On the Trigonal Growth of Ice Crystals

Abstract

Trigonal ice crystals, ice crystals with the basal faces of a regular triangle, are formed by seeding I of using adiabatic cooling as well as by seeding II of using a very cold body. When ice crystals are made by the former seeding and are grown in free fall for about 200 seconds in a dense supercooled cloud, the production rate of trigonal ice crystals depends considerably on temperature of the cloud. At about -7°C it runs up to about 20%. Scalene hexagonal ice crystals, which are obtained in abundance at the same time as trigonal ice crystals, are inferred to be grown from trigonal ice crystals. Rhombic ice crystals, scalene pentagonal ice crystals and trigonal dendrites are also observed occasionally.
These results mean that the growth directions <1120> and <1010> are the first and the second favourable directions respectively in the basal plane for the growth of ice crystals. In free fall growth, therefore, ice crystals having trigonal constructions seem to be obtained because growth toward<1010> initiate at the time of nucleation.