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PROPANE CLOUD SEEDING EXPERIMENT OVERVIEW: WASATCH PLATEAU, UTAH, DURING WINTER OF 2003/04 by Drs. Arlin B. Super and James A. Heimbach, Jr. March 2005 Introduction The Utah Division of Water Resources, with primary support from the Bureau of Reclamation, conducted an experimental cloud seeding program on the Wasatch Plateau of central Utah during the 2003/04 winter. While most cloud seeding has used silver iodide (AgI) for decades, the goal of the Utah experiment was to demonstrate that liquid propane could also be used to seed winter mountain-induced (orographic) clouds to increase seasonal snowfall. Spring and early summer melting of mountain snowpacks is the major source for runoff and stream flow in most areas of the West, utilized by agriculture, municipalities, industry, and aquatic environments. The randomized propane seeding experiment was conducted from mid-December 2003 through March 2004. The experiment and substantial associated applied research is described in considerable detail in a final project report by Super and Heimbach (2005a)1. A shorter version, still containing much more information than provided in this document, can be found in Vol. 37 of the Journal of Weather Modification (Super and Heimbach 2005bf The current document is intended for water users and managers, and members ofthe general public who may desire a briefer and less technical overview of the 2003/04 winter project and its primary results. Accordingly, no equations are used and no references will be cited herein besides the two just noted. Readers interested in more detail, including references to considerable earlier work with propane seeding and related topics, are referred to Super and Heimbach (2005a, 2005b). I Super, A. B. and J. A. Heimbach, Jr., 2005a: Final Report on Utah cloud seeding experimentation using propane during the 2003/04 winter. Final Report from the Utah Division of Water Resources to the Bureau ofRec1amation, Financial Assistance Agreement Nos. 3-FC-81-0875 and 3- FC-81-0923, March 2005,110 pp. plus appendices. 2 Super, A. B. and J. A. Heimbach, Jr., 2005b: Randomized propane seeding experiment: Wasatch Plateau, Utah. Journal of Weather Modification, 37, 32 pp, Silver iodide (AgI), propane and some other substances allow the artificial stimulation of the glaciogenic process by nucleating ice crystals in supercooled clouds made up of tiny droplets in liquid form in spite of having temperatures below the freezing point of bulk water. Ice crystals grow at the expense of surrounding supercooled droplets due to water vapor transfer from droplets to crystals because the latter have a lower vapor pressure. After sufficient growth time, on the order of ten minutes, some larger crystals fall relative to the smaller particles, sweeping out other crystals, termed "aggregation," and/or droplets, called "accretion" or "riming," Previous work showed that propane seeding was effective in clearing supercooled liquid water (SL W) fog over airports. Similar seeding, by releasing liquid carbon dioxide from a moving vehicle, demonstrated widespread conversion of SL W fog to very light snowfall in the Salt Lake City area (Project Mountain Valley Sunshine). Prior laboratory and field experiments demonstrated that high concentrations of embryonic ice crystals result from propane release in SL W cloud or fog. Some brief case studies during the 1990's produced obvious: evidence that propane seeding resulted in light snowfall on the Wasatch Plateau and the Sierra Nevada of California, However, it is unreasonable to extrapolate the results of a few case studies to estimation of the seasonal impact over many storms. A randomized seeding experiment is considered the "gold standard" for statistical testing oflong-term seeding effects, if properly supported by physical observations and reasoning. The logical next stl~p in advancing the emerging technology of propane seeding of winter orographic clouds was to conduct a randomized experiment. This was done during the 2003/04 winter on the same part ofthe Wasatch Plateau used for considerable previous applied research during the 1990s. It was the logical experimental site because routine targeting of st:eding material and frequent existence of SL W cloud had already been demonstrated. Moreover, an all weather road access provided access to the plateau top making it practical to service instrumentation. ,*-r~..s,..i.:<':1