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This book reviews experiments and operations designed to enhance rainfall or suppress hail. Widely varying and often disappointing results have been obtained and in the case of rainfall enhancement appear to have been partly, or even mainly, due to persistent after-effects on rainfall of silver iodide used to increase atmospheric concentrations of ice-forming particles. A hypothesis to explain this phenomenon is advanced that involves interaction of silver iodide with micro-organisms having ice-forming activity. Historical comparisons of rainfall in large areas surrounding three cloud-seeding operations suggest that rainfall was only slightly increased in the targeted areas but greatly increased downwind. A modified experimental design might in future lead to much greater success. A feedback between rainfall and airborne particles of microbiological origin provides a further complication to weather modification attempts. It suggests that the concentration of airborne natural cloud-active particles is often delicately balanced between increasing and decreasing rainfall.
