What is an ice circle, and how does it form?
I've seen ice do a lot of strange things, but I've never seen it do this before: Ice Circle spinning on Middle Fork Snoqualmie River (Vimeo).
What causes ice circles to form?
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1 answer
Ice Disk
That phenomenon is called an ice disc or ice circle. It occurs in slow moving water in cold climates.
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.
Your Ice Circle
This particular ice disk (from your video), was formed because there was a certain climate which is perfect for such phenomenon to build.
"Calm winds this week combined with 6 mornings in a row in the low 20s made for perfect conditions to form the circles," said Q13 Chief Meteorologist Walter Kelley.
From Fox4KC
Why is it so rare?
(...) but I've never seen it do this before(.)
The occurrence of such are absolutely rare since you have to face perfect really specific conditions (water, climate, river etc.).
Ice circles or disc are a relatively uncommon phenomenon. One of the earliest recordings is of a slowly revolving disc spotted on the Mianus River in 1895.
From Fox4KC
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.
Further reading
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