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Diagram

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.

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  • $\begingroup$ The diagrams here are actually super helpful in answering your question. Thank you for including those figures. $\endgroup$ – Zhe Mar 5 '17 at 22:17
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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.

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