Cloud seeding

Cloud seeding attempts to alter cloud microphysics by dispersing materials into a cloud. In glaciogenic seeding, particles such as silver iodide are intended to promote ice-crystal formation in supercooled liquid water; hygroscopic seeding targets liquid droplets by introducing particles that take up water. The expected result depends on the cloud's temperature, moisture, structure and natural precipitation processes. Seeding cannot supply the atmospheric moisture or energy that a cloud system lacks.

The SNOWIE field campaign in Idaho studied winter orographic clouds—clouds formed as moist air is lifted over mountains. In 2018, French and colleagues reported observations that traced a chain from seeding, through ice formation and crystal growth, to snowfall reaching the ground. The study offered unusually direct evidence of the physical process in the particular clouds and conditions observed. It was not a universal test of every seeding method, cloud type or operational programme; estimating how much additional precipitation an operation produces at a broader scale remains challenging.

A cloud must be naturally suitable for the method before seeding can plausibly affect precipitation. Weather varies considerably from one event to another, so simple before-and-after comparisons or selected case studies can confuse natural variability with a treatment effect. The World Meteorological Organization says evidence has advanced for wintertime glaciogenic seeding of orographic clouds, while emphasizing that many operational outcomes are difficult to establish confidently. It also states that there are no demonstrated methods to use cloud seeding to modify tornadoes, lightning danger, floods or other severe weather phenomena. Claims of dramatic, large-scale weather control are not supported by sound scientific evidence.

Programs in different places have pursued precipitation enhancement, hail suppression or fog dispersal. A 2024 U.S. Government Accountability Office assessment described precipitation estimates across reviewed studies ranging from zero to 20 percent and highlighted limits in the evidence, the need for suitable clouds, and uncertainties around wider use of seeding agents. The World Meteorological Organization recommends carefully designed evaluations that combine objective statistical comparisons with physical evidence. Existing assessments have not established significant health or environmental effects from commonly used seeding agents at past operating levels, but potential effects of substantially larger quantities or new agents warrant evaluation.