Abstract
A fairly wide range of 28 naturally‐occurring mineral dusts has been tested for ability to act as ice nuclei. 19 substances, mainly silicate minerals of the clay and mica groups, are found to be effective at −18°C; seven of these are active above −10°C. The most abundant of these is kaolinite with a threshold temperature of −9°C. 10 of the 28 substances, again mainly silicates, are found to become more efficient ice nuclei having once been involved in ice‐crystal formation, i.e. they can be pre‐activated. 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 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 may 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 the poor nucleating properties of some meteoritic material that was tested, it appears that atmospheric ice nuclei are predominantly of terrestrial origin, with the clay minerals, particularly kaolinite, a major source.