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<br />FROM SILVER IODIDE TO RAIN DROPS <br /> <br />The object of cloud seeding for rain increase is to <br />improve the cloud's precipitation efficiency, thus <br />reducing the amount of water wasted into the upper <br />atmosphere. This can be done using common table salt, <br />ammonium nitrate, urea, dry ice, or silver iodide. <br />Silver iodide has been used exclusively in the South <br />Dakota Weather Modification Program. <br /> <br />Silver iodide particles are cagable of producing ice <br />crystals at temperatures of 28 F or colder. One ounce <br />will produce one hundred-trillion (100,000,000~000,000) <br />particles when vaporized by a cloud seeding generator. <br />Silver iodide has been used extensively because of the: <br />larger number of effective ice nuclei produced from a <br />small quantity of the material. One silver iodide <br />crystal per quart of cloud air (containing about 300,000 <br />minute cloud droplets) is generally adequate for rain <br />increase purposes. <br /> <br />A silver iodide crystal is geometrically very similar <br />to ice. When ice crystals occur with unfrozen water <br />droplets in colder than freezing parts of a cloud, the: <br />water vapor prefers to collect on the ice crystal. The <br />ice crystal grows through this process, called deposition, <br />until it reaches sufficient age to slowly fall through <br />the field of surrounding cloud droplets. While falling <br />it collects water droplets causing it to grow larger. <br />Finally the ice particle reaches the melting level in <br />the atmosphere, melts, and falls earthward as a rain- <br />drop. <br /> <br />39 <br />