  We examine the molecular dynamics of crystal growth in the presence of
surface melting and surface impurities, and from this propose a detailed
microscopic model for the growth of ice from the vapor phase. Our model
naturally accounts for many aspects of the experimental data that are otherwise
difficult to explain, and it suggests a variety of measurements that can
provide further confirmation. Although additional work is needed to refine
these ideas, we believe that the combined influences of surface melting and
impurities provide a viable solution to the 60-year-old mystery of why snow
crystal morphologies vary so dramatically with temperature.
