Free-Fall Behavior of Planar Snow Crystals, Conical Graupel and Small Hail

Abstract

Drag coefficients and Best numbers of models of six planar snow crystals, two conical graupel and two conical small-hail particles were determined experimentally in glycerin-water mixtures and salt solutions. The Reynolds number (Re) range covered for the crystals was 0.1 to 200 and for the conical models 10 to 2000. It was found that the drag coefficients of dendritic shapes differed by factors of up to 4 from that of a disc of equal thickness and at an equal Reynolds number. The drag ratio is roughly constant with Re and linearly related to the ratio of the respective surface areas. The drag coefficients of the conical models assumed values between 0.5 and 2.0. During steady fall they decreased with increasing Re; however, as soon as oscillations started this trend reversed. Since tumbling does occur for larger graupel and small hail at Re > 300?1000 their main characteristic motions and frequencies are also discussed. Values for oscillation frequencies are given and the motions are described in detail.