A century of trying to stop hail, with no proof that it works

In the autumn of 1900, at Collina d'Oro, a dozen or so cannons shaped like large metal cones were pointed at approaching thunderstorms. They produced shock waves aimed at the clouds, in the hope of stopping them from dropping large hailstones on the village. It was not an isolated case: at the end of the nineteenth century hail defence systems were widespread across much of Switzerland, and in the summer of 1901 alone those cannons fired around six thousand times.

Marco Gaia of MeteoSwiss recalls that hail's destructive potential has always been known. Even today, in summer, it regularly causes damage running into millions of francs.

In the twentieth century the technique changed but the aim did not. Cloud seeding consists of introducing particles into clouds using rockets fired from the ground or released from aircraft. The substance generally used is silver iodide, which is meant to encourage the formation of many small stones instead of a few large ones, less destructive on the ground.

Four experiments, no confirmation

Switzerland ran four large-scale experiments in this field, the so-called Grossversuch, organised by a federal commission set up by the agriculture section of the Federal Department of Economic Affairs. The first, between 1948 and 1952, tested the effectiveness of the rockets then in use. The second began in 1953 and the third ran from 1957 to 1963: both assessed the dispersal of silver iodide from a network of ground generators, with twenty-three stations in Ticino and neighbouring Lombardy. The fourth, between 1977 and 1982, covered 1,300 square kilometres of the Plateau with Italian and French research groups taking part; the Federal Institute of Technology in Zurich tested a system developed in the Soviet Union, whose officials had reported damage reductions of 70 to 90 per cent. Those figures, Gaia notes, had no scientific basis.

The conclusions were clear: cloud seeding does not significantly affect the size of hailstones. Indications even emerged that the methods used might encourage their formation rather than limit it. "The experiments never managed to demonstrate conclusively the effectiveness of the various methods tested," Gaia sums up.

The Swiss contribution lies elsewhere, explains Ulrike Lohmann, professor of atmospheric physics at ETH Zurich: in the method. Those studies included control groups, as in clinical trials, comparing treated clouds with untreated ones. "In my view this approach is a model to follow," the researcher says, and it holds as a lesson for other countries that turn to seeding.

The most recent experiment ended the same way. Between 2018 and 2023 the insurer Baloise, together with ETH Zurich, flew a specially equipped aircraft out of Birrfeld to disperse silver iodide, with thirty to forty flights a season, aiming to reduce hail damage to cars. In October 2024 Thomas Schöb, head of customer service at Baloise, told SRF that the tests were over: the effect of the flights, he said, "was not demonstrated in a measurable way".

The problem, meanwhile, is growing. Warmer air holds more moisture and feeds stronger updraughts which, according to recent studies, will produce larger stones in Switzerland and other temperate zones. In 2021 the Swiss Hail Insurance Company received more than 14,000 reports of crop damage, with compensation claims of 115 million francs.

Many countries seed clouds all the same, from the United States to China, from Russia to Germany and France. Scientific consensus, though, is missing. As Marine de Guglielmo Weber, a researcher at the Institute for Strategic Research of the Military School in France, points out: "Hail and rain are natural phenomena characterised by high variability. It is very difficult to link them to human activity." The environmental effects of silver iodide, she adds, also remain poorly documented.