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Ice particle multiplication mechanisms are summarized and considered to be incapable of producing the excess of ice crystals over ice nuclei (factors of 102–104) observed in several types of clouds. Contact freezing, sorption and immersion freezing nucleation are re-examined with regard to their temperature and mode of activation. A model of ice phase nucleation based on contact nucleation is advanced. The collection of contact nuclei by drops is considered to result from Brownian motion and phoretic forces. Based on estimated sizes for natural contact nuclei of 0.3–0.5 μm, contact nueleation is predicted to be suppressed in updrafts and enhanced in downdrafts or mixing zones. This correlation is in excellent agreement with observations of first ice in cumuli. Blanchard's drop freezing data are used to deduce concentrations of natural contact nuclei active at −4C of 100–1000 liter−1. These concentrations are noted to be consistent with Hobbs' observations of 1–10 ice particles liter−1 at −4C in an orographic cap cloud.
Kenneth C. Young (Mon,) studied this question.