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

Seeding does not make clouds. It nudges clouds that already exist.

1. What is cloud seeding?

The World Meteorological Organization (WMO) defines weather modification as deliberate intervention in the atmosphere to change local weather. Cloud seeding is the typical technique. It disperses substances into clouds. The aim may be more rain, less hail or thinner cloud.

The WMO says it neither promotes nor discourages the practice. It only encourages sound research and best practice.

• It works on local to regional scales.
• It needs clouds that are already there.
• It is different from climate intervention, which works at global scale.

2. Can seeding make rain from a clear or dry sky?

No. The WMO says weather systems hold too much energy. It says it is impossible to create cloud systems that rain. It also says humans cannot shift winds to bring in water vapour. Claims of such big effects have no sound scientific basis.

The Indian research report gives the conditions for a suitable cloud. For hygroscopic seeding:

• Warm cloud depth of one kilometre or more.
• Upward air speed above 1.5 metres per second.
• Liquid water content above 0.5 grams per cubic metre.
• No rain already falling from the cloud base.
• Relative humidity above 60% in the 2 to 6 km layer.

For glaciogenic seeding, the cloud needs supercooled liquid water. That is water drops below 0 °C. The report gives a minimum of 0.05 grams per cubic metre.

3. What is the difference between hygroscopic and glaciogenic seeding?

The WMO says hygroscopic seeding adds particles to change the number and size of liquid drops. Glaciogenic seeding changes the number and size of ice crystals.

The IITM report adds the practical details.

• Hygroscopic: done in warm clouds. Flares of calcium chloride burn at the cloud base. Particles attract water vapour and form droplets.
• Glaciogenic: done in cold clouds that hold both ice and water. Silver iodide particles go inside the cloud. They raise ice-particle production.

4. Why is silver iodide used for ice seeding?

An IITM page traces the idea to the late 1940s. Langmuir and colleagues found that dry ice dropped into a supercooled cloud turned water to ice. Soon after, they found silver iodide had the same nucleating property.

A 2016 review in Atmospheric Chemistry and Physics calls silver iodide one of the best-studied ice-nucleating substances. It is used for glaciogenic seeding. The same review says its results are complex and sometimes conflicting.

The review says a better match between the crystal pattern and ice raises the freezing temperature. So the “threshold temperature” for silver iodide is not one fixed number in the literature.

A seeded cloud and a natural cloud look alike. Proof needs careful design.

5. How does an aircraft actually seed a cloud?

The IITM report describes the steps. A suitable area is chosen using radar and model data. A seeder aircraft flies to the target. The crew checks cloud properties. If the cloud qualifies, flares are burned.

• Hygroscopic: four calcium chloride flares burn at the cloud base. Each flare tube is 12 cm long and 7 cm wide. Each took 4 minutes to burn in the experiment.
• Glaciogenic: one silver iodide flare per cloud. It burns at the cloud top, or is ejected inside convective clouds.

6. How do scientists prove that seeding caused extra rain?

The IITM report says natural and seeded rain cannot be told apart without scientific evaluation. The WMO asks for a randomised design. Only some suitable events are treated. Seeded events are then compared with unseeded ones.

CAIPEEX followed this. Sealed envelopes with “seed” or “no-seed” were prepared by a group outside the campaign. In 2018-19 it randomised 278 clouds. It evaluated 276 convective clouds. Of these, 151 were seeded and 125 were not.

The WMO also warns that goals of operational programmes are often hard to confirm with confidence.

7. How ready is India to run seeding as a regular service?

The Ministry of Earth Sciences told the Lok Sabha on 23 July 2025 that IITM has built know-how. Work under Mission Mausam includes a 16 m tall cloud chamber. It also includes hired seeding aircraft and drones. Seeding materials and dispensers are being developed with industry.

The reply says operational readiness is held back. India has no aircraft at its disposal. It also needs permissions to operate in areas relevant to weather modification. Most technology can be imported, but more development is needed in India.

Rain does wash particles out of the air. Whether seeded rain does it well is less clear.

8. How does natural rain remove particles from the air?

Rain removes particles in two main ways. Raindrops can collect particles as they fall. Particles can also act as seeds for cloud droplets and leave with the rain.

• A Beijing wetland study in summer found wet deposition (92%) was the more efficient route for PM2.5. Rainfall intensity and particle size changed the efficiency.
• A Tokyo study found that cloud-droplet formation controls the wet removal of small particles. Removal depended strongly on particle size.

9. What did a recent seeding study find about PM2.5 and PM10?

A 2026 study in the Yangtze River Delta, China, looked at silver iodide seeding. Observations came from Xuzhou and Suqian in 2022-23. The study found particle removal rises in a straight line with rainfall intensity above a threshold.

