  We review several aspects of aerodynamics that affect the growth, morphology,
and symmetry of snow crystals. We derive quantitative estimates for
aerodynamical forces that orient falling snow crystals, estimate how air flow
around snow crystals affects their growth rates (the ventilation effect), and
examine how the combination of orientation and growth modification can
stabilize or destabilize different growth behaviors. Special attention is given
to the formation of triangular snow crystals, since it appears that
aerodynamical effects are responsible for producing this unusual morphology,
both in nature and in the laboratory.
