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Ulrich Katz and Roland J. Pilié

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.

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Roland J. Pilié and James E. Jiusto

Abstract

Contrails were produced for laboratory study by burning aircraft fuels under controlled conditions of ambient temperature and humidity at pressure altitudes between 1000 and 300 mb. Observed critical formation temperatures differ from Appleman's theoretical data in a manner similar to that obtained on project CLOUD TRAIL flights. Laboratory experiments with these trails proved that the initial phase of the condensed moisture is liquid and produced strong evidence that, contrary to general belief, the final phase is sometimes liquid. Additional evidence was obtained indicating that Appleman's criterion for a barely visible trail (0.004 g per m3 of condensed moisture) is very nearly correct for ideal conditions of observation such as used in the laboratory, but is probably small by an order of magnitude or more for adverse conditions. By modifying Appleman's theory to allow for the production of a visible quantity of liquid water under adverse viewing conditions, agreement is reached with project CLOUD TRAIL data. Also presented is a simple interpretation of the theory which substantially reduces the labor required to compute critical temperatures for contrail formation.

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