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<br />"1: <br /> <br />.. <br /> <br />""!- ';! <br /> <br />STATUS OF PRECIPITATION AUGMENTATION <br />AND HAIL SUPPRESSION EXPERIMENTS <br /> <br />by <br /> <br />A. S. DENNIS <br />u. S. Bureau of Reclamation <br />Denver CO 80225 <br /> <br />;1 <br /> <br />Prepared for Presentation at Technical Session on <br />Factors Influencing Responses to <br />Deliberate and Inadvertent Weather Modification <br /> <br />AAAS Annual Meeting <br />New Orleans, Louisiana <br />February 20, 1990 <br /> <br />Abstract <br /> <br />National and international scientific bodies have concluded that cloud seeding affects precipitation <br />at the ground, but the great variability of natural precipitation and variations in seeding response <br />make it difficult to measure seeding effects. New atmospheric sensors and computer mQdeling of <br />clouds are being brought to bear to reduce the scientific uncertainties involved. Winter orographic <br />clouds are often seeded with silver iodide or other ice-forming agents to increase their precipitation <br />efficiency; statistical analyses of some such projects indicate increases of the order of 10 percent <br />in snowpack and runoff. The presence of supercooled liquid water is considered an indicator of <br />seedability in orographic clouds; it is observed most frequently in shallow clouds with top <br />temperatures warmer than about _12oe. Seeding convective clouds with supercooled tops can alter <br />their dynamics, producing large changes in rainfall from individual clouds, but resultant increases <br />in rainfall over predetermined target areas have not been scientifically established. The world's <br />best known randomized experiments to test effects of silver iodide seeding on hailstorms were <br />inconclusive, and future hail suppression experiments promise to be very complicated and therefore <br />costly. Nevertheless, operational hail suppression projects continue in many countries on the basis <br />of substantial apparent reductions in crop-hail damage associated with those projects. <br /> <br />1. Introduction <br /> <br />Both the World Meteorological Organization (WMO, 1982) and the American Meteorological <br />Society (AMS, 1984) have adopted statements affirming that, in some situations, cloud seeding <br />increases precipitation at the ground. Their conclusions about precipitation augmentation were <br /> <br />1 <br />