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<br />Appendix B <br /> <br />Uncertainties of Weather Modification for Precipitation Enhancement <br />(Excerpts from the NRC Report) <br /> <br />Although WxMod research has declined significantly since the 19708, much progress has <br />been made in other associated fields such as remote sensing, more accurate and higher <br />resolution precipitation measurement, three dimensional depictions of the structure, <br />airflow, the hydrometeor composition of clouds before and after seeding, computer <br />modeling, Doppler radars such as Next Generation Radar or NEXRAD. There are stilI, <br />however, many areas of uncertainty. <br /> <br />Reliable Data. No complete and rigorous comprehensive study has been made of any <br />precipitation enhancement projects. Part of the reason is the difficulty in locating <br />unaffected control basins for the standard target and nearby control area comparisons <br />since wind variations would cause spillover into adjoining basins. Some studies of <br />individual projects have been made in the past years on certain projects, which have <br />shown increases in water. <br /> <br />The basic properties of clouds make it difficult to keep track of seeded units and to <br />replicate the treatment of successive trials. No two clouds lire ideritical, and clouds are <br />not independent of one another. <br /> <br />Ooerational Precision. It is difficult to target seeding materials to the right place in the <br />clouds at the right time. There is an incomplete understanding of how effective operators <br />are in their targeting practices, since they depend on transport by complex wind patterns <br />and atmospheric mixing to dilute the seeding material (see below). Chemical tracer <br />experiments provided support for current targeting practices. There is also the serious <br />challenge of measurement uncertainties, which complicate separation of the effects of <br />cloud seeding from natural variability. <br /> <br />Concern over Potential Impacts. Questions about potential unintended impacts from <br />precipitation enhancement have been raised and studied over the years by Reclamation <br />and others. Concerns relate to downwind effects (enhancing precipitation in one area at <br />the expense of a downwind area, or "robbing Peter to pay Paul"), long-term toxic effects <br />of silver iodide, and added snow removal costs. In the Project Skywater Progranunatic <br />Environmental Statement (1977) and the Sierra Cooperative Pilot Project Environmental <br />Impact Statement (1981), available evidence did not show that seeding clouds caused a <br />decrease in downwind precipitation; in fact there were increases for up to 100 miles <br />downwind. The potential effects of silver have not been shown to be a problem, as silver <br />and silver compounds have a low order of both acute and chronic toxicity. Industry emits <br /> <br />-1- <br />