  I describe a semi-empirical molecular model of the surface attachment
kinetics governing ice crystal growth from water vapor as a function of
temperature, supersaturation, and crystal mesostructure. An important new
hypothesis in this model is surface-energy-driven molecular diffusion enabled
by a leaky Ehrlich-Schwoebel barrier. The proposed surface-diffusion behavior
is sensitive to facet width and surface premelting, yielding
structure-dependent attachment kinetics with a complex temperature dependence.
By incorporating several reasonable assumptions regarding the surface
premelting behavior on basal and prism facets, this model can explain the
overarching features of the snow crystal morphology diagram, which has been an
enduring scientific puzzle for nearly 75 years.
