Observation of the Falling Motion of Early Snow Flakes

Part I. Relationship between the Free-fall Pattern and the Number and Shape of Component Snow Crystals

Abstract

Free-fall pattern of early snow flakes, which were composed of two to six crystals, were observed by a stereophotogrammetric method.
The considerable part (about 80%) of analysed snow flakes showed the fall pattern of spiral type or rotation type. On the other hand, the stable motion (the fall pattern without horizontal movements) was only observed in the small number of snow flakes composed of two crystals. Whether those snow flakes show the stable motion or not depend on Reynolds number of mean vertical motion, the combined state and the ratio of sizes of two component crystals.
The nondimensional frequency and the period of spiral motion increased in proportion to Reynolds number and the nondimensional amplitude of spiral path, respectively. Moreover, the displacement velocity of spiral axis increased proportionally to the falling velocity of early snow flakes.