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).
The Role of Natural and Anthropogenic Pollutants in Cloud and Precipitation Formation
Since the resurgence of interest in cloud and precipitation physics, stimulated by the demonstration in 1946 of the possibility of cloud modification by artificial nucleation, an ever-increasing amount of research has been reported in the literature on the mechanisms which underlie the formation of clouds and precipitation in the atmosphere. This research has essentially been carried out along two lines of study: studies of the cloud microphysical processes including the phase change of water substance in the atmosphere and the growth of the formed cloud particles to precipitation particles, and studies of the cloud macrophysical processes including all those dynamic processes which govern the formation, growth, dimensions, shape, organization, lifetime, and vigor of the cloud as a whole. In recent years, it has been realized that the cloud dynamical and cloud microphysical processes have to be viewed in a combined manner for a complete understanding of the formation of clouds and precipitation. Thus, much work is currently being done with this aim in mind.
Airborne measurements of the concentration of natural and artificial freezing nuclei
An aircraft-mounted instrument for measuring the concentration of freezing nuclei in the free atmosphere is described. It has been calibrated and its limitations investigated. 
 
Preliminary observations, in flight, of natural freezing nuclei were made. The temperature at which a given concentration of ice crystals formed on natural nuclei varied markedly from day to day, and increased with altitude. 
 
Measurements in flight of the concentration of freezing nuclei in silver-iodide smoke, released from a ground generator, are described. The nucleating properties of small particles produced by a hydrogen burner deteriorated very rapidly, while larger particles produced by a kerosene burner were more durable.