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<br />nature may be partially or totally inefficient in ice crystal and snowfall production. Seeding when <br />nature is producing abundant ice crystal concentrations may not be effective, but there is no <br />convincing evidence that it decreases snowfall with the possible exception of some deliberate <br />attempts to overseed in order to redistribute snowfall patterns (e.g., Hobbs 1975b). Detailed <br />discussion of how the formation of seeded ice crystals within SLW cloud can enhance mountain <br />snowfall is given in several sources (e.g., Dennis 1980), and will not be repeated here. A facts <br />brochure entitled "Weather Modification in the United States" presents the basics of cloud <br />seeding. It is available from the Weather Modification Association and can be downloaded from: <br />www.weathermodification.org/facts.htm <br /> <br />To design an optimum winter orographic seeding project, the following information is desired. <br />How frequently does SLW exist, where and in what amounts? Knowledge of the frequency and <br />amount of excess SL W, not naturally converted to snowfall, is important for estimation of the <br />maximum potential snowfall production by seeding. In other words, does SL W exist often <br />enough and in sufficient amounts that seeding has a reasonable chance of producing meaningful <br />additional snowfall? It is obviously important to know the spatial distribution of SL W, and its <br />vertical temperature distribution, when considering what seeding approaches to apply. Will <br />ground-based releases be routinely transported into the SL W cloud or is aircraft seeding required? <br />Is the SL W zone cold enough for AgI seeding to be effective or should an alternative agent like <br />propane be considered? Similarly, if airborne seeding is required, should seeding be done with <br />acetone generators, pyrotechnic flares or dry ice pellets? <br /> <br />The common operational practices of simply assuming SL W will often be present, and will be <br />effectively seeded by deploying a conveniently-accessed ground generator network can no longer <br />be justified in light of current knowledge. Such simplistic approaches do minimize costs. <br />However, there is little physical evidence, or credible statistical evidence, to support typical <br />claims of about 10% increases in precipitation through the indiscriminate application of valley- <br />based ground generators. <br /> <br />The availability of SL W cloud in excess of that converted by natural processes to snowfall has <br />long been assumed for cloud seeding projects. The classic paper by Ludlam (1955) described <br />extensive low clouds over the mountains of central Sweden, no more than several hundred meters <br />thick, which were composed of SL W cloud droplets without snowfall production. But most <br />orographic clouds in the Intermountain West have higher, colder tops and are composed of <br />varying concentrations of both droplets and ice crystals. In Colorado mountain clouds, typical <br />natural ice crystal concentrations are in the 10 to 100 per liter range while tiny cloud droplets <br />often range from 100,000 to 300,000 per liter (100 to 300 cm-3). Conversion from SL W to snow <br />requires that vast numbers of droplets combine to make a single snowflake. Introduction of <br />relatively few ice crystals into a SL W cloud can initiate snowfall by one or more of three <br />processes: vapor deposition on the crystals because of the greater saturation vapor pressure over <br />liquid than over ice at the same temperature; liquid deposition on the larger crystals (often called <br />"riming" or "accretion") as they fall through and collide with numerous droplets which freeze on <br />the crystals; and aggregation (chaining together) of many crystals into a larger snowflake. More <br />details can be found in Dennis (1980) and other sources. <br /> <br />The large majority of operational and experimental seeding projects have been conducted <br />without routine evidence ofSLW availability. That is, it was assumed but not documented that <br />SL W was often present, in large part because of instrumentation limitations. Prior to about 1980, <br />SL W orographic cloud was manually sampled on an intermittent basis at a few western mountain <br />observatories. Some aircraft sampling was done, usually at altitudes more than 3000 ft above <br />mountain barriers for reasons of safety. The presence of SL W was inferred from observations of <br /> <br />12 <br />