An Evaluation of Hail Suppression Cloud Seeding Effects on Frequencies of Weather Phenomenan in Serbia
AbstractAn attempt is made to estimate the effect of hail suppression by silver iodide seeding in Serbia (eastern part of Yugoslavia), on hail-day frequency and the frequencies of other phenomena related to Cb clouds as observed at synoptic meteorological stations. The weather phenomena related to Cb clouds considered here include: hail, ice pellets, thunderstorms, gust winds, and amount of precipitation. The series of observations from 20 seeded and 20 non-seeded stations covers a 40-year period (1949-1988). The ratio of the average frequencies, in number of days in which these phenomena were observed during the seeding period, and the average frequencies observed during the non-seeding period, were calculated. Averaging was performed first over time for each station, and subsequently over 20 stations. A starting assumption on a positive correlation among frequencies of weather phenomena connected to Cb clouds was proven, as well as the correlation between the frequency of hail occurrences and the frequencies of the rest of the phenomena. The highest correlation coefficient r = 0.72 was found between frequencies of Cb clouds and the hail occurrences. The student’s test for difference between the average hail frequency during the seeding operations and the average frequency before these operations in the same territory shows a reduction in the hail frequency which was statistically significant. That is not the case with differences of mean values of the frequencies of other phenomena for the same periods. Such a difference between the mean hail frequencies in the surrounding areas which had no hail suppression programs for the same periods was statistically negligible. These results suggest that the 20 years of seeding operations in Serbia have had a positive effect.
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