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<br />The final "camp" consists of those who are not necessarily convinced that seeding is effective <br />but regard that view as a reasonable possibility. This camp simply sees application as a sensible <br />bet when uncertainties and large potential gains are weighted against the relatively low cost of <br />application. They are comfortable applying current technology while welcoming any future <br />research which may provide improvements in application. <br /> <br />Some randomized experiments, considered to provide the "gold standard" for credible results <br />when supported by sufficient physical evidence (AMS 1998), have published results indicating <br />that seeding works during particular atmospheric conditions. The most well known randomized <br />experiments are likely the two conducted near Climax, Colorado, during the 1960s. But the <br />results from Climax and some other randomized experiments have been seriously challenged in <br />the scientific literature. It may be surprising to some, but in fact only a limited number of <br />randomized winter orographic experiments have been conducted in the western United States <br />since about 1970. The two best funded long-tenn experiments were the Colorado River <br />Augmentation Pilot Project (1970/71 through 1974/75 winters) and the Sierra Cooperative Pilot <br />Project (1977/78 through 1986/87 winters). While much was learned from these experiments, <br />neither was able to demonstrate that seeding had increased the seasonal snowfall over a target <br />area. Only a few randomized experiments have provided both strong statistical suggestions of <br />seeding success, and sufficient physical documentation like routine targeting of the seeding agent, <br />to still be considered credible. And none of these has been followed up with a confinnatory <br />experiment, necessary to provide "proof' in the sense this tenn is used by scientists and <br />statisticians. The reason for the lack offollow-up experimentation has usually been lack of <br />sustained funding. It is well known that the fields of weather modification in general, and winter <br />orographic cloud seeding in particular, have long suffered from lack of a long-tenn research <br />funding commitment by federal agencies (NRC 2003). <br /> <br />Metric units will be used for several variables in this report, especially when dealing with <br />cloud physics and temperatures. However, English units will be used for a number of others <br />(e.g., distances, elevations, wind speeds, precipitation rates) in order to make the discussion more <br />meaningful to the majority of readers who are likely to have a water resource or engineering <br />background. Sometimes both metric and English units will be given with one or the other in <br />parentheses. This report in its present state is not intended for the peer-reviewed scientific <br />literature which would require almost exclusive use of metric units. <br /> <br />A major intent of this report is to provide readers who have only a general interest in these <br />topics with a practical approach to assimilating the large amount of previously published <br />infonnation and resulting conclusions. Those with a limited knowledge of winter orographic <br />cloud seeding and its history are unlikely to be aware of the considerable research which has been <br />accomplished over many years at several locations. They are also unlikely to be aware of <br />important complexities and uncertainties in this field. <br /> <br />The report is organized in the following manner. Following the executive summary and table <br />of contents is this section, I, which is the introduction. Sections 2, 3 and 4 briefly summarize <br />overviews of the most important findings regarding SL W cloud characteristics, delivery of AgI to <br />SL W clouds, and propane seeding, respectively. The casual reader may find the executive <br />summary and the first four sections sufficient for his needs. <br /> <br />Sections 5, 6 and 7 provide more detailed discussion in support of the stated findings for the <br />respective topics of Secs. 2, 3 and 4. Numerous references are given to individual published <br />investigations supporting the results for each topic. It is important that stated technical findings <br />be based on the considerable body of past research. Otherwise, they may be simply opinions of <br /> <br />3 <br />