Evaluation of the South Dakota weather modification program

Evaluation of the past four years of the state sponsored South Dakota weather modification program was performed through the use of computer gridded monthly rainfall data, daily precipitation data, and crop-hail insurance data. Results indicate that over the four years of seeding the seeded counties received, overall, 6.7% more rainfall than would have been expected based on unseeded county precipitation. Rank testing with 31 years of gridded historical data give a significance level of 1.5% to the results. Analysis of daily rainfall data shows both a 5.4% increase in rainfall frequency and a 2.8% increase in rainfall intensity in the seeded counties. In addition to the rainfall results, crop-hail insurance data shows that the seeded counties received less hail damage than the unseeded counties with significance at the 3.4% level. It is postulated that the high significance levels of the findings are a result of the great number of clouds treated and the large size of the regions participating in the program.