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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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In order to isolate the factors governing the freezing point of cloud and precipitation elements, experiments have been made to measure the effects of physical dimension and of certain ions in solution upon the freezing temperature of water in glass capillaries. The results of these experiments indicate that the freezing point of water in capillaries is independent of the volume or of the interface area, and that the radius of the capillary tubing is the only physical dimension that affects the freezing point of water contained in it. The amount of supercooling necessary to induce freezing can be either increased or diminished by the addition of certain ions in solution. The magnitude and sign of the change in freezing point are functions of the type of ions in solution, the molar concentration of the ions, and the capillary radius.

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