  We describe a comprehensive model for the formation and morphological
development of atmospheric ice crystals growing from water vapor, also known as
snow crystals. Our model derives in part from empirical measurements of the
intrinsic ice growth rates as a function of temperature and supersaturation,
along with additional observations and analyses of diffusion-driven growth
instabilities. We find that temperature-dependent conformational changes
associated with surface melting strongly affect layer nucleation dynamics,
which in turn determines many snow-crystal characteristics. A key feature in
our model is the substantial role played by structure-dependent attachment
kinetics, producing a growth instability that is largely responsible for the
formation of thin plates and hollow columnar forms. Putting these elements
together, we are able to explain the overall growth behavior of atmospheric ice
crystals over a broad range of conditions. Although our model is complex and
still incomplete, we believe it provides a useful framework for directing
further investigations into the physics underlying snow crystal growth.
Additional targeted experimental investigations should better characterize the
model, or suggest modifications, and we plan to pursue these investigations in
future publications in this series. Our model also suggests new avenues for the
continued exploration of ice surface structure and dynamics using molecular
dynamics simulations.
