Cloud seeding is a weather modification technique which is used to disperse fog, increase precipitation levels, and suppress hail. Like many practices which actively alter the Earth's environment, cloud seeding is not without controversy. Some nations have accused each other of “stealing rain” with cloud seeding techniques, while others question the validity of cloud seeding, 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 to describe. Basically, scientists add a substance such as dry ice or silver iodide to the atmosphere, creating nuclei for condensation. Cloud seeding can be accomplished with an aircraft which flies over the region in question, or it can be done with rockets fired from the ground. In both cases, cloud seeding works best when clouds or large amounts of water vapor are already present.

When used to disperse fog, cloud seeding causes the fog 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 which are experiencing droughts, cloud seeding can save crops. Some ski resorts also utilize cloud seeding to increase the amount of natural snowfall, making them more appealing sites to visit.

In hail suppression, cloud seeding reduces the size of hailstones by providing so many nuclei for ice crystals to form on that they cannot reach very large sizes. Since hail can cause serious damage to property and crops, this use of cloud seeding is extremely valuable. It is also appears to be backed up with science, as studies have shown an overall decrease in the average size of hailstones where cloud seeding is used, whereas it is harder to confirm that cloud seeding causes increased precipitation or snowfall.

Changing the weather is part of a larger branch of science known as geoengineering. The ability to alter our 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. However, some scientists are concerned that the effects of activities like cloud seeding may not be fully understood, and that it could be dangerous to interfere with the Earth's weather.