Downwind and upwind effects in the Arizona cloud-seeding experiment

The principal subject of this report is a comparison of precipitation on days with seeding with that without seeding, averaged over those rain gauges that on each particular day were "downwind," "upwind," or to the sides. Two estimates of relevant wind directions are used, based on successive radiosondes at Tucson that bracketed the scheduled time of seeding. By use of these radiosondes, the apparent effects of seeding on rain in downwind localities 90-180 miles (145-290 km) away from target were found to be an apparent 45% loss of rain (P = 0.002) and an apparent 34% loss of rain (P = 0.028), respectively. Other results indicate considerable geographic heterogeneity.