Properties of Columnar Ice Crystals Precipitating front Layer Clouds

Abstract

Single unrimed columnar ice crystals (>200 µm in length) from shallow layer clouds were collected in silicone oil, photographed under a microscope, and melted to determine their mass. These ice crystals were representative of those growing in a water sub-saturated environment in the temperature range ?4 to ?10C. The axial lengths of the crystals were related by the expression D = 6.3 L0.437, where D and L are the lengths of the minor and major axes, expressed in micrometers, and have a nearly constant density of 0.3 gm cm?3. The habit of the ice crystals was very similar to those obtained in the water-droplet-free environment of laboratory diffusion chambers and was elementary or bundles of sheaths. Present observations suggest that bundles originate by the growth of secondary ice crystals on the prism face of the crystal. These secondary ice crystals then grow on the surface of the parent crystal and may grow at a rate faster than, and at the expense of, the parent crystal. The results of these observations were compared with those made on ice crystals growing in clouds supersaturated with respect to water.