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Created 12/26/06 3 <br /> practical to seed only the SLW periods because of these rapid natural changes.) The SLW is <br /> found predominantly over the windward slope of a mountain range, and often extends <br /> considerably upwind of the physical barrier (6, 7). The SLW decreases downwind of the <br /> mountain crest, or region of maximum lift, due to removal by precipitation and/or evaporation <br /> (7, 9, and 10). In the vertical the zone of maximum SLW generally extends from somewhat <br /> below the mountain crest to< 1 km above the mountain crest(8, 9, and 10). <br /> The temperature in the SLW zone varies considerably depending in large part on the <br /> height of the mountain barrier and its geographical location. This indicates, therefore, that the <br /> most appropriate seeding methodology can also vary considerably from one location to another <br /> from storm to storm, and even within the same storm. Studies in the Rocky Mountains have <br /> shown SLW cloud bases typically in the temperature range of-2° to -100 C (9, 10, and 11), with <br /> aircraft measurements and radiometer-inferred estimates indicating temperatures at the top of the <br /> SLW layer are generally -10' to -15' C (10, 11). A general finding was that SLW was abundant <br /> in clouds with tops warmer than about -22' C where natural snowfall was also found to be <br /> generally very light (7, 11). Lack of SLW was at times observed with colder cloud tops and <br /> higher natural snowfall rates, but SLW occurrence has also been observed during periods of <br /> moderate snowfall (9). In the Sierra Nevada cloud base is commonly above freezing and the top <br /> of SLW layer within 1 km of mountain top is generally -120 C or warmer (6, 8). <br /> Several studies calculated the total flux of SLW across a mountain barrier for a winter <br /> season and determined that the total SLW flux, if converted to precipitation, could increase the <br /> observed seasonal snowfall by 50-100% (5, 9, and 12). The overall conclusion of every study of <br /> SLW availability was that significant cloud seeding potential existed in winter storms over <br /> mountainous terrain provided the proper seeding technique could be applied at the appropriate <br /> time and location. <br /> Transport and Dispersion of Seeding Material <br /> In studying the effects of cloud seeding it is important to be able to verify the conceptual <br /> model, or what has come to be termed the cloud seeding "chain-of-events". All or portions of <br /> the chain-of-events have been documented by research studies in the Sierra Nevada of <br /> California, in the Rocky Mountains of Montana, Colorado and Utah, and in the mountains of <br /> northern Arizona. In the initial studies of the BRE in the 1970s (4, 13) steps 1-4 of the <br /> conceptual model were documented a high percentage of the time. With a combination of <br /> surface and upper air wind and temperature measurements, and documentation of seeding plume <br /> locations using an aircraft equipped with an ice nucleus counter, the authors indicated they <br /> "...believed that the BRE has some of the most convincing evidence of successful targeting <br /> obtained in a winter orographic program", and further stated that "data are quite consistent with <br /> the concept that Agl (the seeding material) was transported rapidly up the west slope of the <br /> Bridger Range, crossed the Main Ridge and moved toward the intended Bangtail Ridge target <br /> area In addition, the authors presented some of the first evidence of successful cloud seeding <br /> targeting using the trace chemical analysis of snowfall for silver content. They found that <br /> The current NAIWMC membership includes state agencies in <br /> North Dakota,Kansas,Oklahoma,Texas,Colorado,Wyoming,Utah,Nevada and California <br />