Growth mechanisms to explain the primary and secondary habits of snow crystals

Abstract By estimating the diffusion field 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 fit 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 different 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.