Cloud seeding operations using dry ice dispensed from aircraft have been carried out by the California Electric Power Company in the High Sierra near Bishop, Calif., since February 1948. Limited visual observations suggest that snow showers can be produced from non-precipitating orographic clouds by seeding, the snow reaching the ground at the higher elevations. Comparison of the annual runoff from the seeded watershed with that from adjacent areas for the three years 1948, 1949 and 1950 showed a positive departure in the 1948–49 season significant at the 1% level. Significant departures did not appear during the other two years, but the average for the three years was significant at the 5% level. From the same analysis it might be estimated that the average annual flow during the three-year period was augmented by 9% with the 90% confidence limits being zero and 18%. On the other hand the unusual circulation prevailing during the 1948–49 winter may have been responsible, at least in part, for the departure in flow during this season. Snowpack comparisons did not indicate a significant increase in the Bishop area, but it is shown that such a result is not incompatible with the possibility that additional precipitation was produced which was not reflected in the snow surveys.
* Summary of complete report published as United States Weather Bureau Research Paper No. 36. Washington, D. C.
