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precipitation; the other 50 pct have more. Eleven storms had no precipitation during icing (these storms were included in the last category: >2X the duration of precipitation for computing averages). The average hours of icing per STORM as a pct of precipitation hours was 81 pct. Note that the percentage of hours of mountaintop icing to hours of precipitation are significantly lower than what was derived from the radiometer data (62 pct for icing detector vs 170 pct for radiometer). Although encompassing different winters, this observation suggests that substantial periods of SLW pass above the mountain peaks. It is not well known whether many of these hours of SLW suspended above mountaintop levels constitute a seeding potential because of the chance of intervening dry air between cloud base and the barrier. These results have a bearing on developing a seeding strategy for tapping this SLW. In summary, various methods of observing and quantifying SLW over the Sierra have produced a consistent picture. SLW, although highly variable, is observed during many storms passing the Sierra each winter. Several hundred hours of SLW occur per winter season, in both wet and dry winters. The highest SLW comes during lighter precipitation periods within a storm, generally when the cloud is rather shallow (1 to 2 km in depth). The SLW is concentrated within the first kilometer above the highest terrain, at low concentrations (0.1 to 0.2g/m-3), and at relatively warm supercooled temperatures (0 to -10 OC). 3.3 Exploratory Seeding Trials 3.3.1 Aerial Seeding The SCPP conducted numerous seeding experiments during its 10 yr of operation. Seeding was conducted almost exclusively from aircraft to assure seeding material could be placed directly into clouds. Two separate seeding experiments were conducted during SCPP. The Floating Target Experiment treated convective clouds developing over the foothills of the Sierra with the target area expected to be a box 15 to 45 min downwind from the treatment location. The Fixed Target Experiment treated widespread shallow orographic clouds, attempting to increase precipitation within a specific area along 1-80 where a sophisticated set of instrumentation had been set up to quantify the effects. The Floating Target Experiment seeded the rather weak convective clouds which develop along the Sierra Nevada foothills several hours after passage of a cold front. This experiment was also known by the name of SCPP-1 (Huggins and Rodi, 1985). A portion of this experiment included randomized seeding to assure unbiased analysis of the collected data. A total of 15 d satisfied experimental conditions. Of these 15, 9 were seeded. Results from these experiments showed that seeding produced substantially more ice crystals 15 min after seeding than in the nonseeded cases. Radar echoes developed earlier and precipitation was observed to fall from the seeded clouds earlier than the non-seeded cases. However, because of the limited areal coverage of these cloud types and their infrequent occurrence, seeding of these cloud types was expected to produce less than an additional 0.1 pct of the average annual runoff from the American River. The second and more important experiment was the Fixed Target Experiment, also known as SCPP-2. As has been discussed, radiometer and icing rate data suggested the shallow but widespread orographic clouds constituted the most seedable clouds during storm passage. 9