ICE NUCLEATION BY SILVER IODIDE: I. FREEZING VS SUBLIMATION

Abstract

The ability of silver-iodide particles to act as ice nuclei has been studied under controlled conditions of temperature and humidity. Particles up to 100Å in radius are largely ineffective as ice nuclei unless the humidity, relative to water, exceeds 110 per cent. Consequently, in natural clouds where the relative humidity rarely exceeds 101 per cent, such particles will only be useful if they collide with cloud droplets. An apparatus has been developed which allows the number of ice nuclei in an aerosol to be counted with a probable error of less than 10 per cent. The particular advantage of this apparatus is its continuity of operation, which enables one to count either a rapid succession of aerosol samples or, if the fall-out is prolonged, to follow the variation of fall-out with time. The delayed appearance of ice crystals in a cold chamber is not due to a time lag in the nucleation process but to the time required for aerosol particles to collide with cloud droplets or to migrate to more humid regions of the cloud chamber where condensation can take place on each particle. The activity, as a freezing nucleus, of the silver iodide described herein is much greater than that found by previous authors, particles of radius 75Å being active at ?8C.