SNOWFLAKES. “Ice can form homogeneously by either freezing liquid water, or directly through vapor deposition to the crystalline phase. As is the case for the homogeneous nucleation of liquid water surface tension effects imply that for a given supersaturation an ice-embryo must reach a critical size. For ice-embryo’s smaller than this critical size an increase in the crystal size implies an increase in the change of the free energy of the system.”
http://www.mpimet.mpg.de/fileadmin/staff/stevensbjorn/teaching/skript-5.pdf