Another important point about the pattern formation is the mechanism of morphological instability occurring on the polyhedral snow crystals. When a snow crystal is growing in the atmosphere, the water vapor near the surface is consumed by the growing crystal and the diffusion field conforms around the crystal, as shown schematically in Fig. 9 . The equi-concentration lines should be almost concentric, but the snow crystal has a polyhedral shape. This process also can be simulated by numerical calculations. The development from a circular plate, to a sector crystal through a hexagonal plate, and then to a dendrite is clearly found in this illustration.
Snow crystals are one of the most popular natural products.
Their beautiful and minute structures still include many intriguing mysteries. We have found a possible mechanism to explain the various patterns of snow crystals, but it is still only beginning to satisfy our spirit of inquiry.