The dimensions and aggregation of ice crystals in natural clouds

From ice particles collected in clouds and at ground stations in the Cascade Mountains during two winter seasons, measurements have been obtained of the lengths and widths of needles and columnar crystals and the thicknesses and diameters of hexagonal plates. Best-fit empirical relationships between these dimensions are presented. The particles have also been examined to obtain detailed information on the properties of aggregates of ice particles. The maximum dimensions of aggregates and the probability of the occurrence of aggregates decrease with decreasing temperature, but both exhibit a local maximum near the dendritic growth region. Below −15°C and for particle concentrations less than 0.1 cm−3, aggregates are unlikely to form; above −5°C and for particle concentrations in excess of 1 cm−3 there is more than a 50% chance of the formation of aggregates. The size spectra of aggregates depend mainly on the sizes of their component crystals. The number of component crystals in aggregates of various sizes is given.

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