A chemical crystalline – silver iodide, the most common—is scattered to the clouds to make it heavy. Potassium iodide and dry ice are also used. Of course, everything is studied before scattering the substance; from the track of selected cloud, its density and buoyancy. The dispersion of silver iodide is commonly done by plane.
It’s not only used in promoting rainfall, but also used to improve air quality by downing pollutants hanging in the atmosphere.
Supplying the needed rainfall is its main pro, but, some researchers criticize the use of silver iodide in cloud seeding. According to them, high doses of the substance is toxic, since a part of its compound is silver, a heavy metal classified as toxic. Prolonged exposure can cause argyria, where the skin turns blue or bluish-grey. However, the arguments were blocked by scientists, mentioning the toxicity used in cloud seeding is low in order, and only use 1% of silver – in industry emission– on the atmosphere.
Another is the unpredictable behavior of the clouds. The rainfall could fall to untargeted areas, making it an act of risk and foresight. In other countries, heavy rain, snowfall and hailstorms often happen due to this kind of manipulation.
Well, all things have its cheers and jeers. It’s not only cloud seeding that can save us from the scorch, it is the conservation of water and food that’s prime. Survivability and adaptability depends on us, because if not, we humans would have gone extinct thousands of years ago.
Sources: https://en.wikipedia.org/wiki/Cloud_seeding
http://www.channelnewsasia.com/news/asiapacific/philippines-braces-for/2103586.html
(written by Rex Felix C. Salvador I, edited by Jay Paul Carlos, additional research by Lovely Ann Cruz)