Few would question such doggerel as "no two snow crystals are alike and in infinite variety they fall." Conversely , many might challenge the statement that individual crystals are relatively scarce by comparison with snowflake aggregates, rimed crystals, and irregular forms; but they are. The types of snow occurring in a storm are a function of many variables including updraft strength, temperature, cloud moisture, liquid water content , cloud thickness, and ice nuclei and crystal concentrations. Some of the relationships are obvious, others more obscure, still others unknown. This paper describes ground observations and calculations that attempt to clarify certain snowfall forms in terms of the cloud conditions likely to produce them.

The classical ray optics approximation has been applied to compute the angular scattering of light by finitesized hexagonal ice crystals in the form of columns and plates. The results are presented at a wavelength of 0.55 microm for a random orientation of the crystals either in space or in a plane. The results are also compared to those of ice spheres. For the first time the angular light scattering of platelike crystals and a quantitative description of the 46 degrees halo are given. Contrary to earlier studies, it is shown that both plate- and columnlike ice crystals show a strong backscattering. In agreement with previous studies, ice spheres are found to scatter-when compared to ice prisms-less energy at angles near 90 degrees . With regard to the effect of orientation on the light scattering, it is shown that columnlike crystals randomly oriented in a plane behave rather like spherical particles and not like columns randomly oriented in space.

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