Observations of the Falling Motion of Plate-Like Snow Crystals Part I: The Free-Fall Patterns and Velocity

[摘要] Free-fall patterns and the variations in the vertical and horizontal velocities of unrimed plate-like snow crystals were analyzed by means of a stereo-photogrammetric method.Whether the crystals exhibit a stable falling motion or not mainly depends on the Best number, which involves the mean vertical velocity (fall velocity), and the non-dimensional moment of inertia of the crystals.Unstable fall patterns were roughly classified into three types: nonrotation, swing, and rotation or spiral. For dendritic shaped crystals, the distinction among these types is approximately determined by the combination of the non-dimensional moment of inertia and the Reynolds number, which involves the mean vertical velocity.Although the standard deviation of the vertical velocity for dendritic shaped crystals was very small (1 to 3% of the mean vertical velocity), that of the horizontal velocity was considerably larger, measuring 5 to 20% of the mean vertical velocity. Accordingly, it appears likely that the variation in the horizontal velocity plays an important role in the random aggregation of plate-like snow crystals having almost the same shape and size.

[学科分类] 大气科学