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PROPA:\'E CLOUD SEEDI:\'G EXPERI.\IE:\'T OVER\'IEW: WASATCII Pl.ATEAU.UTAII. DURI~G WI:"TER OF 2003/0-& h) Drs. Arlin B. Super and James A. Heimbach. J.... Introdu{"tion The Utah Division of Water Resources, with primary support from the Bureau of Red am at ion, conducted an experimental cloud seeding program on the Wasatch Plateau of central Utah during the 2003/04 winter. While most cloud sel'ding has used silver iodide (AgO for decades. the goal ohhe Utah experiment was to demonstrate that liquid propane could also be used to seed wimer mountain-induced (orographic) clouds to increase seasonal snowfal1. Spring and early summer melting of mountain snowpacks is the major source for runolT and stream flow in most areas of the West. utilized by agriculture. municipalities. industry, and aquatic environmems. The randomized prormne seeding experiment was conducted from mid, December 2003 through \Iarch 2004, The experiment and substantial associ.lIl'd applied research is described in considerable dl'tail in a tinal projlX"t report by Super and Ileimbach (2005al1. A shorter ve:-rsion. still containing much more infonnation than providl'd in this document, can be found in Vol. 37 of the Journal of Weather \lodification (Super and lleimbach 2005bl~. The current document is intended for waler users and managers, and members of the general public who may desire a briefer and lc-ss technical o\ervkw of the 2003,04 winter project and its primal)' results. Accordingly. no equations are used and no references wilt be cited herein bc-sides the tv..o just noted. Readers intereste:-d in more detail. including referencc-s to considerable:- earlie:-r work with propane seeding and relatl.d topics, are referred to Super and Heimbach (2005a, 2005b). t Supo:r. A. B. and J. A. HeImbach. Jr.. 2oo5a: Final Rcpon on L'lah cloud !>Cl>Jing e\penmenlallon uSing propane Jurmg the 200J'().t Winl<.'f. Final Repon from the L:lah Di\ i~lon of Waler Ro:sourccs to the Bureau of Redamallon, Financial A~mtance Agrecment1'\os. 3.FC-lI.I -Oll.75 and 3- FC-lI.I-092J. \1arch 2005, 110 pp. plus appendIces. 2 Super. :\. B. .l.nd 1. A. HeImbach. Jr" ~005b: Randomized propane j;(.-edmg e'penment: Wasatch Plateau. Utah. Jvurnal vlWe<llher .\fodijir:uIlQn. 37. 32 pp. .\larch 2005 Silver iodide (Ag!), propane and some other substances allow the artificial stimulation of the glacioge:-nic process by nucleating ice crystals in supercooled clouds made up of tiny droplets in liquid fonn in spite ofha....ing temperatures below the freezing point of bulk water. lee crystals grow at the expense of surrounding supercooled droplets due to water vapor transfer from droplets to crystals because the laller have a lower vapor prc-ssure. After sufficient growth time, on the order of ten minute-s, some larger crystals fall relative to the smaller panicks. sweeping out othe:-r crystals, termed "aggregation," andlor drop1eb.. called "accretion" or "riming." Previous work showed that propane seeding was effecti\'e in clearing supercooled liquid water (SL Wj fog o\er airports. Similar s~ding, by releasing liquid carbon dioxide from a moving vehicle, demonstrated widespread conversion of SL W fog to very light snowtafl in the Salt Lake City area (Project ~lountain Valley Sunshine). Prior laboratory and field experiments demonstrated that high concentrations of embryonic ice crystals result from propane release in SL \V cloud or fog. Some brief case studies during the 199O's produced obvi~)us evidence that propane seeding resulll-d in light snowfall on the Wasatch Plateau and the Sie:-rra Nevada of California. lIowever, it is unreasonable to extrapolate the re:-sults ofa few case' studies to estimation of the seasonal impact over many stom15. A randomizl-d seeding experiml-nt is considered the "gold standard" for statistical testing oflong.tenn seeding effects, if properly supponed by physical observations and reasoning. The logical next step in advancing the emerging technology of propane seeding of winter orographic clouds was to conduct a randomized experimem. This was done during the:- 200)/{)4 "imeron the same part of the \Vasatch Plateau u5e:-d for considerable previous applied research during the 1990s. It was the logical experimental site because routine targcting of seeding mate:-rial and frequem exiStence of SL \V cloud had already been demonstrated. \lore-over. an all weathe:-r road access provide:-d access to the plateau top making it practical 10 sen'ice inslrumentalion.