Cloud Seeding: How Artificial Rain Is Used to Fight Air Pollution

Every winter, Delhi’s air turns toxic and officials look for anything that might help. One idea keeps coming back: artificial rain. Aircraft fly into the sky and release chemicals into clouds, hoping to trigger rainfall that washes pollutants out of the air. UPSC’s 2025 prelims asked exactly which chemicals this process uses.

A ground-based silver iodide generator, the same seeding agent used in aircraft-based cloud seeding. (Photo: Esteban9, Wikimedia Commons, Public Domain)

Cloud Seeding: Chemicals & Delhi’s 2025 Trial

Seeding Sorties, Delhi 2025 (IIT Kanpur)

Silver Iodide

Primary

Ice-Nucleating Agent

Potassium Iodide

Supplementary

UPSC 2025 Answer, Part 2

Sodium Chloride

Field Trials

Used in Real Delhi Flares

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📑 Contents

1Must Know
2Good to Know
3Test Yourself
4Great to Know
5Current Affairs
6Previous Year Question

✊ Must Know

1. What Is Cloud Seeding?

Aim Cloud seeding is a way to boost rainfall from clouds that already exist. It cannot create rain from a clear sky.
Method Aircraft or ground generators release chemicals into clouds. These chemicals act as tiny particles that water droplets or ice crystals can form around.
Purpose in India Delhi has tested cloud seeding to wash pollutants out of the air during winter smog. States have also used it to fight drought.

2. Silver Iodide: The Primary Ice-Nucleating Agent

Why It Works Silver iodide’s crystal structure closely resembles ice. This makes it an efficient trigger for ice-crystal formation inside a cloud.
Temperature It can nucleate ice at relatively warm sub-zero temperatures, as high as about -3°C. Most seeding operations use it between -5°C and -20°C.
UPSC 2025 Silver iodide is the first component of the correct answer to Q85 — option (a), silver iodide and potassium iodide.

3. Potassium Iodide: The Supplementary Seeding Agent

Role Potassium iodide is used alongside silver iodide as a supplementary seeding agent in some cloud-seeding formulations.
The Trap UPSC’s wrong options swap in silver nitrate, potassium nitrate, or potassium chloride. None of these three combinations is the standard seeding pair.
Answer The correct pairing, per UPSC’s own 2025 answer key, is silver iodide and potassium iodide — option (a).

📘 Good to Know

1. Two Ways to Seed a Cloud: Ice Nucleation vs Hygroscopic

Glaciogenic This method adds ice nuclei, like silver iodide, to cold supercooled clouds. Ice crystals form, grow heavier, and fall as rain or snow.
Hygroscopic This method adds large salt particles to warmer, low-altitude clouds. The salt attracts water and speeds up droplet growth until rain falls.
Choice Which method gets used depends on the cloud’s temperature and height. Cold, tall clouds suit glaciogenic seeding; warmer, shallow clouds suit hygroscopic seeding.

2. How the Seeding Process Actually Works

Flares An aircraft carries seeding flares, each weighing around 2 to 2.5 kilograms. It ignites and releases them into or below a target cloud.
Requirement The cloud must already carry enough moisture, generally around 50 percent, for seeding to trigger real rainfall.
Limit Delhi’s November 2025 trial saw thin cloud cover with under 20 percent moisture. This kept the resulting rainfall very small.

3. Real Trials Often Use Sodium Chloride Flares Too

Field Practice Delhi’s actual 2023-2025 field trials used silver iodide together with sodium chloride, or common salt, flares.
Exam vs Field UPSC’s textbook answer for Q85 stays silver iodide and potassium iodide. Real trials can vary by vendor and by which flares are available.
Why It Matters Both facts are true at once: the standard exam-answer pairing, and the operational chemistry actually flown over Delhi.

Test Yourself

🏅 Great to Know

1. Does Cloud Seeding Actually Cut Air Pollution?

Evidence Scientists have not established firm proof that cloud seeding reliably lowers air pollution. Studies on this specific use remain limited.
Delhi’s Report The Delhi government said its 2025 trials reduced particulate matter at some targeted sites. Independent researchers have questioned how strong that effect really was.
Key Constraint Cloud seeding only works when clouds with real moisture are already present. It cannot manufacture rain during a fully dry spell.

2. Cloud Seeding Beyond India

UAE The United Arab Emirates runs a large Rain Enhancement Programme. It seeds clouds regularly to boost the country’s scarce fresh water supply.
China China has used cloud seeding for decades, including a large operation to clear skies before the 2008 Beijing Olympics.
United States Several US states, including Colorado, use silver iodide seeding to boost mountain snowpack for later water supply.

📰 Current Affairs

1. Delhi’s 2023-2025 Cloud-Seeding Trials

Approval The Delhi cabinet approved five cloud-seeding sorties with IIT Kanpur, at an outlay of about ₹3.21 crore. (Source: News on Air)
First Flight The first trial flew over parts of Delhi using a Cessna 206H aircraft carrying silver-iodide-based flares. (Source: Deccan Herald)
Result Thin cloud cover and low moisture limited actual rainfall, though Delhi’s government reported some drop in particulate matter at target sites. (Source: Phys.org)
Next Round IIT Kanpur is now planning fresh cloud-seeding trials over Delhi for summer 2026, pending fresh regulatory approval. (Source: Careers360)

2. India’s Older State Rain-Enhancement Programmes

Origins India first tried cloud seeding in the 1950s, using ground sprayers over Delhi and a longer seeding programme across north India.
Tamil Nadu Tamil Nadu ran cloud-seeding operations during droughts in 1983, 1984-87, and 1993-94.
Karnataka Karnataka launched “Project Varshadhari” in 2017, flying seeding aircraft over Bengaluru, Gadag, and Yadgir districts.
Other States Maharashtra and Andhra Pradesh have also run their own drought-relief cloud-seeding operations over the years.

📝 Previous Year Question

UPSC CSP 2025 — Chemicals Used in Artificial Rainfall

UPSC 2025 The correct answer is (a) silver iodide and potassium iodide. Silver iodide is the primary ice-nucleating agent; potassium iodide is the supplementary seeding agent. The other three options swap in silver nitrate, potassium nitrate, or potassium chloride, none of which is the standard seeding pairing. See UPSC CSP 2025 GS Paper I, Q85.

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