Properties of Pyrotechnic Nucleants Used in Grossversuch IV

Abstract

The characteristics of the pyrotechnic nucleant Silverspare, containing only 2% by weight of silver iodide and produced in the USSR are investigated and compared with other widely used materials. It was found that it had a high ice nucleation effectiveness of 2.1013g?1 of AgI at ?10°C. The rate of nucleation is high so that ice crystal production practically ceases within two minutes after burn. Silver-spare aerosol has a pronounced bimodal size distribution with AgI particles in the small diameter mode at d = 0.025 ?m and KCI particles constituting the mode at d = 0.13 ?m. The large amount of hygroscopic material present, leads to a large fraction of active CCN in the smoke. It is shown that the contact nucleation mechanism is too slow to explain the high nucleation rate of J = 8.4 ? 10?3 s?1 for Silverspare and that the fast-acting deposition (sorption) and/or condensation-freezing mechanisms are dominant. As expected, the effectiveness per gram of PbI2 of the lead iodides Alfaplom and S-50 is lower by an order of magnitude. The pyrotechnics NEI-TB1 and the French Hailless had a much lower effectiveness in the ETH cloud chamber which uses a sugar solution with a sensitive time of ?100 s. This is shown to be due to considerably smaller rates of nucleation, because these particles act predominantly by contact nucleation with its characteristic time lag. The high effectiveness of Silverspare and the high rate of nucleation make this pyrotechnic well suited for hail suppression work and dynamic seeding which both call for overseeding a certain cloud volume in a short time. The deactivation of Silverspare smoke in contact with liquid water above ?5°C is considerable. It therefore should not be used for cloud-base seeding at warm temperatures.