On the Orientation of the Principal Axis of Frozen Water Droplets

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Abstract
Freezing experiments were carried out from a point of view of the mechanisms of origin of polycrystalline snow crystals, namely, spatial dendrites, radiating assemblage of dendrites, combination of bullets and so on. Especially, a detailed examination was made on the number of component crystals and the angle between c-axes of neighbouring crystals when water droplets of 0.4-1.7mm in diameter were frozen under controlled rates of cooling of -12°C/h, -30°C/h and -60°C/h. The results were summarized as follows; (i) Most of the water droplets froze in a range of -17--23°C among the rates of cooling selected. (ii) The number of the component crystals was 2-3 for the cooling rate of -12°C/h and -30°C/h and 5-6 for the cooling rate of -60°C/h. (iii) There were two peaks on the frequency distributions at 20°-0°and 60°-80°for the angle between c-axes of the neighbouring crystals. (iv) Under the condition of annealing, a remarkable peak around 20°-30° diminished and another peak at 60°-80°remained showing an increasing prominent. (v) Examples suspected to be penetration twinning were obtained and the angle between the neighbouring crystals of 13 out of 18 examples was found concentrated at approximately 70°. These experimental results seem to explain the 70° of the angle between spatial branches and bullets of natural polycrystalline snow crystals and further appear to support the calculated results based on twinned structures of snow crystals by Kobayashi et al (1976).

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