Measured Ice Crystal Capacitances: the Failure of the Electrostatic Analogy

Laboratory measurements of mass growth rates of ice crystals for temperatures from -20^oC to -60^oC have been used to calculate the capacitances of hexagonal plates, columns, and bullet rosettes. When compared with various theoretical models of ice crystal capacitances derived from electrostatic theory concepts, the theoretical values significantly over estimate the mass growth rates in almost every case, typically by a factor of 2-6, depending on the habit. As with in situ crystals, the majority of simple hexagonal crystals grown in the laboratory are asymmetric in shape, with prism facets of different widths, and basal facets with asymmetric cross sections. Additionally, most facets contain irregular steps, layers, and/or dislocations which likely lead to varying vapor densities over strained surfaces, causing supposedly equivalent facets to experience different accommodation coefficients and different growth rates. On occasion some facets appear not to grow at all until a critical ice supersaturation is reached, sometimes approaching water saturation. In application to the real atmosphere where irregular crystals are the norm, the electrostatic analogy cannot give reasonable results. Laboratory measured capacitances provide a more realistic basis for modeling ice crystal growth. Measured capacitances for simple hexagonal shapes and the more dominant irregular polycrystalline habits are presented.