Precipitation Results from a Pyrotechnic Cumulus Seeding Experiment

Abstract

In an attempt to specify the changes in precipitation produced by alteration of cloud dynamics, airborne seeding with silver iodide pyrotechnics was carried out in South Florida during May 1968. Emphasis was placed on altering cloud dynamics and on increasing precipitation as a by-product of the dynamic alteration. Nineteen clouds were studied; 14 were seeded and 5 unseeded (controls) as dictated by the randomized seeding instructions. Each of the 14 clouds received approximately 1 kg of AgI smoke. Seeding was found to be effective in promoting increased cloud growth; the average growth difference between the seeded and control clouds was 11,400 ft, significant at the 0.5% level. The induced growths took many forms and in many cases were produced in clouds containing significant amounts of natural ice. A 10-cm radar with iso-echo contouring was used to infer changes in precipitation. Analysis indicates that seeding increased rainfall an average of 100?150 acre-ft 40 min after the seeding pass, an increase of over 100%. The result is changed little by using an alternate analysis scheme or by including five additional control clouds selected after the program. The rainfall increases would probably have been greater it calculations had been possible for entire cloud lifetimes. The significance of the rainfall results ranged between 5 and 20% based on two-sided statistical tests. Comparison between radar and raingage rainfall demonstrates that the rainfall calculations are probably underestimates by no more than 30%. The Z-R relation used in the rainfall calculations was equally valid for the seeded and control clouds. The amount of rain from the seeded clouds was positively correlated with the maximum top growth following seeding. The seeded rainfall increases were apparently the result of larger and more lasting clouds that were the by-product of the dynamic invigoration. The natural glaciating behavior of the experimental clouds would appear to preclude the ?colloidal instability? approach to rainfall augmentation from Florida cumuli.