TIME-DEPENDENT CHARACTERISTICS OF THE HETEROGENEOUS NUCLEATION OF ICE

Abstract

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.