seeding has been studied and practiced in the United States for at least five decades. Over this period, research investment by agencies of the federal government has waxed and waned. Early experiments conducted by the U.S. Weather Bureau in the late 1940s showed sufficient promise that federally sponsored efforts were scaled up in the 1950s with programs overseen by the Weather Bureau, the U.S. Air Force, and the National Science Foundation, all of which supported cloud seeding research into the 1960s and 1970s. The mid-1970s marked a high point of federal support for cloud seeding, and the National Weather Modification Act of 1976 spurred federal research efforts and mandated a Department of Commerce Weather Modification Advisory Committee to coordinate research among federal agencies. In this same time frame, assessments were made of scientific progress made over the preceding decade and a half. The assessments include a series of reports from both the National Research Council (NRC) and the National Science Board that concluded that experimental evidence for cloud seeding had not yet definitively established its scientific efficacy (NRC, 1964, 1966, 1973; NSB, 1966). The National Research Council subsequently (in 2003) issued a report on the prospects of cloud seeding and other weather modification techniques, concluding that:
There is still no convincing scientific proof of the efficacy of intentional weather modification efforts. In some instances there are strong indications of induced changes, but this evidence has not been subjected to tests of significance and reproducibility. This does not challenge the scientific basis of weather modification concepts. Rather, it is the absence of adequate understanding of critical atmospheric processes that, in turn, lead to a failure in producing predictable, detectable and verifiable results (NRC, 2003).
In 2004 the Weather Modification Association (WMA) assessed the NRC report from the perspective of those involved in operational weather modification (Orville et al., 2004). This review supported many of the NRC report’s recommendations but also included some criticisms; specifically, the WMA claimed that the NRC report did not adequately account for recent field applications for precipitation enhancement and hail suppression. Since the NRC and WMA reports were issued, some scientists have sought common ground with operators to develop a cloud seeding program that would include scientifically controlled watershed experiments (Garstang et al., 2004).