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)