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<br />Executive Summary <br /> <br />The feasibility of using artificial ice nucleation, commonly called "cloud seeding," is <br />considered as a means of increasing seasonal snowfalls from Colorado mountain-induced <br />(orographic) clouds. The report relies primarily on review of numerous scientific articles <br />published in the open, peer-reviewed technical literature, with some use of relevant internal <br />project reports. Where possible, emphasis is given to articles based on investigations of Colorado <br />and Utah orographic clouds, believed to be similar since the states adjoin with Utah mountains <br />generally upwind of Colorado mountains. Considerable winter orographic cloud research has <br />been conducted in both states. <br /> <br />Three main topics are considered. These are, first, the availability, abundance and locations of <br />supercooled liquid water (SL W) cloud which must exist for cloud seeding to enhance snowfall. <br />Second, the complex subject of how and where to release seeding agents in order to effectively <br />seed SL W cloud is discussed at length. Consideration of this topic relies heavily on observational <br />tracking studies of plumes of seeding agents and/or resulting seeded ice crystals, and of tracer gas <br />plumes. The stability of the lower atmosphere, especially at lower elevations within mountain <br />valleys, is given special attention because most winter orographic cloud seeding projects have <br />used valley-based silver iodide (AgI) generators. The stability of the lower atmosphere is critical <br />to whether or not such AgI is transported over intended mountain target areas thousands of feet <br />higher than the valleys. Finally, the suitability of the emerging technology of propane seeding for <br />Colorado clouds is addressed. Propane seeding has the advantage of producing large quantities of <br />seeded ice crystals in clouds only slightly colder than ooC but not cold enough for AgI to be an <br />effective ice nucleant. Such clouds, in the approximate temperature range of -I to -SoC, will <br />sometimes be referred to as "mildly supercooled." They are common over windward mountain <br />slopes in the western United States. <br /> <br />The effectiveness of AgI is dependant on temperature. Depending on the particular chemical <br />makeup of the AgI seeding agent, and whether it is released within or below SLW cloud, AgI can <br />become effective from about -6 to -SoC, with efficiency increasing as the temperature lowers. <br />After this "threshold temperature" is reached, at which a small but detectable fraction of a large <br />population of AgI ice nuclei create ice crystals, further cooling of the SL W cloud will result in <br />orders of magnitude more effective AgI ice nuclei until-l2OC or even colder. Obviously, SLW <br />cloud temperature is an important factor in AgI seeding effectiveness. Propane releases, on the <br />other hand, can produce abundant ice crystals as wann as -I oc with little temperature dependence <br />below -20C. A disadvantage of propane seeding is that it must be released within SL W cloud or <br />just beneath in ice saturation conditions. Consequently, high altitude remote-controlled <br />dispensers are required for propane seeding. <br /> <br />Supercooled liquid water cloud observations suggest that excess amounts are available <br />frequently enough to readily support the modest snowfall increases usually associated with <br />successful seeding, that is, approximately 10% on a seasonal basis. In other words, availability of <br />SL W cloud in excess of that naturally converted to snowfall does not appear to be a limitation for <br />successful winter orographic cloud seeding. The primary SL W cloud zone is over the windward <br />slopes and crests of mountains within about 3000 ft of the terrain. This is the obvious zone to <br />target with seeding agents and/or seeded crystals. Fortuitously, it is also the zone in which <br />ground-released seeding plumes are concentrated. <br /> <br />Aircraft seeding is not recommended for the majority of Colorado's mountains for a number of <br />reasons including high expense, difficulties in filling the volume of SL W cloud with seeding <br />