Why does the crystal sublime more quickly at the corners where there is a higher vapour gradient?

[![Diagram](https://i.stack.imgur.com/UFCEy.png)](https://i.stack.imgur.com/UFCEy.png)



> 
> 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.


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