What conclusions did studies of winter cloud seeding in the mountainous western United States reach?

Studies regarding winter cloud seeding in the mountainous western United States have historically yielded varying conclusions, often complicated by the lack of randomized experimental controls in early private or commercial operations.1

In 1957, the U.S. Advisory Committee on Weather Control concluded that silver iodide seeding increased precipitation from winter supercooled clouds by 10 to 15 percent. A 1966 panel from the National Academy of Sciences reached a similar conclusion using largely the same data. However, many statisticians later rejected these optimistic findings, arguing that the absence of randomized procedures made the data unreliable for testing efficacy.1

Modern scientific consensus remains nuanced. While silver iodide is still routinely used to increase snowpack in the region, evidence indicates that ice-nuclei seeding can produce different outcomes depending on meteorological conditions:
* Increases: In some cases, precipitation may increase by some tens of percent.12
* Decreases: In other circumstances, seeding may lead to decreases of a similar magnitude.12
* Ineffectiveness: In many situations, seeding has no measurable effect.12

Currently, it is often impossible to specify the exact conditions under which these positive or negative effects will occur.1