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 growt h rates of the various forms, and also of ice spheres, as they fall th rough atmospheres with specified profiles of temperature and supersatu ration, reveal that the transitions from plates to stellar dendrites t hat grow only between -12 and -16-degrees-C, and from plates to column s below -25-degrees-C, permit the more effective release of precipitat ion from layer clouds. The interruption of the plate regime by the gro wth of columns and needles between -8 and -3-degrees-C leads to the fo rmation of larger precipitation elements that better survive evaporati on below cloud base than would plate crystals, by virtue of having muc h higher collection efficiencies for supercooled cloud droplets.