Hopi Indians in the southwestern United States still invoke rain by sacrificing golden eagles and dancing with live rattlesnakes between their teeth. While this is happening, others have adopted a scientific method to make it rain.
In 1946, Vincent Schaefer and Irving Longmuir of General Electric's Research Laboratories in Schenectady, United States, began their work, which proved that rain could be artificially provoked.
Clouds are made up of millions of particles of water too small to precipitate as rain. Only when drops of a quarter of a millimeter or more form do they cause a fine drizzle. The smallest droplets evaporate before reaching land.
One way to make the droplets grow is to turn them into ice. In a cloud with ice particles and water droplets, the former grow rapidly when water vapor is absolved. Since the temperature of the clouds is almost always lower than the freezing point, one would expect the drops to freeze easily. But the water can be below that point without freezing.
This is because the water in the clouds is absolutely pure; it has no dust or contaminants that can form the center of an ice crystal. If particles are added, the droplets freeze, grow rapidly to a size large enough to precipitate, and then melt as the temperature rises to form rain.
Schaefer and Longmuir proved that adding particles to clouds, usually silver iodide, could produce rapidly growing ice crystals. These particles are launched from airplanes, by rockets or by air currents that raise them from the surface.
In the former Soviet Union, 70 mm artillery pieces were used to shoot silver iodide particles into the clouds. Although the technique can work when the clouds have the right temperature, it is impossible to know how much it would have rained without resorting to artifice. And this raises questions about the cost-effectiveness and efficiency of the method.
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