Snow crystals

The growth of the snow crystal is an important example of crystal growth not attributable to the presence of screw dislocations. More than 400 years of study of the habits of these crystals culminated in the work of Nakaya establishing the complex and abrupt changes of crystal habit with temperature and humidity, and emphasising the essentially three-dimensional skeletal form of these crystals. These facts are explicable in terms of crystal growth by surface nucleation, which at high supersaturation becomes a periodic process rather than a sporadic one, occurring at the corners of a crystal face and thus able to inhibit nucleation at the adjoining comers of other faces. Only so can one account for the observed abruptness of habit change. The other qualitative features of the dendritic habits then become explicable.