Abstract Individual crystals are grown under controlled conditions at temperatures between −0.6 and −20°C at rates as low as 10 −4 g/year and supersaturations as low as 6.5×10 −7. The transition between the kinetic growth form and the equilibrium form is clearly distinguished at temperatures between −2 and −10°C where the equilibrium form is a well-rounded plate with an aspect ratio of about 2.5. At temperatures below −11°C the equilibrium form is a hexagonal prism of about the same aspect ratio. This transition coincides with the rapid increase in surface roughening on the prism faces at temperatures above −10°C. The equilibrium form is a fully rounded particle just below 0°C although we had expected the fully rounded particle to prevail down to at least −5°C. Furthermore, there are unresolved differences between these experimental results and observations of crystals from the seasonal snow cover where particles are fully rounded at slow growth rates and low temperatures.