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Created 12/26/06 2 <br /> Cloud seeding programs with the purpose of increasing snowfall over mountainous <br /> terrain are based on results of research conducted over the past 40+ years, which has in large part <br /> been funded by federal agencies such as the U.S. Bureau of Reclamation and the National <br /> Oceanic and Atmospheric Administration. The research studies range from randomized <br /> statistical experiments conducted over entire drainage basins to highly detailed physical <br /> experiments conducted over the scale of individual seeding plumes from ground-based or aircraft <br /> seeding platforms. For cloud seeding programs conducted in mountainous regions of the western <br /> U. S. the following research results are the most applicable and have been used in the design of <br /> many of the ongoing operational snowfall augmentation programs in the U.S. and elsewhere. <br /> Cloud Seeding Conceptual Model <br /> As an introduction to the research results it is worthwhile to restate the generally accepted <br /> conceptual model for successful wintertime cloud seeding, which has evolved over the past 40 <br /> years, but is not substantially different now compared to its description in the design of the <br /> Bridger Range Experiment (BRE) in Montana (4). As stated, the model is based on seeding by <br /> an ice nucleant such as silver iodide (AgI) which has ice-forming capability at temperatures <br /> below about-5°C. The model is depicted as follows: <br /> 1)Seeding material must be successfully and reliably produced. <br /> 2) Seeding material must be transported into a region of cloud that has supercooled <br /> liquid water(SLW). <br /> 3) Seeding material must be dispersed sufficiently in the SLW cloud, so that a significant <br /> volume is affected by the desired concentration of ice nuclei (IN) and a significant <br /> number of ice crystals (ICs)are formed. <br /> 4) The temperature must be low enough (depends on seeding material used) for <br /> substantial ice crystal formation. <br /> 5)ICs formed by seeding must remain in an environment suitable for growth long enough <br /> to enable them to fall into the target area. <br /> Documenting the Chain of Events <br /> Availability of SLW for Cloud Seeding <br /> Before discussing experiments that have documented the steps in the conceptual model, it <br /> is important to note that numerous studies in mountainous regions of the western U. S. have <br /> examined the temporal and spatial availability of SLW and its temperature range. Successful <br /> cloud seeding depends on there being an excess of SLW in winter storms, and that the SLW <br /> exists at low enough temperatures for seeding material to be effective. Analysis of aircraft, <br /> microwave radiometer and mountain top icing measurements have shown some consistent SLW <br /> characteristics in wintertime storms. The more important ones are as follows. SLW is present at <br /> some stage of nearly every winter storm (5, 6), but also tends to exhibit considerable temporal <br /> and spatial variability (e.g. 5, 6, 8, 9, 11). A given storm passage may result in a number of <br /> SLW periods interspersed with other periods with none. (This implies that it is generally not <br /> The current NAI'WMC membership includes state agencies in <br /> North Dakota,Kansas,Oklahoma,Texas, Colorado,Wyoming,Utah,Nevada and California <br />