FORMATION MECHANISMS OF EIGHTEEN-BRANCHED SNOW CRYSTALS

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During the work involving snow crystals of low temperature types at Inuvik (68°22′N, 133°42′W), Northwest Territories, Canada, eighteen-branched snow crystals were observed under relative warm temperature conditions on January 8 and 11,1986. The formation mechanisms of these crystals were discussed in terms of rotation twinning on the basis of the concept of coincidence-site lattices which was first introduced into the formation mechanisms of twelve-branched snow crystals by KOBAYASHI and FURUKAWA (J. Cryst. Growth, 28,21,1975). However, the mechanism of rotation twinning could not be always applied to the eighteen-branched snow crystals. A new idea based on freezing experiments of supercooled water droplets was introduced to the formation (UYEDA and KIKUCHI, J. Meteorol. Soc. Jpn., 56,43,1978). These experimental results showed that when the supercooled water droplets were frozen by means of contact with ice crystals, the supercooled water droplets froze and had several parallel straight cracks surrounding the droplets in a vertical direction to the principal axis of the frozen droplets. Further, numerous tiny air bubbles appeared around the cracks. Therefore, it was considered that when the cracks were formed simultaneously on the surface of droplets, a horizontal rotation around the cracks arose. After which the dendritic branches grew from the droplets in the temperature conditions suitable for dendritic growth. As described above, it was considered that multibranched snow crystals including twelve- and eighteen-branched snow crystals were formed from the freezing of supercooled cloud droplets.

FORMATION MECHANISMS OF EIGHTEEN-BRANCHED SNOW CRYSTALS

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