Abstract
Polyhedral ice crystals are grown on a growth substrate at a low air pressure of 40Pa at -7, -15 and -30°C at relatively low supersaturations. The measurements of the normal growth rates of ice crystals versus supersaturation and in situ observations of ice crystal surfaces are carried out. It is inferred that the {0001} and {1010} faces of ice crystals grown under these conditions grow by the mechanism proposed by Burton, Cabrera and Frank (BCF mechanism). The habit change of ice crystals with temperature is equivalent to the temperature dependence of the condensation coefficient al of the {0001} and {1010} faces. In this paper, the formation mechanism of non-hexagonal snow crystals observed in the upper atmosphere and in Antarctica is discussed.
