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Evaluation Aller. Watcr Y car 1990 Water Y car 2003 Seeding EfTect 0(%) +9.4 +3.3 90% Confidencc Intcrval Low (%) High (%) +6.0 + 13.0 -0.1 +6.8 Prob 0 ~O% 100.0 94.4 7. Preliminary ph)'sical analyses It is tentatively postulated that thc decrease in seeding effectivcness that started in about 1990 and, perhaps, the other significant changes in trend apparcnt in Figs. 3a and 3b may be duc to (a) a change in metcorological (seedability) conditions, and/or (b) a change in seeding operational procedurcs (seeding strategy and/or seeding agent fomlUlations It is beyond the scope of data currently available to this author to explore explanation (b). The following discussion will explore the feasibility of explanation (a), albcit in a preliminary way, The rawinsonde observations for Oakland, CA were assembled for each day of each year from 1951 to 2003. These data "'ere sorted to isolate the days with prccipitation in the Sierra Ncvada Mountains using the South Yoscmite Entrance (SYE) precipitation-mcasuring station, an unseeded area. to represcnt thc Sierra Ncvada Mountains in gcneral. The days of prccipitation probably represent days of sceding reasonably well although it likely includes somc days when seeding did not take place and excludes some days of seeding when no prccipitation occurred. Thc rawinsonde snundings for each of the days on which precipitation occurrcd wcre analyzed to producc parameters that might bc indicativc of seedability, such as precipitablc watcr over various depths of the atmosphere. the hcight of various temperature levels, thc temperaturc of various heightlcvcls and the wind speed and direction at 700 mb. All of the operational seeding programs rely on ground generators to deliver thc bulk of thc silver iodidc seeding agent and supplement it as conditions pemlit with aircrall sccding. Super (1999) has shown that the cloud's supercooled liquid water (SLW), the fuel for the seeding process, is usually concentrated near the mountainous terrain wherc the temperatures arc relatively wann and that high altitude releases of silver iodide from ground generators rcsult in a marked enhancement of ice crystals. When silver iodide was released from valley ground generators and it was transported to cloud levels, thc resulting ice crystal fomlation was usually too limited for significant snowfall enhancement. Thus. the key to successful seeding of such clouds is to produce the appropriate quantity of ice crystals in the right part of the cloud, the part containing the SLW, at the right time so the enhanced snow falls on the intcnded target Thus, it is necessary to release, from high altitudc ground gencrators, silver iodide chemical complexes that activate rapidly to producc rclatively high concentrations of ice crystals at relatively wann temperatures, Presumably. each of thc operational secding programs ha\'c tuned their seeding procedures (ground generator contiguf3tions and seeding chemicals) to the prcvailing topography and metcorology of their arcas of operation as evidenced by the success of the seeding operations up to about 1990 (sec Table 4). But what if thc meteorological conditions change and the seeding procedures are not adjusted accordingly? Sincc the _50C level rcprescnts the wannest temperature at whieh silver iodide will activate and create ice crystals. the height of thc _50C level was chosen for investigation, Figure 4 shows that the hcight of the _50C level during the Octobcr-March pcriod when most of the precipitation occurs has. on average. increased from about 9 I 60 ft in 1951 until 10,550 n in 2003, reaching an average height of about 10.200 n in 1990. This finding is consistent with the observation that the fraction of water year streamllow occuning in the October-March period has 44