Water shortages, dry vegetation, declining rivers… Faced with the drought that many countries in the northern hemisphere have been experiencing since the beginning of summer, the temptation to control the weather is great. This is what China is trying to do, which sends projectiles into the clouds to make rain fall on the Yangtze River, reports the American channel CNN . While a severe drought affects part of the country, the rain would indeed be welcome to remedy the lack of water.
China’s maneuvers to manipulate the weather have particular resonance in many countries in the northern hemisphere, including France, which are suffering from too hot and too dry a summer in a context of climate change. Yet this technique, called “cloud seeding,” has been around for decades. Here are five questions to understand how countries try to make it rain or shine.
1. What is seeding?
First of all, you have to understand how a cloud is formed and what it contains. Most clouds form when an air mass containing water vapor rises and cools. The water vapor condenses and turns into droplets or ice crystals depending on the temperature, and creates the cloud. But these droplets sometimes remain too small to fall as rain. Cloud incense, a technique discovered in the 1940s, helps clouds coalesce and grow so they can fall as rain or ice.
For this, it is necessary to artificially introduce products into the clouds. In “cold” clouds, the seeding consists of launching aerosols, most often silver iodide, into them to form ice cores that will fall more easily in the form of snow or rain. In “hot” clouds, scientists add salt which allows the droplets to coalesce and fall.
2. How does it work?
There are several ways to send these products to the clouds. “It’s a very expensive method, but you can fly over the area you want to treat with an airplane that has torches on its wings”, lists Andrea Flossmann, professor at Clermont Auvergne University and co-responsible for the group of weather modification experts for the World Meteorological Organization (WMO).
Rockets can also be used, as is currently the case in China, especially in Hubei province. Finally, there is a method operated on the ground by generators which spray a mixture of acetone and silver iodide into the air. A burner allows the mixture to rise into the air.
3. How effective is it?
“There are few cases where cloud seeding has been scientifically proven to work,” says Andrea Flossmann. And for good reason, the effectiveness of seeding is difficult to prove. “No cloud is the same and the natural variability is very important. We don’t know if it would have rained naturally when we observe rain after sowing,” continues the meteorologist. “I don’t believe it can save us. I doubt that we will be able to do it in sufficient quantity to fight against the effects of global warming, ”she concludes.
However, conclusive results have been obtained in mountainous regions, where clouds are formed by the movement of air above the mountains, notes a WMO report . “On a relief, the natural variability of the cloud is less. The cloud, stuck against the relief, is forced to rise”, notes Andrea Flossmann. The increase in precipitation would then be 10 to 15%.
Cloud seeding to combat drought is therefore far from being a panacea, especially since this technique only works on certain clouds. But above all, this technique applies to clouds formed naturally from the humidity present on the ground. Thus, if it can make it possible to increase water resources, “this technique can do nothing against drought, because it requires the availability of clouds”, notes the WMO report. “No one can create or drive away a cloud. »
4. Which countries change the weather this way?
China has been allocating colossal resources to cloud seeding programs for decades. The country used this method in particular to try to avoid the rain during the 2008 Olympic Games in Beijing. The former USSR, now Russia, also claims to use these techniques to avoid rain during major events such as parades.
The United Arab Emirates launched a vast international program in 2015 to fund research in the field, while the country is one of the driest in the world. Other countries such as the United States, Morocco or even Israel are trying to cause rain artificially.
In France, cloud seeding is used to avoid hailstorms, which can sometimes be destructive to crops. Several companies have positioned themselves in this niche, but it is the National Association for the Study and Fight against Atmospheric Blights (Anelfa) which is the benchmark. Created in 1951, this association seeks to reduce the damage caused by hailstorms by reducing the size of the hailstones. If used well, this technique could halve the damage caused by hail, according to the association.
5. Does this technique involve any risks?
“Silver iodide is toxic in large quantities, notes Andrea Flossmann. But for the moment, the concentrations observed on the ground remain below the thresholds given by the WHO. It is also difficult to find recent studies on the environmental impact of such processes.
On the other hand, the artificial modification of the weather can create geopolitical tensions. “For now, even though there is not yet hard evidence that cloud seeding works, there are fears from neighboring countries as soon as a cloud seeding program is announced. explains Andrea Flossmann. Countries may therefore fear that their neighbor will “steal” their water or, on the contrary, that it will be responsible for flooding. “The potential for problems is great,” summarizes the scientist. In 2018, in the midst of a drought, an Iranian general accused Israel of “stealing” its clouds and therefore the rain.