  In this paper, we show theoretically that there exists quasi-liquid on the
edges and vertices of snow crystals between -4C and -22C, while the basal and
prism faces have no quasi-liquid layers. Investigating the macroscopic theory
of quasi-liquid and applying to the edges and vertices of the crystal, we find
that the quasi-liquid becomes unstable above the critical supersaturation
point, which is above the water saturation point. The thickness of this
unstable quasi-liquid layer continues growing indefinitely. We interpret this
behavior as corresponding to continuous production and overflow onto
neighboring faces in a real system. We hypothesize that the unstable growth of
snow crystals originates from the edges and vertices, and it is due to the
overflow of quasi-liquid from the edges and vertices onto the neighboring
faces, which are rough and lack quasi-liquid. Our hypothesis accounts for the
qualitative behavior of the relations between the morphological instability and
the water saturation in the snow phase diagram.
