Snow Crystal Habit Changes Explained by Layer Nucleation

Critical supersaturations have been measured for the vapor growth of ice crystals on both the basal and prism faces between 2168 and 20.48C. The values are low: approximately constant at 0.4% for the prism face, less for the basal face between 238 and 298C, but greater at higher and lower temperatures. The transitions between tabular (platelike) and columnar growth habits that occur near 238 and 298C are thus directly understandable in terms of layer nucleation as the growth mechanism, without explicitly considering the surface migration of water molecules or spiral steps. These low values of critical supersaturation are consistent with a disordered ice surface, but not with a surface melt layer, even at 218C.