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<br />agent or resulting seeded crystals, and operational problems in an icing environment with the <br />associated question of safety. <br /> <br />Use of valley or foothill AgI generators is the least expensive and most widely used seeding <br />method. However, there are serious concerns about how frequently this approach actually <br />produces sufficiently high seeded crystal concentrations to result in meaningful snowfall rates. <br />Some storm phases are unstable and appear capable of providing the necessary vertical transport <br />over mountain barriers. However, valley-based inversions appear to be common during many <br />Colorado winter stonns which will trap and pool the AgI below the SL W cloud zone. Therefore, <br />valley seeding will not provide routine seeding but only unpredictable intennittent seeding when <br />SL W is available. Moreover, operational seeding projects which use valley generators commonly <br />have overly large crosswind distances between generators, resulting in large un seeded gaps <br />between seeding plumes when vertical transport is sufficient. Generator outputs may also be too <br />low to result in sufficiently high concentrations of seeded crystals. This is of a particular problem <br />of special concern with relatively warm SLW cloud because of the large temperature dependence <br />of AgI effectiveness. <br /> <br />Silver-in-snow concentrations have not exceeded natural background levels in a number of <br />operational project target areas which used valley seeding. Failure to increase silver <br />concentrations can be interpreted to mean failure to target with enough silver iodide for effective <br />seeding. It is acknowledged that relatively few operational projects using AgI have been sampled <br />for silver-in-snow levels wherever AgI generators were located. For that matter, relatively few <br />operational projects have provided any physical evidence of successful seeding. <br /> <br />It is well documented that high altitude releases of either AgI or propane, from sites more than <br />midway from the valley floor to the mountain crest, result in routine transport of AgI and/or <br />seeded crystals over mountain barriers. This is the much preferred method for seeding the <br />primary SL W zone over the windward slopes and crestlines. The ability of high altitude seeding <br />sites to routinely seed orographic cloud cannot be overemphasized. Plume widths are typically <br />20-25 degrees wide with plume tops about 2000 ft above crestline elevations. Depending upon <br />local terrain, high altitude seeding sites should have a crosswind spacing of about 2-3 miles to <br />achieve plume overlap before the crestline is reached, assuming such seeding sites are located on <br />minor ridges extending a few miles upwind of primary crestlines. High altitude seeding is <br />admittedly a more expensive and logistically difficult approach than siting and using manual <br />generators only where roads exist in mountain valleys. However, high altitude releases can be <br />counted on to actually seed the clouds whenever SL W is present. Silver-in-snow concentrations <br />from a number of experiments and projects which used high elevation AgI releases have been <br />well above natural background levels. This finding does not prove that seeding actually increased <br />the snowfall because scavenging by natural snowflakes can bring AgI particles to the surface. <br />However, increased silver-in-snow levels do document that AgI plumes were transported over the <br />intended target area, a necessary step for seeding to have any chance of snowfall enhancement. <br /> <br />Supercooled liquid water temperatures have been observed at a number of mountain-top <br />locations in Colorado and Utah. The observations indicate AgI seeding can be effective some <br />portion of the time but that SL W temperatures are too wann for AgI seeding during other periods. <br />However, it would be more appropriate to consider SL W temperatures some distance downslope, <br />where seeded crystals should be produced to provide sufficient crystal growth times before the lee <br />subsidence region is reached where cloud droplets rapidly evaporate. But windward slope <br />observations have not been common. Consequently, it should be recognized that use of <br />mountain-top temperatures may overestimate the frequency of AgI seeding potential. <br /> <br />ii <br />