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Results from research programs conducted on summertime cumulus <br /> clouds are encouraging but somewhat variable. Part of the resulting uncertainty <br /> is due to the variety of climatological and microphysical settings in which <br /> experimentation has been conducted. Other important factors include the spatial <br /> scale at which the investigations are conducted and the seeding mode. Projects <br /> which relied upon introduction of glaciogenic seeding material targeted for <br /> specific clouds or portions of clouds that met certain criteria (based essentially <br /> upon the stage of development of the clouds) have generally indicated positive <br /> seeding effects, ranging between 50% and 100% for individual clouds and on the <br /> order of 50% for clusters of convective clouds. <br /> Evaluations of operationally conducted summer precipitation <br /> augmentation programs present a difficult problem due to their non-randomized <br /> nature and the normally large temporal and spatial variability present in <br /> summertime rainfall. Recognizing these evaluation limitations, various methods <br /> for the evaluation of such programs have been developed and used, ranging in <br /> scale from individual clouds to floating targets of varying sizes to area-wide <br /> analyses. The results of many of these evaluations, at the single cloud scale <br /> through floating target areas up to 1,700 km2 have indicated a positive seeding <br /> effect in precipitation. Area-wide effects can be more difficult to discern due to <br /> the large temporal and spatial variability in summertime rainfall noted earlier. In <br /> some instances, apparent positive effects of seeding have also been noted <br /> outside the specific targets. Thus, the apparent effect of seeding is not <br /> necessarily confined to the directly-treated clouds. The physical mechanisms <br /> leading to those effects outside the directly-treated clouds are not yet fully <br /> understood. <br /> Technological advances have aided summer precipitation augmentation <br /> programs. These include fast-acting silver iodide ice nuclei, new hygroscopic <br /> seeding formulations, sophisticated radar and satellite data processing and <br /> analysis capabilities, airborne cloud physics instrumentation and continued <br /> improvements in numerical modeling. <br /> Hail Suppression <br /> The capability to suppress damaging hail continues to improve. Attracted <br /> by potentially large benefit-to-cost ratios, many countries are conducting <br /> programs where hailstorms are seeded to reduce the damage caused by hail. <br /> While there are a number of concepts regarding the formation and mitigation of <br /> hail, the most common treatment method for hail suppression involves the <br /> addition of high concentrations of ice nuclei (usually silver iodide smoke particles) <br /> into the new growth regions of storms from aircraft or ground-based sources <br /> to manipulate the hail embryo formation process and thus limit the growth of <br /> hailstones. <br />