ON THE FACET-SKELETAL TRANSITION OF SNOW CRYSTALS - EXPERIMENTS IN HIGH AND LOW GRAVITY

A Laboratory investigation of the influence of air velocity on the growth of columnar ice crystals from the vapor over the range -3 to -5°C shows that the linear growth velocity increases and that columns transform to sheath crystals or needles as air velocity increases from a few cm s-1 to 40 cm s-1. Comparison with a similar transition of plates to dendrites shows that, macroscopically, in both cases the facets sprout rounded tips at a critical velocity which is lower for higher ambient supersaturation. Studies in low gravity (20 s KC 135 aircraft parabola) show that chamber scale convection under normal gravity may have significant influence on growth even in the absence of an imposed air velocity. Falling snow crystals, both plate like and column like, become more skeletal in shape as they grow and fall with increasing velocity. This development depends critically on temperature (±0.5°C) and demonstrates that snow crystal shape is even more dependent on environmental growth conditions than previously thought. © 1990.