Abstract
A large cloud chamber of about 15 meter high made possible to observe ice crystals grown in free fall. During the experiments carried out by using several seeding methods it was found that the seeding method to use a cold body produced not only usual ice crystals of hexagonal shape but also several kinds of very interesting skeleton ice crystals of non-hexagonal shape. As a cold body a metal rod chilled by liquid nitrogen or by dry ice was used. The temperature range at which those crystals were found were -1.9--11.1°C and <-20°C. These ice crystals are thought started their growth at the special initial conditions of extraordinary high super-saturation. Their growth modes are mainly determined by air temperature as shown by the fact that those grown at high temperature (-1.9--11.1°C) and those grown at low temperature (<-20°C) were very different in appearance.
From a crystallographic point of view the formation of these skeleton ice crystals of nonhexagonal shape presents the three important facts as follows. One is that initial condition of ice crystal formation is possible to affect the later growth habit seriously, another is that an ice dendrite growing in the ‹112ι› direction is possible to be understood as a growth along skeleton arm and the other is that the growth of both inside plane and pyramidal plane are possible. (The definitions of skeleton arm and inside plane are presented in section 3)
