UPSC Prelims 2025 Q85: Cloud Seeding Chemicals — Explained

Artificial way of causing rainfall to reduce air pollution makes use of

silver iodide and potassium iodide
silver nitrate and potassium iodide
silver iodide and potassium nitrate
silver nitrate and potassium chloride

Answer: (a) silver iodide and potassium iodide

Statement-wise explanation

a: correct — Silver iodide is the standard cloud-seeding agent. Its hexagonal crystal lattice is very close to that of ice, so it acts as an ice nucleus in supercooled clouds (below 0 °C), and ice crystals grow at the expense of water droplets until they fall as precipitation. Silver iodide does not dissolve in water or acetone on its own. It is dissolved in acetone with an alkali iodide such as potassium iodide (or sodium iodide), and the solution is burnt in generators to release fine AgI particles.
b: incorrect — Silver nitrate is not a seeding agent. It is a soluble silver salt and does not form ice-like crystals. It is a laboratory reagent from which silver iodide is made (AgNO₃ + KI → AgI + KNO₃). Potassium iodide is right, but the silver compound is wrong.
c: incorrect — Silver iodide is right, but potassium nitrate is not its partner. Potassium nitrate is a fertiliser and an oxidiser, and it is the by-product left over when silver iodide is made from silver nitrate and potassium iodide. It plays no part in dissolving or dispersing silver iodide.
d: incorrect — Neither compound is the pair used for silver-based seeding. Silver nitrate is not an ice nucleus. Potassium chloride is a hygroscopic salt that can serve as a condensation nucleus in warm-cloud seeding, but common salt (sodium chloride) is the usual choice. Pairing it with silver nitrate does not describe any standard seeding mixture.

How to eliminate

Every option pairs a silver compound with a potassium compound, so the question tests which silver salt and which partner salt. Step 1: silver iodide is the most widely known cloud-seeding chemical. If you know only that, you can drop the two options that use silver nitrate, leaving two. Step 2: silver iodide has to be dissolved before it can be sprayed or burnt, and the salt that does this shares its anion, iodide (AgI forms a soluble complex with KI or NaI). Potassium nitrate has no such role. It is simply the waste product of the reaction AgNO₃ + KI → AgI + KNO₃. Note that the wrong options are built from the other chemicals in that same reaction. The question cannot be solved from absolute words or structure alone. You need to know that silver iodide is the seeding agent and that iodide salts go with it.

Know more

Cloud seeding is a form of weather modification. Chemicals are added to a cloud that already exists to speed up how its droplets or ice crystals grow into raindrops. It cannot create a cloud. Most clouds have plenty of water vapour but too few effective nuclei, and seeding supplies those nuclei.

Type	Cloud targeted	Agents	How it works
Glaciogenic	Cold clouds holding supercooled water (below 0 °C)	Silver iodide (dissolved with KI/NaI in acetone), dry ice (solid CO₂)	AgI's lattice resembles ice and triggers freezing. Dry ice chills the air so ice forms. Ice crystals grow at the expense of droplets and fall.
Hygroscopic	Warm convective clouds	Sodium chloride, calcium chloride, potassium chloride	Salt particles absorb water and form large droplets that collide and coalesce into rain.

The modern technique dates from 1946, when Vincent Schaefer seeded clouds with dry ice. Bernard Vonnegut then identified silver iodide as an ice nucleus. Seeding is delivered by aircraft-mounted flares, ground generators or rockets.

Why it is linked to air pollution. Falling rain removes particulate matter from the air by wet deposition (washout). For this reason Delhi has tested seeding against winter smog. The difficulty is that Delhi's winter air is dry. In the October 2025 IIT Kanpur trials, flares carrying silver iodide and salt compounds produced little rain because cloud moisture was only about 15–20%, against the roughly 50% needed.

India's research base. The Indian Institute of Tropical Meteorology (IITM, Pune) ran CAIPEEX over the rain-shadow region in 2018–19. It reported rainfall enhancement of about 18% on average over a 100 sq km area downwind of seeding, and up to about 46% at some locations. It also found that results depend heavily on the cloud.

In the news

On 10 May 2025, about two weeks before Prelims, the Delhi Cabinet approved (Decision No. 3200) a cloud-seeding pilot with IIT Kanpur titled 'Technology Demonstration and Evaluation of Cloud Seeding as an Alternative for Delhi NCR Pollution Mitigation'. This made 'artificial rain against air pollution' a live current-affairs topic.

NCERT Concepts to Revise

Cloud seeding: The question asks which chemicals are used in cloud seeding, the deliberate modification of a cloud to produce rain.
Cloud condensation nuclei: Seeding chemicals act by supplying nuclei. Knowing what nuclei do helps you judge which compounds could plausibly work.
Condensation: Seeding speeds up condensation or freezing of vapour into droplets and ice, and that process is what makes rain fall.