Weather warfare is the use of weather modification techniques such as cloud seeding for military purposes.
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' Operation Popeye 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.
A research paper produced for the United States Air Force written in 1996 speculates about the future use of nanotechnology to produce "artificial weather", clouds of microscopic computer particles all communicating with each other to form an intelligent fog that could be used for various purposes. "Artificial weather technologies do not currently exist. But as they are developed, the importance of their potential applications rises rapidly." Weather modification technologies are described in an unclassified academic paper written by airforce officer-cadet students as "a force multiplier with tremendous power that could be exploited across the full spectrum of war-fighting environments."
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". In contrast, the "Consultative Committee of Experts" established in Article VIII of the Convention has stated in their "Understanding relating to Article II" that any use of environmental modification where this is done "as a means of destruction, damage or injury to another State Party, would be prohibited.". Furthermore, they conclude in the same paragraph that "military or any other hostile use of environmental modification techniques, would result, or could reasonably be expected to result, in widespread, long-lasting or severe destruction, damage or injury.", meaning that all signatories are expected to abstain from using weather modification to cause harm at any scale. Importantly, the language of the treaty does not overtly condemn military use of weather modification when it does not directly cause harm, such as the United States' use of weather modification in the siege of Khe Sanh, discussed above. Because of the limitations of the treaty, and the fact that it applies only to signatory states, weather warfare is not a thing of the past, and may continue to play a role in warfare throughout the twenty-first century.