Some recent investigations on the nature and properties of natural and artificial freezing nuclei

Abstract

Listen

Translate article

Working on the hypothesis that atmospheric ice-forming nuclei are largely of terrestrial origin, the nucleating ability of various types of soil particles and mineral dusts has been investigated. Of the thirty substances tested, twenty-one, mainly silicate minerals of the clay and mica groups, were found to produce ice crystals in supercooled clouds and also on supercooled soap films at temperatures of − 18° C, or above, and of these, ten were active above − 12° C. The most abundant of these is kaolinite with a threshold temperature of − 9° C. Ten natural substances, again mainly silicates, were found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they could be pre-activated or «trained». Thus, ice crystals grown on kaolinite nuclei, which are initially active at −9° C, when evaporated and warmed to near 0° C in a dry atmosphere, leave behind nuclei which are thereafter effective at − 4° C. Particles of montmorillonite, another important constituent of some clays, and which are initially inactive even at −25° C, may be pre-activated to serve as ice nuclei at temperatures as high as −10° C. It is suggested that although such particles can initially form ice crystals only at cirrus levels, when the ice crystals evaporate they will leave behind some «trained» nuclei which may later seed lower clouds at temperatures only a few degrees below 0° C. On this hypothesis, the fact that efficient nuclei are occasionally more abundant at higher levels would not necessarily imply that they originate from outer space. Indeed, in view of our tests on products of stony meteorites, produced both by grinding and vaporization, which show them to be ineffective at temperatures above − 17° C, it seems likely that atmospheric ice nuclei are produced mainly at the earth's surface, the clay minerals, particularly kaolinite, being a major source.