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<br />Seeding Material and Delivery Techniques. When a seeding opportunity <br />is identified, it must be treated effectively to produce maximum <br />seeding effects. Since the bounds of an opportunity are finite <br />in both time and space, it is critical that the seeding system be <br />able to introduce the proper concentration of environmentally-safe <br />seeding material into the cloud at the appropriate time and place. <br /> <br />Weather modification research in the Colorado River Basin and other <br />regions has revealed the existence of a negative as well as a <br />positive opportunity. Therefore, the seeding system must be <br />capable of treating only the recognized opportunity for increasing <br />precipitation. This requirement precludes seeding from the surface <br />until selective targeting by ground generators can be perfected. <br />Recent observations of typical orographic storms show they are not <br />as simple as once thought and, in fact, are sufficiently complex to <br />preclude generalization about the transport of nuclei and their <br />destination. Moreover, it has been established that silver iodide <br />can persist in the atmosphere for days under certain conditions, <br />while seeding opportunities come and go in hours. <br /> <br />Consequently, the San Juan Operational Cloud Seeding Project will <br />use aircraft to accurately target seeding effects. Dry ice will <br />be used to avoid persistent effects. These techniques will insure <br />verification of the physical hypotheses and establish certainty of <br />effect in the shortest time. Aircraft seeding permits the widest <br />range of efficient operational designs. However, concurrent <br />transport and diffusion studies of surface generated seeding <br />material are strongly recommended in another subbasin to determine <br />whether the ground-based seeding and ta~geting questions can be <br />resolved. If so, ground-based seeding would be a more cost- <br />effective alternative. <br /> <br />Total Area Effects. "Extra area" effects are a definite public and <br />scientific concern. The public perceives a "robbing Peter to pay <br />Paul" problem inherently tied to the augmentation of precipitation <br />over a given area. Public meetings in western Colorado invariably <br />resulted in unequivocal requests for priority research to answer <br />extended area questions. Scientists also recognize that additional <br />information is essential to improve existing confidence levels about <br />downwind phenomena. <br /> <br />A recent workshop on Total Area Effects of Weather Modification <br />evaluated ~ posteriori analyses of past winter cloud seeding <br />projects. ~/ Results indicate that precipitation changes in <br />extended areas tend to be of the same sign (increase or decrease) <br />and generally the same magnitude as target area effects. There is <br />very little evidence that precipitation increases in one area will <br />result in corresponding and compensating decreases in downwind or <br />other areas. <br /> <br />V-7 <br />