In one aircraft experiment in Suqian, rain fell at 0.1 to 0.3 mm per hour after seeding. Over three hours, PM10 fell 25% to 29%. PM2.5 fell 10% to 16%.

The model showed the effect was local.

• Across the whole 9,000 sq km domain, PM fell by less than 1%.
• Over 100 sq km with strong updrafts, the fall was 4% to 5%.
• Over 40 sq km, it was more than 10%.

10. What does the Government of India say about seeding as a pollution fix?

In July 2025 the Ministry of Earth Sciences said the matter is “an area of research“. It said hazards and the pollution benefit must be studied using the CAIPEEX guidelines. It called this a prerequisite for any government policy on regulation and future use.

On 5 February 2026 the Ministry told the Rajya Sabha that Delhi’s activity is at trial stage. The aim is one more option, as an emergency measure, during episodic high-pollution events. It applies only when suitable cloud cover is present.

The same weather that traps smoke also lacks the right clouds.

11. What makes Delhi-NCR's air so bad after the monsoon?

The Government says many sources add up. They include vehicles, industry, and dust from construction and demolition. Road dust, biomass burning, waste burning and landfill fires also add to it.

In the post-monsoon and winter months, the weather traps the pollution. The reply names lower temperatures, lower mixing heights, inversion and stagnant winds. Stubble burning and firecrackers worsen it.

12. What did experts say in 2024 about seeding in Delhi's winter?

Delhi’s environment minister wrote to the Union Government. The letters were dated 30 Aug, 10 Oct, 23 Oct and 19 Nov 2024. The Centre asked IMD, CAQM and CPCB for views.

The expert opinion, shared with Delhi on 30 Oct 2024, listed four problems.

• Winter clouds come with Western Disturbances, which are short-lived. When low clouds form, natural rain often falls anyway.
• Clouds above 5-6 km cannot be seeded because of aircraft limits.
• The cold, dry winter generally lacks the cloud conditions seeding needs.
• A dry layer below the cloud could evaporate the rain before it reaches the ground.

The reply also noted doubts about efficacy and effects of the chemicals. CAQM held a stakeholder meeting on 27 Nov 2024. The reply is dated 5 Dec 2024.

13. How does CPCB label air quality?

The National Air Quality Index (AQI) turns many pollutant readings into one number, a name and a colour. It has six categories: Good, Satisfactory, Moderately Polluted, Poor, Very Poor and Severe.

The worst pollutant sets the overall AQI. For PM2.5, the 24-hour ranges in µg per cubic metre are:

• Good: 0-30. Satisfactory: 31-60.
• Moderately Polluted: 61-90. Poor: 91-120.
• Very Poor: 121-250. Severe: above 250.

Delhi backed a pilot with IIT Kanpur. The flights came in October 2025.

14. Who approved the Delhi project, and what did it cost?

The Cabinet approved it under Cabinet Decision No. 3200 dated 10.05.2025. The Delhi Environment Department and IIT Kanpur run a pilot. It is titled “Technology Demonstration and Evaluation of Cloud Seeding as an Alternative for Delhi NCR Pollution Mitigation”.

All India Radio reported five trials at a total outlay of ₹3.21 crore. Of this, ₹2.75 crore was the estimate for the trials, at ₹55 lakh each. IIT Kanpur handles planning, aircraft, chemical dispersal, modelling and logistics.

By 5 February 2026, Delhi had paid ₹37,93,420 as the first instalment to IIT Kanpur.

15. What was the original plan for the sorties?

An Akashvani report on the plan gave a flight window of 4 to 11 July (2025). The report said IIT Kanpur had sent its flight plan to IMD Pune. A proposal had also gone to the DGCA for a backup window.

The plan had five sorties over low-security airspace in north-west and outer Delhi. Each sortie would last about 90 minutes. Each would cover about 100 sq km. The sources checked do not say why the flights moved to October.

16. What flew over Delhi in October 2025?

DD News reports a first trial flight on 23 October. An Akashvani report of 24 October 2025 also says the first trial flight was the day before, so 23 October. Operations were planned for 28-30 October. On Tuesday, 28 October, IIT Kanpur flew a Cessna aircraft. It entered Delhi from the Meerut side.

Environment Minister Manjinder Singh Sirsa named the areas covered.

• Khekra
• Burari
• North Karol Bagh
• Mayur Vihar

He said eight flares were used. Each weighed 2 to 2.5 kg. One flare burned for 2 to 2.5 minutes. He said the clouds had 15-20% humidity. Poor visibility delayed the flight from the 12:30 pm schedule.

17. Which chemicals did the Delhi flares carry?

DD News says the aircraft carried “salt-based and silver iodide flares“. No official page checked gives the exact recipe or the amounts.

