I guess my mental picture of, they start as large irregular pieces of ice and gradually become smaller spheres, is wrong. Rather the point is that the water is quite literally 0C and so the pieces of ice are both shrinking and growing continuously. At which point, whatever initial condition of the piece of ice gradually becomes irrelevant, and the limiting shape is somehow determined by the eddy. I wonder what factors determine its size; perhaps all such water ice balls limit to this size?
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