Spinning Ice Discs
The latest Mystery Photo is of an ice disc – a large disc of ice spinning in a river. It’s thought that this relatively rare natural phenomenon is likely caused by cold, dense air coming in contact with an eddy in a river, forming discs ranging anywhere from 3 to 650 feet in diameter.
While eddies contribute to the spinning, they are not the only cause. If they were, small discs would spin faster than big discs, and this is not the case. Discs of all sizes rotate at roughly the same rate. One would also expect that discs in still water, where there aren’t any eddies, wouldn’t start spinning, but they do.
The melting of the ice disc contributes to its spinning as well. When an ice disc starts to melt, the melted ice water is denser than the ice, and thus sinks below the disc. This movement causes the water to spin, which in turn spins the disc. (Thanks to Martha Kent for photo submitted by Paula Kelley)
Naturally Curious with Mary Holland 
My guess is that such discs spin in opposite directions in the Northern and Southern hemispheres, just like flushed toilets!
December 20, 2017 at 8:46 am
Thanks for putting this out there Mary as I realize you didn’t take the shot of this most unusually large ice disc.
I can’t recall ever seeing one this large before…
Thanks again,
Bill Farr… 🙂
December 20, 2017 at 9:14 am
The disc at thr base of our dam forms every year as a result of small clusters of slush which fall from the house roof. The clusters spin individually before joining together at night when the temps are lowest. It’s edges are trimmed by the edges of ice on the stones of the dam.
December 20, 2017 at 9:17 am
Thanks. Learning new stuff every day.
December 20, 2017 at 9:18 am
I still think it’s an alien invasion
December 20, 2017 at 9:51 am
Mary, I am wondering if their rotation is affected by the Coriolis effect, counter-clockwise in the Northern Hemisphere.
December 20, 2017 at 10:07 am
That’s phenomenal, Mary. I can’t believe I’ve lived here more than 80 years and have never heard of an ice disk, much less seen one. I wonder if they spin in the opposite direction south of the equator? – Mike
December 20, 2017 at 10:44 am
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December 20, 2017 at 11:37 am
Absolutely fascinating Mary – have never seen one of these though I’ve been a limnologist for over 40 years.
December 20, 2017 at 12:40 pm
In Brattleboro there’s a place where the sidewalk goes over a substantial brook not far from where it hits the CT River. One day my mom and I saw a whole flock of ice disks lurking in the backwaters there. Like those giant tropical (I think) high-edged pads. Very nifty. So thanks, Mary. I’m glad to find out something about them.
December 20, 2017 at 6:46 pm
I am guessing this disk formed on the Mad River, north of Waitsfield, east of Route 100, about 700-feet north of the intersection of Tremblay Road and Route 100; where the Mad River swings close to Route 100. These circles form here repeatedly. Very nice.
December 20, 2017 at 9:37 pm
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cool
December 24, 2017 at 2:07 pm