Seven Years of Weather Modification in Central and Southern Utah

A post-hoc statistical evaluation of an operational cloud seeding project, designed to enhance winter snowfall in the mountainous sections of central and southern Utah, was based on comparison between several control areas and multiple target areas. Linear regression equations were developed for each control-target area combination based on average January-March precipitation, 1956-1973; from 1974 through 1980 the target was seeded consistently during these months. Ratios of observed to calculated precipitation are presented for the target areas and several sub-target areas based on the predictions using the control area precipitation as predictor for the seeded years. Significance of the results was determined using a one-tailed Wilcoxon-Mann-Whitney ranking test.  On the basis of this evaluation it is concluded that seeding has been successful in increasing the January-March precipitation within the intended target areas over the seven year seeded period. The results vary depending on the control area, but all are positive. Indicated increases range from about eight percent to twenty-eight percent within the various target sub-sectors. Over the total Primary Target precipitation increases of between 13 and 20 percent are indicated. These results appear to be highly significant in most of the target areas. Some evidence of positive extra-area effects are noted "downwind" from the target, but with less statistical significance than the target area.