An Investigation of the Relative Importance of Vapor Deposition and Contact Nucleation in Cloud Seeding with AgI

Abstract

In order to determine the relative frequency of contact and deposition nucleation in AgI seeded supercooled clouds, laboratory experiments were carried out in which the supercooled cloud drops were tagged with insoluble fluorescent particles. Ice crystals resulting from seeding this cloud with AgI were examined microscopically to locate fluorescent particles. Presence of such a particle in the center of an ice crystal was interpreted as evidence that freezing of a drop through contact nucleation initiated crystal formation. Absence of a tracer particle from an ice crystal center was considered to be indicative of crystals formed by the vapor deposition mechanism. Evaluation of the experimental data (limited, so far, to the temperature range of ?15 to ?17C) indicate that only 2?4% of the ice-forming AgI particles in the 100?500 Å diameter range act as contact nuclei. Several auxiliary experiments were devised to show the validity of the experimental methods employed. The results are discussed in view of drop-nuclei collision efficiencies and other investigators' findings.