| description abstract | Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., ?lucky draws? or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan. Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north target area, and when the south target area was seeded, the appearance of decreases in rainfall due to seeding was produced. Target-control (as contrasted with crossover) evaluations of the second Israeli experiment for the north target area alone foundered when control stations were selected from a relatively small region of anomalously low seed/no-seed ratios that was situated within a much larger region of high seed/no-seed ratios, which included Lebanon, Jordan, and most of Israel. Thus, the north target area seed/no-seed ratios are not an isolated, seeding-induced anomaly. On the contrary, it is the low seed/no-seed ratios of the northern coastal control stations, selected after the experiment began, that are anomalous in a regional context and are virtually the only stations that yield an apparently statistically significant effect due to seeding in the north target area. It is concluded that neither of the Israeli experiments demonstrated statistically significant effects on rainfall due to seeding. Considerations of the rainfall climatology of Israel, recent reports concerning the microstructure of clouds in Israel, aid the relatively small amount of seeding carried out in the first Israeli experiment support the view that seeding was unlikely to have had significant effects on rainfall. Contrary to previous reports, clouds in Israel contain large cloud droplets, precipitation-sized drops, and considerable concentrations of natural ice particles at quite high temperatures, all of which should obviate attempts to increase rainfall by artificial seeding in wintertime air masses. | |
