The United States has been secretly seeding clouds over North Vietnam, Laos and South Vietnam to increase and control the rainfall for military purposes. Government sources, both civilian and military, said during an extensive series of interviews that the Air Force cloud seeding program has been aimed most recently at hindering movement of North Vietnamese troops and equipment and suppressing enemy antiaircraft missile fire. The disclosure confirmed growing speculation in Congressional and scientific circles about the use of weather modification in Southeast Asia. Despite years of experiments with rainmaking in the United States and elsewhere, scientists are not sure they understand its long term effect on the ecology of a region. The weather manipulation in Indochina, which was first tried in South Vietnam in 1963, is the first confirmed use of meteorological warfare. One well-informed source said that Navy scientists were responsible for developing a new kind of chemical agent effective in the warm stratus clouds that often shielded many key antiaircraft sites in northern parts of North Vietnam. The chemical, he said, "produced a rain that had an acidic quality to it and it would foul up mechanical equipmentlike radars, [and] trucks." "This wasn't originally our planning," the official added, "it was a refinement." Apparently, many Air Force cloud-seeding missions were conducted over North Vietnam and Laos simply to confuse or "attenuate" ... the radar equipment that controls anti aircraft missiles.