THE SHAPES OF SNOW CRYSTALS - FITNESS FOR PURPOSE

Snow crystals exhibit six quite sharp changes of habit between 0 and -25-degrees-C as between hexagonal plates, columns/needles and stellar dendrites. The questions arise as to why, and do these changes, unique in crystal physics, serve a useful purpose? Calculations of the growth rates of the various forms, and also of ice spheres, as they fall through atmospheres with specified profiles of temperature and supersaturation, reveal that the transitions from plates to stellar dendrites that grow only between -12 and -16-degrees-C, and from plates to columns below -25-degrees-C, permit the more effective release of precipitation from layer clouds. The interruption of the plate regime by the growth of columns and needles between -8 and -3-degrees-C leads to the formation of larger precipitation elements that better survive evaporation below cloud base than would plate crystals, by virtue of having much higher collection efficiencies for supercooled cloud droplets.