Further Tests on the Ice Nucleation Potential of Meteoritic Material
SCHAEFER1 was unable to detect any trace of ice-crystal formation when he vaporized a sample of iron–nickel meteorite by electric spark in a supercooled cloud—even in clouds as cold as −25° C. Mason and Maybank2, who ground up and vaporized three samples of stony meteorites, and Mason3 with three further samples also reported that they were unable to detect any production of ice-crystals at temperatures above −17° C This communication records the results of tests on the nucleating properties of a further thirteen samples of meteoritic materials. These comprised iron–nickel types (siderites), stony type (aerolites), plus two samples of the intermediate (siderolite) variety (all obtained from the American Meteorite Laboratory, Denver).
Ice‐nucleating properties of some natural mineral dusts
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.
Some recent airborne measurements of freezing nuclei over Southern England
Daily measurements of freezing-nucleus concentrations at 10,000 ft over southern England during January 1956 revealed no well-marked peaks. The results also indicated, however, that the earth's surface was not the main source of freezing nuclei.