisconsin River between Sauk City & Prairie du Sac.
MADISON, Wis. (WMTV) - You can hear it in Mike’s voice - he’d seen it on TV, but never before in person. Mike happened upon an ice circle (or ice disk) while walking along the Wisconsin River on Saturday. It was, of course, on the coldest morning of the year so far, - so he was freezing! But, his video shows a winter phenomenon which can be explained by physics -- the same physics in the atmosphere.
WHAT IS AN ICE CIRCLE?
An ice circle or ice disk is a spinning piece of ice found in ponds, rivers, lakes, or other partially-frozen bodies of water. They often look like giant saucers spinning on top of the water’s surface. They’re found during the cold season - and only in places which reach frigid temperatures - like Canada, the northern United States, or right here in southern Wisconsin!
CAUSE: SHEAR
The term shear is used in meteorology to describe something changing speed & direction over a certain distance. Typically, we use this term when describing the wind. As you go up in the atmosphere, wind changes speed & direction. In Severe Weather Season, significant wind shear sometimes occurs in the lower parts of the atmosphere - helping to create rotating supercell thunderstorms & tornadoes.
In this instance, we’re talking about the speed of the water on the Wisconsin River. In Mike’s video, the current appears stronger out in the middle of the River as opposed to the water closer to shore. The result is a sort of “eddy” or whirlpool effect. The faster water farther out causes the ice to rotate. As the ice continues to rotate, any sharp or pointy edges are smoothed out as the ice piece brushes against the surrounding ice.
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