The Israel 4 Cloud Seeding Experiment: Primary Results
Yoav Benjamini, Amir Givati, Pavel Khain, Yoav Levi, Daniel Rosenfeld, Uri Shamir, Ayal Siegel, Assaf Zipori, Baruch Ziv, David M. Steinberg
Abstract
Abstract After 38 years of operational cloud seeding for rain enhancement in northern Israel, the Israel 4 experiment was conducted to reassess its effect on rainfall and provide a basis to evaluate its utility. Operational seeding started after two randomized experiments, the second ending in 1976, found a large and statistically significant effect of cloud seeding on rainfall. Observational studies in later years raised doubts as to the magnitude of the effect, possibly because of changing climatological conditions. A carefully designed randomized experiment was conducted from 2013 to 2020. A unique feature of the design was the use of forecast rainfall on target, rather than rainfall in an unaffected area, as a control variate to attenuate variability. The Israel 4 experiment was stopped a year earlier than planned, because the result was disappointing: a 1.8% increase, p value = 0.4, and 95% confidence interval of (−11%, 16%). These results led to a decision by the Israel Water Authority to stop operational seeding. Significance Statement The recent cloud seeding experiment in northern Israel did not show a significant rainfall increase—unlike the sequence of seeding experiments conducted in Israel in the previous century.