Why cannot cloud seeding greatly change the rainfall from a large storm system?

Cloud seeding cannot significantly alter the rainfall from a large storm system because the practice is inherently limited to the scale of individual clouds or small groups of clouds.1

Large storm systems, such as squall lines that can span hundreds of miles, are composed of numerous cloud formations that collectively produce massive quantities of precipitation—often measured in trillions or quadrillions of liters. In contrast, cloud seeding operations typically affect less than 1 percent of the total water budget in a given area.1

Because these large-scale weather systems are driven by complex atmospheric dynamics and produce precipitation on a scale that dwarfs the output of any single seeded cloud, human intervention through seeding is unable to change the total rainfall in an appreciable manner. Furthermore, while seeding may coax up to 20 percent more precipitation from clouds already capable of producing rain under optimal conditions, it remains impossible to specify the exact meteorological conditions under which such efforts will be effective, or to determine how much rain would have fallen naturally without the intervention.12