The growth of small aggregates of ice crystals has been observed between -6°C and -25°C by mounting an ice sphere in a moving cloud of ice crystals. The density of the aggregate formed increased with increasing temperature, and observations of the aggregate growth showed that the bonds between ice crystals permit folding of crystal towers. The higher the temperature, the more folding was noted. The proportion of the ice crystals in the path of the aggregate that became attached to it was temperature-dependent, showing a maximum collection efficiency at -11°C. Plates formed aggregates at a greater rate than did columnar crystals ; hence, when the cloud composition changed from plates to columns as the temperature increased above -11"C, the amount of aggregation diminished. These data and other evidence are interpreted as indicating that the aggregation of the ice crystals depends upon the existence of a liquid f i l m on the ice surfaces. The film thickness is greater at higher temperatures.
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