The second phase of the Cloud Physics Project was carried on in the vicinity of Wilmington, Ohio, during the spring and summer of 1948. The organization, facilities, and general mode of attack were the same as those used in the first phase of the project. In this second phase of the study, the basic objective was to determine in definite quantitative terms the practical limits and economic importance of cloud modification processes in producing precipitation from cumuliform clouds.

The basic objective of this Project was to determine in definite quantitative terms the practical limits and general utility of cloud modification processes in producing or suppressing precipitation and increasing the visibility from flying aircraft. In order that the effects of the induced modifications may be clearly separated from those occurring naturally, a closely coordinated attack using all available measuring facilities was adopted.

In the summary, suggestions are made relative to the inclusive of the theory discussed in this paper in the day-to-day analysis of the weather. Indications are also given for the extension of the hydraulic analogy beyond the point covered in this paper.

Cloud seeding operations using dry ice dispensed from aircraft have been carried out by the California Electric Power Co. 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 3 years 1948, 1949, and 1950 showed a positive departure in the 1948-49 season significant at the 1 percent level. Significant departures did not appear during the other two years, but the average for the 3 years was significant at the 5 percent level. From the same analysis it might be estimated that the average annual flow during the 3-year period was augmented by 9 percent, with 90 percent confidence limits being zero and 18 percent. 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 the season. Snow pack comparison 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.