Cloud seeding is a method of spraying chemical contaminants in clouds to induce rainfall. The spraying is usually done from airplanes but also from land based equipments. The contaminants, often silver iodine, act as magnets for water vapour in clouds, inducing the formation of water droplets heavy enough to fall to the ground.
This is an old technique. It has been developed in the 1950s in the U.S.A., and since then has been used in the American Midwest, during the Vietnam war (resulting in the ENMOD Convention), in Australia, in South Africa, and even in experimental programs over Alberta, Canada (circa 1980s?). In fact, there is even a statute in Québec regulating cloud seeding, the Act respecting the artificial inducement of rain.
Cloud seeding has remained a marginal activity for decades because of a black box problem. Basically, science is not advanced enough to establish satisfactory correlations between the seeding and rainfalls: it is impossible to track all the dynamic variables in the atmosphere and isolate seeding as the definitive causa causans for precipitations in the area around the seeding activity.
However, cloud seeding has been the object of renewed interest in recent years due to increased water scarcity. The FAO and African countries have looked at cloud seeding to secure water for crops, The Israelis are experimenting with new techniques to induce cloud formation by changing land cover...
Why this renewed interest? Cloud seeding essentially participates from the same process as rainwater harvesting. This is an 'escalation to the extremes' (borrowing from von Clausewitz) between competing users of a limited resource to secure access to water by getting ever closer to the ultimate upstream point of the hydrological cycle.
Legal doctrine (from authors like Edith Brown Weiss in international law and Ray Jay Davis in American law) has approached the legal issues raised by artificial rain inducement. Such studies might become very useful in a context of growing water scarcity and increased use of weather modification techniques.
For example, AlterNet reports of a small community in California that has passed an Ordinance to prohibit cloud seeding on its territory.
NOTE: This is completely off topic, but the article mentions that Coca-Cola extracts groundwater near that municipality but does not have to disclose the quantities withdrawn... because of the company's right to free speech!!! It's always surprising to see how far American law has gone to divorce the morally just from the legally right.