Why is it difficult to measure how much rain would have fallen without cloud seeding?

Determining how much rain or snow would have fallen without cloud seeding is difficult because precipitation is highly variable in both time and space. Because of this natural complexity, it is not possible to answer that question satisfactorily based on physical theory alone.1

Several factors influence the amount of precipitation a cloud produces, including its horizontal dimensions, cloud-top temperature, microphysical properties, and the strength and persistence of its updrafts. Because these variables are so complex, scientists have struggled to develop quantitative rainfall forecasts with enough accuracy to evaluate seeding schemes.1

Furthermore, because clouds are inherently dynamic, cloud thickness alone does not provide a unique specification of how much rain or snow will fall. While mathematical models are being developed to simulate these systems, the lack of randomized control procedures in many private or commercial operations has historically made it difficult for statisticians to verify the efficacy of these projects or isolate the specific impact of seeding from natural weather patterns.1