Possible Influence of Desert Dust on Seedability of Clouds in Israel

Abstract

Experimental and operational cloud seeding in Israel has been conducted since 1961 by aircraft seeding of silver iodide (AgI) at cloud-base level along a line upwind of target areas. The first experiment, Israeli-1 (1961?67), had a crossover design. Its overall seeding effect was an enhancement of 15% of the rainfall in the target areas, significant at 0.9%. The second experiment, Israeli-2 (1969?75), had a generally similar crossover statistical design to that of Israeli-1 but with some modifications that allowed a separate evaluation of the north target area alone. The seeding effect obtained for the north alone was 13%, significant at 2.8%. Based on this, clouds in northern Israel have been seeded operationally since 1975, while randomized experimental seeding is continuing in the south as Israeli-3. Recent analyses of both targets of the Israeli-2 experiment indicated that rainfall was not enhanced in the south target area. Preliminary and intermediate analyses of Israeli-3 do not indicate rain enhancement in the south either. Reanalyses of the experiments, stratified by observations of dust haze, show an increase of 26% in Israeli-2 north on the 202 ?no-dust? days and no effect on the remaining 182 ?dust? days. According to Israeli-2 south and Israeli-3, the indicated seeding effects in the south were also more positive by 16% on the ?no-dust? days than on the ?dust? days. The statistical evidence suggests that dust, blown from the Sahara-Arabian deserts bordering the target area to the south (or something else that is associated with the dust), plays an important role in the natural precipitation processes such that seeding is beneficial only when this dust is absent.