The charge on an AgI particle, freely suspended in an air current of terminal velocity, has been measured at various temperatures, humidities and cooling rates. The wall of the flow pipe serves as a source or sinkfor water vapor, simulating neighbor droplets and ice particles in a cloud. The data show that the AgI particle sorbs water below the dew point and thereby acquires negative charge, that the sorbed water freezes at 0C, and that the electrification is reverisble. There are sudden changes in charge at the onset of sorption at the dew point and at freezing and melting, but the largest changes take place in prolonged and extensive sorption or desorption. The electrification follows an exponential rate law, indicating an autocatalytic process. Ice formation at 0C was also obtained with a dense cloud of AgI and correspondingly small condensate droplets in the expansion chamber. It is concluded that nucleation occurs at 0C, and that super-saturation pertains to the growth of the ice to detacable size. The data are compared with data in the literature. It is concluded that contradictions among published data are the results of differences in experimentalconditions, especially substrate effects in experiments with supported drops. The mechanisms of nucleation,growth and electrification are discussed.

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