"Hygroscopic cloud seeding, which was developed in South Africa in the early 1990s, involves dispersing water-absorbing salts (a mixture of sodium, magnesium, and potassium chlorides) into parts of clouds; the salts then collect water vapour and grow bigger and faster in size before falling as rain. Earlier seeding efforts used silver iodide, whose molecular structure is very similar to ice, but in the 1990s, a shift was made to the non-polluting hygroscopic salts. Under some conditions, ground-based seeding (which may employ anti-aircraft guns or rocket launchers) is cheaper and more optimal; but once you get up to or above the connective clouds, airplanes become necessary."
"By 1999, 40 countries had become involved in cloud seeding efforts, and randomised, double-blind experiments were starting to prove that cloud seeding did in fact work. Currently, estimates for how much seeding increases the amount of rainfall range between 15 and 40%."
"Some weather modification projects are very questionable, says Terblanche. Russia and China still invest in hail suppression, which lacks all scientific support, and some of their cloud-seeding techniques remain controversial. Other countries often launch short-term weather interventions in response to political pressure, rushing into projects based on shaky science."
