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<br />immediately fonning ice crystals when temperatures were below -6oe as suggested by Finnegan <br />and Pitter (1988). It is probably not coincidence that seeding site temperatures of -6oe <br />correspond to a ridgeline temperature near _9oe for in-cloud lapse rates. <br /> <br />With the exception of in-cloud releases, the authors are unaware of solid field evidence <br />showing AgI cloud seeding to be effective with SL W cloud temperatures wanner than about -8 or <br />-9oe. This should not be surprising once calculations take into account typical generator outputs, <br />along-the-wind spacing and horizontal and vertical transport and dispersion of AgI plumes from <br />generators to the target area. It has been shown that seeded crystal concentrations need to exceed <br />about 20 L-1 over the target to result in more than trace snowfall rates as far back as the classic <br />paper by Ludlam (1955) and as recently as Super and Heimbach (2005a). <br /> <br />4) There is an obvious large "temperature window" from ooe to at least -6oe (for forced <br />condensation freezing) and, more commonly, about -8oe, that cannot be effectively seeded with <br />conventional fonnulations of AgI and typical generator networks. Yet mildly supercooled cloud, <br />wanner than -6 or -8oe, frequently exists in the primary SL W zone near the windward slope and <br />crestlines of mountains throughout the West. Of course, the frequency of mildly SLW cloud <br />varies with geographic location and altitude. The frequency ofwann temperatures was found to <br />be so high over the northern Sierra Nevada of California that a new winter orographic seeding <br />method was developed (Reynolds 1989, 1996). The new approach adapted propane dispensers, <br />previously used for SL W fog suppression over airports, for use in high mountain terrain by <br />configuring them to be remotely controlled by radio. Expansion of liquid propane causes <br />homogeneous nucleation I by local chilling of the air below -40oe (equal to -400F). Reynolds <br />(1996) noted that the temperature of SL W in the Sierras, detennined from mountain top icing <br />meters, was warmer than _4oe about 80% of the time. Since ground-released AgI seeding plumes <br />are usually concentrated within about 2000 ft (600 m) of the terrain, AgI plume tops would not be <br />cold enough (-80C) for effective seeding about 80% of the time. Even this value is an <br />overestimate because seeded crystals need to be formed some distance upwind of the mountain <br />top, as will be discussed. Propane has been shown to be an effective seeding agent at <br />temperatures slightly below ooe, and has little temperature dependence at temperatures colder <br />than -2oe according to the laboratory and field investigations of Hicks and Vali (1973). <br /> <br />The ability of propane seeding to create meaningful snowfall when SLW cloud is too warm <br />for AgI effectiveness is well-demonstrated by Fig. 1, taken from Super and Holroyd (1997). An <br />extended period with SL W available but very limited natural snowfall, until the end of the <br />experiment, was first seeded with propane and later with AgI, both released from the same high <br />altitude site. The large increase in seeded crystals is obvious during the propane seeding, but AgI <br />seeding was ineffective. The mountain top sampling location temperature was near -4oe during <br />the entire period. <br /> <br />5) A small-scale randomized propane seeding experiment was conducted in central Utah <br />during the 2003/04 winter described in detail by Super and Heimbach (2005a, 2005b). Propane <br />was released from the same high altitude location used in Fig. 1, known to routinely pass over the <br />Wasatch Plateau by the same mountain-top observatory used in constructing that figure. Many <br />prior plume tracking experiments verified the routine transport and dispersion during seeding site <br /> <br />I Homogeneous nucleation occurs when water droplets freeze without the presence of foreign material. <br />This will occur spontaneously at temperatures below about -40oC. Expansion of liquid propane produces <br />temperatures well below -40oC in a small volume just downstream of the spray nozzle. Heterogeneous <br />nucleation of ice requires a foreign surface or substrate for water molecules to collect. Silver iodide is <br />especially effective because it has a crystalline structure nearly identical to that of ice. <br /> <br />9 <br />