The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (Geneva: 18 May 1977, Entered into force: 5 October 1978) prohibits "widespread, long-lasting or severe effects as the means of destruction, damage or injury". However it has been argued that this permits "local, non-permanent changes".

The United States Space Preservation Act of 2001 bans the use of exotic weapons systems to damage climate, weather, and tectonic systems so as to induce damage or destruction.

Prior to the Geneva Convention, the United States used weather warfare in the Vietnam War. Under the auspices of the Air Weather Service, the United States used cloud seeding over the Ho Chi Minh Trail, increasing rainfall by an estimated thirty percent during 1967 and 1968. It was hoped that the increased rainfall would reduce the rate of infiltration down the trail.

With much less success, the United States also dropped salt on the airbase during the siege of Khe Sanh in an attempt to reduce the fog that hindered air operations.

http://en.wikipedia.org/wiki/Weather_warfare