  With climate modeling predicting a raise of at least 2 C by year 2100, the
fate of ice has become a serious concern, but we still do not understand how
ice grows (or melts). In the atmosphere, crystal growth rates of basal and
prismatic facets exhibit an enigmatic temperature dependence, and crossover up
to three times in a range between 0 and -40 C. Here we use large scale computer
simulations to characterize the ice surface and identify a sequence of novel
phase transitions on the main facets of ice crystallites. Unexpectedly, we find
that as temperature is increased, the crystal surface transforms from a
disordered phase with proliferation of steps, to a smooth phase with small step
density. This causes the anomalous increase of step free energies and provides
the long sought explanation for the enigmatic crossover of snow crystal growth
rates found in the atmosphere.
