We have seen that there is considerable evidence suggesting that anthropogenic activity, either in the form of constructing major urban areas, changing natural landscape to agricultural and grazing areas, or emission of particles and gases, has contributed to changes in weather and climate on the regional scale. There is a diversity of views on the demonstration of this evidence, however, with a distinction between deliberate and inadvertent weather and climate changes. It is curious, for example, that the scientific community has accepted the rainfall change results obtained in studies such as METROMEX as being valid, yet has questioned the validity of cloud-seeding-induced changes in rainfall inferred from well-designed, randomized cloud seeding experiments.

The answer to this paradox lies in human psychology. As an example, Dr. Stan Changnon described a conversation he had with Dr. John Tukey, one of the world's leading statisticians, following a meeting of the Weather Modification Advisory Board in the 1990s. Stan asked John why the statisticians had been very critical of attempts to prove planned weather modification of clouds and rainfall was successful, yet were not so critical of inadvertent weather modification (i.e., the cities are not randomized). John looked up at Stan and said, “Well, Stan, in the end it is just a lot more believable that a big city can cause clouds, rain, and hail than it is that a small amount of seeding material can.”