Crystallographic Orientation and Crystal Size in Cylindrical Accretions of Ice

Abstract

A wind tunnel investigation was carried out to study the crystallographic orientation and the size of individual crystals grown from accreted droplets on slowly rotating cylinders. The experiments covered dry and wet growth, air temperatures between ?2 and ?25C, wind speeds from 10?40 m sec?1, and droplet spectra with representative diameters of 20, 40 and 80 ?. It was found that dry and wet growth differ by the crystallographic orientation of the c axes, this tending to be normal to the growing surface in dry growth and parallel to it in wet. The crystal size was mainly dependent an the air temperature. A remarkable transition from large to small crystals was found in the range of air temperatures between ?15 and ?24C. This transition was more gradual at the lower end of this temperature range in wet or nearly wet growth conditions and/or at the highest wind speed. Relatively small crystals were observed in very spongy accretions at air temperatures as high as ?3C, when the 20 ? droplet spectrum was used. A reduction of liquid water content caused a transition from relatively small to large crystals under these conditions.