The UPSC option pairs silver iodide with potassium iodide. The Delhi report names silver iodide with salt-based flares. These are two different statements. Neither source says potassium iodide was in the Delhi flares.

18. What did the trial achieve, according to official sources?

The official pages checked give no measured rainfall figure. They give no measured change in PM2.5 either. The Ministry’s reply of 5 February 2026 only says the activity “is at trial stage”.

After the 28 October flight landed in Meerut, the minister said IIT Kanpur believed rain could come anywhere between 15 minutes and 4 hours. He said he hoped for positive results. He also said a longer plan could follow if the results were good.

News reports of trace rain and small PM2.5 drops exist. They were not checked, so they are not used here.

IITM’s CAIPEEX experiment is India’s main evidence.

19. What did CAIPEEX IV find at Solapur?

CAIPEEX IV ran in the rain-shadow region of Solapur, Maharashtra. The IITM report gives the headline numbers.

• Rain rose by about 46 ± 13% at some rain gauges downwind.
• Radar showed about 18 ± 2.6% more rain in a 100 sq km area downwind.
• The result held above the 95% confidence level.
• The report estimates about 867 million litres of extra water.

The study years differ by source. The PIB of 23 Jul 2025 says 2018-2019. The PIB of 5 Feb 2026 says 2017-2019. The report gives 2018-19 for the field data. Its summary text says 2017-19.

The method used was hygroscopic seeding of isolated convective clouds. Clouds seeded early in their growth gave more rain and lasted longer.

20. Did silver iodide seeding work in the same experiment?

The report says glaciogenic seeding did not give significantly different rain at the seeding location. Seeded and unseeded clouds looked alike there. Some extra rain showed downwind, helped by the weather pattern.

The report also advises hygroscopic seeding in the monsoon season. It says this gives more rainfall and longer-lived clouds.

So the best Indian evidence is for warm-cloud salt seeding in the monsoon. It is not for silver iodide seeding in winter.

21. How much does seeding cost?

The CAIPEEX report gives an operating cost of ₹15 lakh per seeding flight. One flight gave about 8.4 million litres of extra water. That works out to about 18 paise per litre.

It says seeding may not end drought alone. It may help manage drought through catchment-scale projects. It adds that costs could fall with Indian aircraft and equipment.

Delhi’s ₹55 lakh per trial is for a different goal and set-up. The two figures are not a like-for-like comparison.

22. How long has India tried cloud seeding?

IITM’s history page and FAQ list the milestones.

• 1951: Tata firm used ground-based silver iodide generators over the Western Ghats.
• 1957-66: a north India programme with ground-based salt generators. It showed about 20% more rain.
• 1973-86: IITM’s Baramati experiments in Maharashtra (1973-74, 1976 and 1979-86) showed about 24% more rain.
• 2003-16: Andhra Pradesh seeding programme.
• 2012: three-river-basin project in Karnataka.
• 2017 and 2019: the Varshadhare project in Karnataka.
• 2019: Maharashtra Rainfall Enhancement Project.

The FAQ says these programmes used area seeding. So firm conclusions are hard.

Many countries seed clouds. Evidence and safety are still debated.

23. How widely is seeding used, and where is the evidence strongest?

The WMO says operational programmes run in more than 50 countries. Its statement carries information only up to June 2024. The IITM report says more than 56 countries, naming Australia, China, Russia, Thailand, the UAE and the USA.

The WMO says the strongest recent evidence is for wintertime glaciogenic seeding of orographic clouds. These are clouds lifted by mountains. It says the signal is masked in very complex clouds such as supercells.

24. Is silver iodide safe for people and the environment?

The WMO says published studies show no significant impact on human health or the environment. This holds for silver iodide and the other common agents used so far. It adds a condition. A much larger amount, or a new agent, should be evaluated first.

It also says downwind and ecological effects have been suggested in some studies. These need more investigation.

The IITM report says CAIPEEX IV did not attempt a detailed environmental assessment. The Ministry also lists the hazards as a research question.

UPSC asked one question in the 2025 Prelims.

25. What did UPSC CSP 2025 ask about artificial rain?

Question 85 of General Studies Paper I (Set A, printed page 37) read:

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

• (a) silver iodide and potassium iodide
• (b) silver nitrate and potassium iodide
• (c) silver iodide and potassium nitrate
• (d) silver nitrate and potassium chloride

Answer: (a) is the pair widely reported, but the key is not confirmed. UPSC has not published an answer key for this paper; its answer-keys page (checked 8 Oct 2026) lists none.

The evidence supports silver iodide as the standard ice-seeding agent (IITM, WMO). Options (b) and (d) lack it. No official page checked names potassium iodide as a seeding agent.

A 2016 review does say potassium iodide and silver nitrate are used in the lab to make silver iodide particles. That is a likely reason these salts appear as options.