A THEORY OF SNOW-CRYSTAL HABIT AND GROWTH

Abstract

Strong evidence, available from the experiments of Nakaya and his co-workers and bolstered by an analogue experiment, supports the argument that type of snow-crystal growth is determined principally by the excess of the ambient vapor density over that at equilibrium with the ice crystal at its own temperature. Changes in crystal habit occur when this vapor-density excess is sufficient to overcome the inhibitions, first to edge growth and secondly to corner growth, that must exist if the edges and corners have higher surface vapor-densities than the flat faces of the crystal. Equilibrium vapor density for liquid water exceeds that for ice by the order of 10 per cent; slight supersaturation, such as can exist in water cloud, will increase this excess considerably. Ventilation has been neglected in analyzing the Nakaya experiments. Its effect was probably slight, since the ventilation was due to natural convection. The effect should be much greater in the atmosphere, where the ice crystal is falling freely. Some such factor must surely be introduced in the case of the natural atmosphere as compared with the laboratory experiments, for with decreasing air pressure the psychrometric values change, and the available vapor-density excess drops off, varying roughly as the cube root of the pressure. The presence of water cloud modifies the diffusion field surrounding an ice crystal, the effect increasing with the size of the crystal. Thus cloud can make a significant contribution to the rate of growth of ice crystals by sublimation. Better spectra of cloud-droplet sizes are needed to evaluate this more fully.