  I examine the molecular dynamics of ice growth from water vapor, focusing on
how the attachment kinetics can be augmented by edge-dependent surface
diffusion. Although there are significant uncertainties in developing an
accurate physical model of this process, it is possible to make some reasonable
estimates of surface diffusion rates and admolecule density enhancements,
derived from our basic understanding of ice-crystal growth processes. A
quantitative model suggests that edge-dependent surface diffusion could
substantially enhance terrace nucleation on narrow faceted surfaces, especially
at the onset of surface premelting. This result supports our hypothesized
mechanism for structure-dependent attachment kinetics, which readily explains
the changes in snow crystal growth morphology with temperature depicted in the
well-known Nakaya diagram. Many of the model features described here may be
amenable to further quantitative investigation using existing computational
models of the molecular structure and dynamics of the ice surface.
