We present a new microscopic model of growth and sublimation (g/s) of ice crystals in the atmosphere. This model is based on the assumption that the flux of vapor to the crystal surface is uniform over each flat crystal face. It thus differs fundamentally from the standard “capacitance” model for crystal growth, in which the mixing ratio is assumed uniform at the surface. In the new model the surface influence on growth is calculated self‐consistently in terms of local environmental conditions, again differing sharply from the standard models in which this influence is either ignored or assigned a uniform, externally prescribed value. The new model leads to predictions of the evolution of ice crystal shape as well as mass. We find that predicted g/s rates are generally smaller than those predicted by the earlier models. The general trends both in g/s rates and in crystal hollowing predicted by the model are consistent with field and laboratory observations. The values of certain surface parameters needed for application of our model must be found from experiment. We review and compare the relevant laboratory experiments on ice crystal g/s rates and show their lack of mutual consistency. Therefore the surface parameters inferred from these experiments are necessarily uncertain. We show that the surface parameter values can be inferred from observations of crystal hollowing, since our model allows the prediction of environmental conditions at which hollowing should occur.

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Critical supersaturations have been measured for the vapor growth of ice crystals on both the basal and prism faces between Ϫ16Њ and Ϫ0.4ЊC. The values are low: approximately constant at 0.4% for the prism face, less for the basal face between Ϫ3Њ and Ϫ9ЊC, but greater at higher and lower temperatures. The transitions between tabular (platelike) and columnar growth habits that occur near Ϫ3Њ and Ϫ9ЊC are thus directly understandable in terms of layer nucleation as the growth mechanism, without explicitly considering the surface migration of water molecules or spiral steps. These low values of critical supersaturation are consistent with a disordered ice surface, but not with a surface melt layer, even at Ϫ1ЊC.

By estimating the diOE usion ®eld adjacent to growing snow crystals and using a variety of ice crystal growth data, it is shown that most observations of snow crystal habits above ¡22¯C are explained by dislocation-promoted growth at small sizes and low supersaturations, but otherwise layer nucleation controls the growth habits in agreement with the Knight±Frank theory. The formation of capped columns, crown crystals, and hollowed crystals, and the growth rates of needles and dendrites also ®t predictions of the theory. The analysis suggests that dendrites at water saturation retain small facets at their growing tips. Below ¡22¯C, the available data together with predicted trends of the edge energy show that spiral steps and layer nucleation can explain why both tabular and columnar forms grow at the same temperature but diOE erent supersaturations. Other growth mechanisms that were proposed in the past, such as step speed variations, surface phase transitions and adatom migration across crystal edges are incapable of explaining the wide variety of available habit data. } 1. Introduction Growth shapes of faceted crystals are determined by the relative growth rates of their faces and thus controlled by surface kinetics and bulk transport of both material and heat. Hence, growth shapes are much more diverse than equilibrium shapes, which are determined by surface free energies. Of all single-component vapourgrown crystals studied to date, few have shapes (habits) as sensitive to environmental conditions and probably none has been studied as long as the snow crystal. Their variable habits were noted at least as far back as Descartes (Frank 1982) and their usual sixfold symmetry was ®rst recorded around 135 BC (Needham and Gwei-Djen 1961), but how the habit depends on environmental conditions was not found until 1936 (Nakaya and Sekido 1936). Further work by many scientists on how the snow crystal habit depends on temperature and supersaturation in an atmosphere of air is summarized as ®gure 1. Despite many attempts over their long history of study, a complete, widely accepted theory for their growth habits has not emerged. This paper shows that the interplay between two mechanisms can explain both ®gure 1 and also how crystal habit depends on the ambient gas pressure.Snow crystal shape is classi®ed into primary and secondary habits. The primary habit of single crystals depends on their aspect ratio G: the ratio of the maximum length 2c along the c axis h0001i to the maximum width 2a along h11 · 2 20i (®gure 2).

Abstract. The build-up of intrinsic Bjerrum and ionic defects at ice-vapor interfaces electrically charges ice surfaces and thus gives rise to many phenomena including thermoelectricity, ferroelectric ice films, sparks from objects in blizzards, electromagnetic emissions accompanying cracking in avalanches, glaciers, and sea ice, and charge transfer during ice-ice collisions in thunderstorms. Fletcher's theory of the ice surface in equilibrium proposed that the Bjerrum defects have a higher rate of creation at the surface than in the bulk, which produces a high concentration of surface D defects that then attract a high concentration of OH − ions at the surface. Here, we add to this theory the effect of a moving interface caused by growth or sublimation. This effect can increase the amount of ionic surface charges more than 10-fold for growth rates near 1 µm s −1 and can extend the spatial separation of interior charges in qualitative agreement with many observations. In addition, ice-ice collisions should generate sufficient pressure to melt ice at the contact region and we argue that the ice particle with the initially sharper point at contact loses more mass of melt than the other particle. A simple analytic model of this process with parameters that are consistent with observations leads to predicted collisional charge exchange that semiquantitatively explains the negative charging region of thunderstorms. The model also has implications for snowflake formation, ferroelectric ice, polarization of ice in snowpacks, and chemical reactions in ice surfaces.