Observations of the Falling Motion of Plate-Like Snow Crystals

Part II: The Free-Fall Patterns and Velocity Variations of Rimed Crystals

Abstract

Free-fall patterns and the variations in the vertical and horizontal velocities of rimed plate-like snow crystals were analysed by means of a stereo-photogrammetric method. The following main results were obtained:
1) Unstable fall patterns were classified into three types: nonrotation, swing, and rotation or spiral.
2) Considering the riming amount of crystals, the distinction among these types was approximately determined by the combination of the nondimensional moment of inertia (stability number) and the Reynolds number, which involves the mean vertical velocity.
3) The standard deviation of variation in vertical velocity was very small (less than about 3% of the mean vertical velocity). On the other hand, the standard deviation of variation in horizontal velocity was considerably large (4 to 15% of the mean vertical velocity).
Accordingly, it is likely that the variation of horizontal velocity plays an important role in the random aggregation of rimed plate-like snow crystals having almost the same shape and size.