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Cloud seeding is a weather modification technique used to disperse fog, increase precipitation levels, and suppress hail. Like many practices that actively alter the Earth's environment, it is not without controversy. Some nations have accused each other of “stealing rain” with these techniques, while others question the validity of the practice, arguing that it cannot be tested in controlled conditions, so its results are hard to view objectively.
The process involved in cloud seeding was developed in 1946, and it is actually rather complex. Basically, scientists add a substance like dry ice or silver iodide to the atmosphere, creating nuclei around which water vapor can condense. This can be done with an aircraft that flies over the region in question or with rockets fired from the ground. In both cases, the process works best when clouds or large amounts of water vapor are already present.
When used to disperse fog, these substances are added to the air to cause the water vapor to condense and precipitate out, clearing the air around airports and other places where good visibility is crucial. Cloud seeding may also be able to increase precipitation by encouraging the formation of rain drops around the particles added to the air. In areas that are experiencing droughts, this practice might save crops. Some ski resorts also use it to increase the amount of natural snowfall, making them more appealing sites to visit.
This technology is also used to reduce the size of hailstones by providing so many nuclei for ice crystals to form on that they cannot reach a very large size. Since hail can cause serious damage to property and crops, this can be very helpful. It is also appears to be backed up with science, as studies have shown an overall decrease in the average size of hailstones where this practice is used, whereas it is harder to confirm that it causes increased precipitation or snowfall.
Changing the weather is part of a larger branch of science known as geoengineering. The ability to alter the environment could be extremely useful, especially if global warming threatens levels of rainfall or alters weather patterns so that some regions of the Earth experience extreme droughts. Some scientists are concerned that the effects of activities like cloud seeding may not be fully understood, however, and that it could be dangerous to interfere with the Earth's weather.