Researchers from the federal technology institute ETH Zurich. are trialling the use of Russian-made ‘Oblako’ rockets to reduce hail damage in the Romoos region, in the canton of Lucerne, June 23, 1977.
KEYSTONE/PHOTOPRESS-ARCHIV/Str
Switzerland pioneered the artificial modification of clouds, known as cloud seeding, which many countries now practise. However, recent experiments to reduce hail damage have not produced the desired results.
Around ten cannons, shaped like large metal cones, are trained on a dark mass gathering on the horizon. It is the autumn of 1900 and, in the Ticino village of Collina d’Oro in southern Switzerland, a group of scientists is attempting to control the forces of nature.
The cannons generate shock waves that travel towards the storm clouds. The aim is to prevent the clouds, as they advance towards the village, from unleashing large hailstones.
Since then, the stakes have grown. Climate change is warming the atmosphere and fueling stronger storm updrafts that produce larger hailstones. In Switzerland and elsewhere, scientists and insurance companies are still searching for a way to blunt the damage – despite a century of cloud-seeding experiments that have yet to prove this method works.
The destructive potential of hail has long been recognised, explains Marco Gaia of the Federal Office of Meteorology and Climatology (MeteoSwiss). At the end of the 19th century, anti-hail systems were widespread across much of Switzerland. In the summer of 1901 alone, hail cannons were fired around 6,000 times.
Today, summer hail regularly causes millions of francs in damage in Switzerland.
Switzerland is among the pioneers in combating hail through cloud seeding. This technique involves releasing particles into clouds using rockets launched from the ground or from aircraft. The projectiles release substances – usually silver iodide – to encourage the formation of many smaller hailstones, which are less destructive than fewer larger ones.
The tests carried out in Switzerland, including large-scale experiments between the 1950s and 1980s, have helped to advance scientific knowledge in this field. But the data remains incomplete.
“They have never been able to conclusively demonstrate the effectiveness of the various methods tested,” Gaia tells Swissinfo.
>> Dozens of countries use cloud seeding to induce artificial rain, with limited results:
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Four major cloud-seeding experiments against hail in Switzerland
In their efforts to fight hail, the Federal Department of Economics’ agriculture division set up a federal commission tasked with studying hail formation and funding experimental research.
The commission organised four large-scale experiments (known as Grossversuch) in Switzerland. The first, between 1948 and 1952, aimed to verify the effectiveness of the rockets used at the time to counter the formation of hail.
Pyrotechnician Karl Mueller, regarded as ‘the inventor of the anti-hail rocket’, photographed at work at the Mueller pyrotechnics factory in Kreuzlingen, Switzerland, in 1942.
KEYSTONE/PHOTOPRESS-ARCHIV/Str
The second experiment began in 1953, and the thirdExternal link took place between 1957 and 1963. Both assessed the effectiveness of seeding storm clouds by dispersing silver iodide from a network of ground-based generators. There were 23 sites spread across Ticino and nearby Lombardy, Italy.
The fourthExternal link test was conducted between 1977 and 1982 over an area covering 1,300 km² on the Swiss plateau, north of the Alps. Research groups from Italy and France also took part. The federal technology institute ETH Zurich tested a system developed in the Soviet Union, which appeared to promise spectacular results. Soviet testers reported a reduction in damage of around 70–90%, but these figures did not have a scientific basis, Gaia says.
A generator used to disperse silver iodide into the atmosphere.
MeteoSvizzera
Cloud seeding in Switzerland: a benchmark for other countries
From a scientific perspective, the conclusions of the four experiments in Switzerland were clear: cloud seeding does not significantly affect the size of hailstones. In fact, as Gaia points out, there were indications that the methods used might even promote, rather than limit, the formation of hail.
Nevertheless, these studies played an important role because they helped to establish methodological foundations for rigorously carrying out this type of experiment, says Ulrike Lohmann, a professor of atmospheric physics at ETH Zurich. They also included control groups – similar to clinical trials – comparing treated clouds with untreated reference clouds.
“In my view, this approach represents a model to follow. It is also an important lesson for other countries: if cloud seeding is used, it is essential to be able to compare results with a situation where such intervention has not been applied,” says Lohmann. The researcher has been conducting cloud-seeding experiments for years, not to combat hail, but to studyExternal link physical processes in clouds, particularly the formation of ice crystals.
Bigger hailstones and more damage with climate change
Rising temperatures due to climate change are expected to lead to more severe hailstorms in Switzerland and other temperate regions of the world, according to a recent studyExternal link. Damage caused by larger hailstones is likely to increase considerablyExternal link.
In 2021, a year when numerous hail events took place, the Swiss Hail insurance company received more than 14,000 reportsExternal link of damage to crops . Compensation claims reached CHF115 million ($142 million).
According to an expert from the Allianz insurance group, in recent years hailstorms worldwide have generated more insured lossesExternal link than all other natural hazards combined.
No more anti-hail flights
However, even the most recent cloud-seeding experiments conducted in Switzerland have not produced encouraging results. The insurance company Baloise, in collaboration with ETH Zurich, tested cloud seeding between 2018 and 2023 across large areas of German-speaking Switzerland. The aim was to reduce hail damage to cars.
During the hail season, from May to September, a specially equipped aircraft took off from Birrfeld airfield in the canton of Aargau to disperse silver iodide particles into storm clouds. Around 30–40 flights were carried out each season.
“The effect of the flights [on the formation of large hailstones] has not been demonstrably measurable,” Thomas Schöb, head of customer services at Baloise, told Swiss public television SRF in October 2024, announcing the end of the tests.
No scientific consensus on effectiveness of cloud seeding
Many countries and regions continue to rely on seeding storm clouds to protect the most vulnerable agricultural areas. The United States, China, Russia and several European countries, including Germany and France, have launched programmes aimed at reducing hail damage.
Many countries and regions continue to bet on cloud seeding to protect their most vulnerable agricultural areas. The United States, China, Russia, and several European countries, including Germany and France, have launched programmes to try and reduce hail damage.
Some programmes, such as the one conducted in the Canadian province of Alberta, claim to have reduced insured hail damage by up to 50%External link. According to a studyExternal link in North Dakota (United States), cloud seeding may have reduced the size of hailstones.
Despite these results, there is still no scientific consensus on the real impactExternal link of this technique on hail and other types of precipitation. Available data are limited and often ambiguous: cloud seeding may reduce the extent of areas affected by hail at ground level, but it could also increase the size of hailstones, according to a recent studyExternal link in which Lohmann participated.
As Marine de Guglielmo Weber, a researcher at the Institute for Strategic Research of the Military School in France, explainsExternal link, hail and rain are natural phenomena characterised by very high variability. “It is very difficult to link them to human activity and to understand what would have happened in the absence of intervention.”
The environmental impact of silver iodide is also poorly documented, Weber notes.
A better understanding of hail – and of the interaction between atmospheric particles and clouds – will therefore be needed to determine whether humans can truly tame one of nature’s most destructive weather phenomena.
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