PARIS — Persistent drought in France’s southern Occitanie region and increasingly destructive episodes of hail are giving us a glimpse of the future effects of global warming. And there is a great temptation to want to control the weather, particularly rain and hail.
In France, cloud seeding to suppress hail has been used for a long time. The National Association for the Study and Fight Against Atmospheric Plagues (ANELFA) has developed a technique that reduces the size of hailstones by sending silver iodide into the clouds.
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“Ice crystals develop on more particles contained in a cloud. There is no more water, so the hailstones are smaller,” explains Sandra Scavennec, coordinator of the Prévigrêle network, one of ANELFA’s members. ANELFA uses “terrestrial vortex generators,” which send silver iodide from the ground using updrafts. Others, such as French company Selerys, rely on balloons to disseminate the particles, while some, in other countries, spread them using airplanes.
“Around 50 countries in the world practice cloud seeding,” explains Marine de Guglielmo Weber, a researcher at the Institute for Strategic Research at the Military School (IRSEM), who wrote her thesis on the topic. “While France currently only uses it to combat hail, others, such as the United States, the Sahel countries or the Gulf States also employ this technique to increase rainfall.”
A controversial tool
China is undoubtedly the world leader in this technology, devoting huge investments to it. The country used cloud seeding for example, to make it rain before the opening ceremony of the Beijing Olympics in 2008, or to combat drought in the Yangtze River region in 2022.
“China is currently implementing the world’s largest drought relief program on the Tibetan Plateau,” de Guglielmo Weber says. For a time, the UAE was suspected of causing the Dubai floods in March 2024 — a claim authorities have denied.
There is no scientific consensus on the issue.
Yet the technology is controversial. First, its effectiveness remains debated. “There is no scientific consensus on the issue,” de Guglielmo Weber says. “Rain and hail are natural phenomena with very high variability. It is very difficult to link them with human activity and to know what would have happened without intervention.”
Second, its impact on the environment is still poorly documented. ANELFA claims, with studies to support it, that the quantities of silver iodide used are too small to be harmful. But de Gugliemo Weber is more cautious.
“Certainly, if silver iodide had massively toxic effects, we would have already noticed it,” she says. “But we don’t know the effects of an accumulation over time, nor those of an interaction with other substances found in soil. A study published in 2016 also showed the particles’ harmful effects on microorganisms, which should be investigated further.”
Slowing Vietnamese troops
Finally, cloud seeding raises a regulatory issue. The United States used it during the Vietnam War, to cause flooding and slow down Vietnamese troops. The UN then adopted a convention (ENMOD) in 1976 prohibiting its use for military purposes.
But even for civilian purposes, exploiting this technology can cause tensions. In 2018, an Iranian general accused Israel of manipulating clouds to prevent rain from falling in Iran.
Some are now calling for a legal status for clouds. French lawyer and novelist Mathieu Simonet even wants to see them added to the UNESCO World Heritage List. In a context of dwindling water resources, the question of regulating cloud seeding is already being discussed.
