Fall Patterns and Fall Velocities of Rimed Ice Crystals

Abstract

Fall patterns of timed snow crystals between 0.02 and 0.5 cm in size were studied using a stereoscopic camera system with stroboscopic illumination. Relationships between terminal fall velocities and crystal dimensions were determined for graupel, rimed columnar crystals, rimed plate crystals, rimed capped columns, rimed broken branches, and aggregates of rimed crystals. For each crystal form the characteristic fall pattern is discussed and Reynolds numbers, drag coefficients and Best numbers are presented. Measured densities for graupel and for densely rimed columns are also reported. It is concluded that oscillatory and rotatory motions are quite common for rimed crystals although the mean fall attitude is mostly the same as for unrimed crystals of similar habit. Fall velocities of rimed crystals are on the average twice those of unrimed crystals.