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<br />Orographic clouds in the mountainous western states are often associated with passing <br />storm systems. Wind flow over a mountain barrier causes the orographic lift to either produce <br />the cloud or enhance cloud developmcnt associatcd with a migratory feature sllch as a cold <br />frontal system. In silu and remote observations of SL W in orographic clouds (Reynolds. 1988) <br />have indicated significant periods of the occurrence of SL W with passing winter storms. These <br />studies have indicated that the preferred location for the formation of zones of SL W is over the <br />windward slopes of the mountain barriers at relatively low elevations (typically reaching only the <br />approximate height of the mountain barrier). Super. 1990. reporting on measurements ofSL \\' <br />observed in \vintcr research programs in the \'.:estern U.S. states that. "There is remarkable <br />similarity among rcsearch results from the various mountain ranges. In general. SLW is available <br />during at least portions of many storms. It is usually concentrated in the lower layers and <br />especially in shallow clouds with wann tops." Another quote from Super (1990) says: "The <br />tcndency for greatest SLW content near the wind\\:ard slopes ofa barrier is clearly shown by <br />Hobbs (1975) from a composite of22 aircraft missions over the Cascade Mountains. and by Hill <br />(1986) based upon 57 vibrating wire sondes over the Wasatch Mountains of Utah. 110lroyd and <br />Super (1984) examined data from many aircralt passes over the flat-topped Grand ~tcsa of <br />Colorado and showed that SL W was concentrated over the windv.'ard slope and barrier top. with <br />higher water contents nearer the surface." <br /> <br />The basic consideration in the development of the design ofa \\inter orographic cloud <br />seeding project is to develop a seeding methodology that will tap this reservoir of SL W to <br />convert water droplets into snowflakes that otherwise would be lost through evaporation ovcr the <br />downwind side of the barrier. In other words. \"..e wish to improve the efficiency of the natural <br />storm system in producing precipitation that reaches the ground. <br /> <br />"If SL W clouds upwind of and over mountain barriers are routinely seeded to produce <br />appropriate concentrations of seeding ice crystals. exceeding 10 to 20 pcr liter of cloudy air. <br />snowfall increases can be anticipated in the presencc or absence of natural snowfall. It has been <br />repcatedly demonstrated with physical observations that sulTiciently high concentrations of seeding <br />agent. effective at prevailing SLW cloud temperatures. will produce snowfall \vhen natural <br />snowfall rates are negligible. Seeded snowfall rates arc usually light. on the order of I mm/hr or <br />less. consistent with median natural snowfall rates in the intermountain West (Super and Holroyd. <br />1997):. <br /> <br />Figure 1 provides a schematic of how cloud seeding using ground generators on a <br />mountainous winter time program is thought to \\'ork. <br />