Operation Popeye was a highly classified weather modification program in Southeast Asia during 1967–1972. The cloud seeding operation during the Vietnam War ran from March 20, 1967 until July 5, 1972 in an attempt to extend the monsoon season, specifically over areas of the Ho Chi Minh Trail. The operation was used to induce rain and extend the East Asian Monsoon season in support of U.S. government efforts related to the War in Southeast Asia.
Operation Popeye (also known as Operation Intermediary or Operation Compatriot) was a top secret campaign of weaponized weather modification during the Vietnam War, from March 20th 1967 until July 5th 1972. It got its beginnings three years after the enactment of Project Stormfury (1962 – 1983) and 30 years after the first known US weather modification operations called Project Cirrus, which began in February 1947.
The intent of Operation Popeye was to extend the monsoon season over North Vietnamese and Viet Cong resupply routes throughout southeast Asia, particularly the Ho Chi Minh Trail.
Operation Popeye was a large and long running operation that successfully manipulated weather by seeding clouds, via aircraft, with silver and lead iodide. The crews, all from the 54th Weather Reconnaissance Squadron, were rotated into the operation on a regular basis from Guam. Inside the squadron, the rainmaking operations were code-named “Motorpool.” On average they were able to extend the monsoon season 30 to 45 days.
From the 1974 Senate hearings on weather modification
A classified rainmaking program was conducted in SEASIA from 1967 to 1972 which employed air dropped silver and lead iodide seeding units to increase normal monsoon rainfall. (US Senate, Subcommittee on Oceans and International Environment; 20 March 1974; p. 88)
Objective: Increase rainfall sufficiently in carefully selected areas to deny the enemy the use of roads by: 1. Softening road surfaces 2. Causing landslides along roadways 3 Washing out river crossings 4. Maintain saturated soil conditions beyond the normal time span. (US Senate, Subcommittee on Oceans and International Environment; 20 March 1974; p. 89)
The initial operation area was over parts of Laos and North Vietnam. It was then extended to include parts of South Vietnam and Cambodia. In total, the Pentagon admitted that US C-130 aircraft operating from Udorn Royal Thai Air Force Base flew 2,602 missions and expended 47,409 cloud seeding units. (US Senate, Subcommittee on Oceans and International Environment; 20 March 1974; pp. 101-105). The Pentagon said the project cost $21.6 million.
Operation Popeye first came to public light in March 1971 by a reporter named Jack Anderson who published a story based on a secret 1967 memo from the Joint Chiefs of Staff to President Johnson. The memo read: “Laos operations – Continue as at present plus Pop Eye to reduce the trafficability [sic] along infiltration routes & Authorization requested to implement operational phase of weather modification process previously successful tested and evaluated in some area“. From the declassified top secret Pentagon Papers (Item number 4 – Read across the document)
After Jack Anderson made the public aware of Operation Popeye, the US Senate pressured military leaders at the Pentagon to provide details of the operation. Melvin Laird who was secretary of Defense at the time, denied that the U.S. was modifying the weather in Vietnam. In 1973 the US Senate began legislation on S.RES.71, a resolution expressing the sense of the Senate that the United States Government should seek the agreement of other governments to a proposed treaty prohibiting the use of any environmental or geophysical modification activity as a weapon of war, or the carrying out of any research or experimentation directed thereto.
In 1974 The US Senate was able to investigate Operation Popeye in detail at: Hearings before the Subcommittee on Oceans and International Environment of the Committee on Foreign Relations United States Senate Ninety-Third Congress, second session on the need for an international agreement prohibiting the use of environmental and geophysical modification as weapons of war, and briefing on Department of Defense weather modification activity, January 25 and March 20, 1974.
Documentation and transcripts of the hearing are shown here in three parts. You can also view a video of a US veteran who flew on the missions in Vietnam, he subsequently blew the lid off of this highly confidential experiment and other documentation of “OP POP” (emphasis mine) at the same link.