A century after its first experiments, Switzerland still cannot tame hail

Around 6,000 cannon shots in one summer show how seriously Swiss communities once took the threat from the sky. In 1901, anti-hail systems were widespread across the country. A year earlier, in Collina d’Oro, Ticino, about ten large metal-cone cannons had been aimed at a storm front. Their shock waves were supposed to disrupt clouds before they reached the village.

The equipment has changed. The uncertainty has endured. Switzerland helped pioneer cloud seeding, a form of weather modification that releases particles into clouds from rockets, aircraft or ground-based systems. Yet a century of trials has not established that these interventions reliably prevent hail damage.

The issue is gaining practical importance. Summer hail already causes millions of francs in damage in Switzerland, affecting crops, buildings and vehicles. Climate change adds pressure by warming the atmosphere and strengthening the updrafts inside severe storms. Those rising currents can keep ice particles aloft longer, allowing larger hailstones to form.

The country’s search for a technical fix now sits between established atmospheric science and an expensive gamble. Researchers, farmers and insurers have reason to keep investigating. They also have a long record of results that remain difficult to interpret.

The first federal trial ran from 1948 to 1952. It formed part of a wider effort to replace local anti-hail practices with controlled experiments. The agriculture division of the Federal Department of Economics created a federal commission to study how hail forms and to finance research into ways of stopping it.

The commission organised four large-scale experiments, known as Grossversuch. The early work focused on rockets and other methods intended to counter hail formation. Later Swiss trials, including research carried out between the 1950s and 1980s, expanded the country’s knowledge of hail-producing storms. The experiments also exposed the difficulty of measuring success in a naturally chaotic environment.

A hailstorm does not arrive under laboratory conditions. Storm cells vary in size, movement and internal structure. Damage can be severe in one valley and limited a few kilometres away. A trial must establish what would have happened without intervention, while accounting for wind, temperature, moisture and the storm’s changing path.

That evidence has never reached a decisive conclusion. Marco Gaia of MeteoSwiss says the tested methods have not been shown conclusively to work. The finding does not erase the value of the research. It sets a clear boundary around what Switzerland can currently claim.

Silver iodide is intended to multiply ice particles inside a storm. In the standard cloud-seeding theory described by Swiss researchers, rockets or aircraft release particles into suitable clouds. Silver iodide can act as an ice-forming nucleus, encouraging supercooled water to freeze. The objective is to create many smaller hailstones rather than fewer large ones.

The proposed benefit is straightforward: smaller stones are more likely to melt or fall with less destructive force before reaching the ground. Reaching that result in a live thunderstorm is considerably harder. The seeding material must enter the right part of the cloud at the right moment, while the storm’s powerful updrafts and turbulent circulation continue to evolve.

A successful intervention would also need to be distinguished from ordinary variation. Some storms weaken, turn or miss populated areas without human assistance. Other storms produce hail in one location while rain falls nearby. Those patterns make before-and-after comparisons unreliable unless experiments are large, carefully controlled and repeated.

The Swiss record therefore supports a cautious description of cloud seeding. It is a plausible physical intervention under specific conditions. It is not a reliably proven shield for Swiss homes, vineyards or fields.