A Possible Mechanism for Contact Nucleation

A contact nucleation mechanism is suggested, by which ice embryos formed on a nucleus in vapor are able to nucleate supercooled water on contact. Using conventional nucleation theory, the activity of a contact nucleus is calculated as a function of the contact angle of ice on the nucleus in vapor, and as a function of nucleus size. It is predicted that the threshold supercooling required for deposition nucleation should be about 2.3 times as great as that required for contact nucleation (for nuclei >0.1 μm radius), and this prediction is found to be in reasonable agreement with the limited experimental evidence. A relation between the deposition and contact nucleus concentrations in air samples is predicted.