Cloud seeding is the process of "sowing" clouds with tiny foreign particles, such as dry ice, silver iodide, or even salt, in order to induce precipitation or otherwise modify the weather. The practice is perhaps more common than one might think, but it is not without its critics.
In order to understanding cloud seeding, let's first look at how clouds form and precipitation is produced. Tiny particles of dust and salt are constantly present in the atmosphere. When warm air rises, it will cool off and form water molecules. These water molecules will attach to the tiny particles to form cloud droplets. Clouds form when billions of these cloud droplets come together, and when clusters get heavy enough they fall to the earth as rain or snow.
Cloud seeding involves spreading these particles in much greater concentration than would be found naturally. This can be accomplished by generators on the ground, by shooting canisters up into clouds with rockets or anti-aircraft guns, or by dropping the particles from an airplane.
In addition to producing rain, cloud seeding may be used for other types of weather modification. Airports will use this technique to reduce the amount of fog near the runways, and ski resorts will often use it to produce more snow. Cloud seeding can even be used to reduce the size of hail during thunderstorms, or to suppress rain.
There is some debate over whether cloud seeding is actually effective. Many researchers have failed to produce statistically significant results, and even if rain does fall after seeding the clouds, it is difficult to predict how much rain would have fallen if nature had been allowed to take its course. In cases of extreme drought, which would likely be the most useful application, cloud seeding is not even an option. Rain cannot be produced from clear skies; there must be clouds present.
There is also controversy surrounding the use of certain chemicals commonly found in cloud seeding. Silver iodide is a non-organic chemical that has been found to be toxic to fish and wildlife. It is not generally considered to be harmful to humans in the amounts found after cloud seeding; however it is possible that it could be toxic to humans with prolonged or repeated exposure, even if it is in small doses.
While cloud seeding may seem to be of great value with many practical applications, perhaps more research needs to be done in order to determine if the practice is worth the effort and expense.