Crystal Growth in the Presence of Surface Melting and Impurities: An   Explanation of Snow Crystal Growth Morphologies
  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. 
Kenneth G. Libbrecht