Silver iodide is used in many regions of the world to prevent damage caused by hail. While some claim that this compound can melt hail and promote liquid precipitation in areas where rain is needed, others swear that silver iodide disintegrates storms and prevents rain. Who is right?
A magical element for modern agriculture
Throughout history, humans have used various tricks to avoid the unwanted effects that hail precipitation has on their crops. Primitive tribes, equipped with a "prehistoric technology", have resorted to devices such as ritual sacrifices (of animals or humans) or the equally effective method of placing a sorcerer in the middle of the fields to avoid disaster with his dances and shouts.
But modern agriculture, which uses technology to improve productivity through more or less sophisticated irrigation systems, herbicides and genetically modified plant species, has incorporated silver iodide as a "magical element" that can minimize the negative effects of hail falling on fields. However, not all farmers support the use of this compound. And even more interesting, they do not even agree on its effectiveness.
Silver iodide is a chemical substance, well known to those who develop their own photographs at home, highly insoluble in water, and with a crystalline structure similar to ice. Sprayed over a cloud, it acts as condensation nuclei, that is, particles around which water vapor condenses. As we learned as children, clouds are made of water vapor, so if we can somehow "spray" them with this chemical to provide a large number of condensation nuclei, we will promote the formation of water droplets that, once they reach sufficient size, will fall by gravity. This is the principle that those who advocate the use of silver iodide as a useful agent put forward, since it would make it possible for small clouds or clouds that for some reason have not formed large droplets to precipitate as rain.
How it reduces hail
This method can also reduce the effects of hail. In a "normal" cloud, hail forms from a small water droplet, initially generated around a condensation nucleus, to which water from the surrounding vapor is added. As time passes, the hail grows in size. If meteorological conditions allow, these ice pellets increase greatly in size before falling to the ground, destroying crops when they do. With these data, we might think that if a cloud is sprayed with silver iodide, we would increase the number of condensation nuclei, so the amount of hail formed would be greater, which would discourage the use of the chemical compound. But it is not so.
José Luis Sánchez, professor at the University of León, based on campaigns carried out since 1997 in the Ebro valley, especially in Lleida, has prepared a comprehensive study on the subject. According to Sánchez, once the iodide comes into contact with suspended droplets, what it does is help freeze them, that is, it accelerates the formation of hailstones, but ironically this process prevents large stones from gradually forming. "The numerous hail embryos compete for the available water and cannot grow. It is not that there are fewer, but the opposite: there are more, but they are smaller, which reduces the damage", he explains.
What the experts say
From the conclusions obtained by this and other experts, it follows that "cloud seeding" is a useful method, although not definitive. In some cases, when the appropriate clouds are available, the probability of precipitation can be increased or the impact of hail on crops can be reduced. But obviously silver iodide does not work miracles.
To carry the compound to the clouds, airplanes are generally used, although for cost reasons, "stoves" that burn the iodide on the ground and rise by their own temperature to the cloud can also be used. Small rockets can also be used. In any case, the result is more or less the same.
A controversial reputation
In Spain, Maite Torà, from the Agrupació de Defensa Vegetal de les Terres de Ponent, who has long been in charge of the iodide generators in the province of Lleida, has said that "the only way to see if it works is to keep evaluating. And do it well: you cannot water one cloud and not another". One of the problems is that the iodide has been used in inherently dry years, which has generated a negative image: "There is drought, so someone must be blamed". This situation is possibly what has caused many farmers not to believe in the system, or to directly oppose it.
When the controversy turns dangerous
In fact, the controversy over the effectiveness of the system is so great that in some countries it reaches extremes that are at least dangerous. For example, in the area of Los Llanos, located in the southeast of the Argentine province of La Rioja, in the past residents have denounced the existence of a supposed airplane that "has the necessary technology to prevent storms and rain, and also pollutes the environment with silver iodide."
Carlos Del Moral, a lawyer in the area, expressed on that occasion that "these flying devices are harmful to the people of La Rioja, so I invite my fellow provincials to shoot them down, for example, with an accurate shot from a .22 rifle," while also offering a monetary reward to those who achieve it. Really, a true madness.
The scientific reality
Beyond particular cases, the scientifically proven fact is that silver iodide can indeed help reduce the size of hailstones or, with appropriate clouds, increase the amount of precipitated water. Of course, seeing an airplane flying over storm clouds and then not raining does very little to improve the image that some farmers have of the system. Obviously, if the airplane had not seeded the cloud, it would not have rained either, but it is always easier to accept misfortunes when we have someone to blame.
