TIME-DEPENDENT CHARACTERISTICS OF THE HETEROGENEOUS NUCLEATION OF ICE

The dependence of heterogeneous nucleation of supercooled water drops on temperature and on the duration of supercooling has been studied. The rain-sized drops were placed on an oiled aluminium surface; thermoelectric refrigeration was used to provide constant cooling rates and to maintain constant temperatures. Once-distilled tap water and melted snow and hail were investigated. Only the first two of these showed definite time-dependent properties.The experimental results are inconsistent with the hypothesis that all drops have equal probability of nucleation (stochastic hypothesis), and also with the hypothesis that the freezing temperature of a drop is the characteristic temperature of one of the impurities contained in the drop ("singular" hypothesis). The results are interpreted in terms of a model in which the characteristics of a particular nucleating site determine a range of possible nucleation temperatures for that site, and within that range of less than 2 degrees the probability per second for a nucleation event increases rapidly with increasing supercooling.