Explanation

Cloud seeding is a weather modification technique used to artificially induce rainfall — increasingly employed in cities to combat severe air pollution by washing out pollutants like dust and particulate matter from the atmosphere.

In cloud seeding, chemicals such as silver iodide (AgI) and potassium iodide (KI) are dispersed into clouds via aircraft or helicopters. These substances act as ice nuclei, around which water vapour condenses and freezes, eventually forming ice crystals heavy enough to fall as precipitation.

Cloud seeding is broadly classified into two types. Hygroscopic cloud seeding introduces salt particles at the base of liquid clouds, encouraging water droplets to combine and grow heavy enough to fall as rain. Glaciogenic cloud seeding releases substances like silver iodide or dry ice into supercooled clouds, triggering the formation of ice crystals that lead to precipitation.

The other options — silver nitrate, potassium nitrate, and potassium chloride — are not used in cloud seeding operations and serve entirely different chemical purposes.