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<br />calculate how many nuclei should be added and what the resulting <br />precipitation rate will be. <br /> <br />Treatment. - The most common device for seeding orographic clouds <br />is the ground-based generator that burns a solution containing sil- <br />ver iodide. The generators are placed where the seeding material <br />will be released into air that will flow up the mountain and into <br />the cloud. They are not usually placed in valley locations because <br />temperature inversions common in winter trap the seeding material <br />near the ground in air that may never be lifted up the mountainside. <br />The number of generators and their location relative to the moun- <br />tain barrier are determined by the size of the target area and the <br />.expected range of windspeed and direction during seeding operations. <br />The amount of material released from the generators is intended to <br />provide ice crystals in concentrations of 10 to 100 per liter in the <br />cloud seeding effectiveness depends on the location of the gener- <br />ators relative to the target area and on the prevailing wind. <br />Therefore, the generator burn rate will vary from project to project <br />and, ideally, from cloud to cloud. Often, several rows of gener- <br />ators will be installed, with the more distant ones used with strong <br />winds and the nearer used with light winds. <br /> <br />There are several variations of ground-based silver iodide seeding. <br />Another conunon seeding device is the pyrotechnic flare. The com- <br />bustion material of the flare is solid rather than liquid, and the <br />chemical composition of the nuclei produced may be slightly differ- <br />ent, but the seeding material gets to the cloud in the same way. <br />Either the liquid-burning generator or the flare can be remotel)' <br />controlled from the project headquarters or manually operated. <br /> <br />Seeding material s other than silver iodide can be used with gr01l.lnd- <br />based generators. Lead diiodide has been used extensively in the <br />Soviet Union but is not used in the United States. Certain organic <br />materials have been tested, using ground-based releases, but thl~se <br />materials are not routinely used for seeding. <br /> <br />Orographic clouds also may be seeded from the air. Silver iodide <br />generators and pyrotechnic flares similar to the ground-based dl~vices <br />can be mounted on aircraft and the aircraft flown in or slightly <br />upwind of the clouds. As an alternative, pyrotechnic flares CIDl be <br />dropped into the clouds from above. These flares are designed to be <br />consumed completely as the flare falls down through the cloud. There <br />is little danger of the flare starting a fire on the ground or of <br />hitting something on the ground. Targeting the effects of seeding <br />can be more precise if aircraft are used, but the costs are higher. <br /> <br />3 <br />