Evaporation of Submicroscopic Ice Crystals

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Abstract
Experiments, designed to interpret certain meteorological observations, provide evidence that very small ice crystals may have evaporation rates much lower than predicted on the basis of the vapor pressure of macroscopic ice. Small ice crystals, formed by spontaneous sublimation in nuclei‐free air, were suspended in a strongly desiccating atmosphere for a time far longer than should have been sufficient for their complete evaporation. Though they decreased in size below the minimum observable optically, their presence was demonstrated by their ability to nucleate visible ice particles when exposed to a supersaturated atmosphere. In representative experiments, ice‐forming nuclei were observed in a 250‐cm3 chamber three hours after the admission of nucleus‐free ice particles of about 1‐micron radius, the chamber containing an atmosphere at —20°C with water vapor pressure at about 50 percent of saturation with respect to macroscopic ice. There was some evidence, however, that the ice‐forming residues diminish in effectiveness at a low time rate.