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The crystal responds to the larger vapor gradients near the edges by sublimating faster there and thus rounding the crystal.
J. Nelson, J. Atmos. Sci., 1998, 55, 910-119.
The change in shape according to the gradient makes intuitive sense to me but I can't for the life of me figure out why it does this. Maybe I'm being really foolish here, but surely a higher concentration of vapour at the corners would decrease the rate of sublimation there.
The vapor gradient reflects the fact there is much more space at the corner than in the middle of a face. The vapor builds up near the surface but because there is much more space near a corner, the vapor pressure drops off, where as the vapor pressure near a point on the middle of a face is reinforced by sublimation at points near it.
