Cloud seeding

Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Beyond NCERT

Meaning

Cloud seeding is a type of weather modification (changing local weather on purpose). Particles such as silver iodide, dry ice or salt are spread into clouds that are already there. This can change how much rain or snow falls, reduce hail, or clear clouds [5].

• It matters because India has tested it against drought and winter smog, and the Ministry of Earth Sciences now has a research programme on it.
• It cannot make rain from a clear sky. Its gains are small and depend on having suitable clouds [5].

Explanation

How it works (cause → process → result)

• Cause: clouds often hold water that does not fall.
• Cloud droplets are very tiny. They stay floating in the air.

• Rain or snow falls only when droplets or ice crystals grow big and heavy.

• Process: seeding gives the water something to grow on.

• Particles are spread into a suitable cloud.

• Water vapour and droplets gather on these particles, so bigger drops or ice crystals form.

• Result: more of the cloud's water reaches the ground.

• This happens only if the cloud has enough moisture and the right temperature.

Two types of seeding

Type	Clouds targeted	Seeding agent	How it works
Glaciogenic seeding	Cold clouds: colder than 0 °C, with supercooled water (water that stays liquid below 0 °C)	Silver iodide, dry ice (solid CO₂)	Silver iodide has a crystal shape like ice, so ice forms on it. The ice crystals grow, become heavy and fall [6]
Hygroscopic seeding	Warm clouds: convective clouds (clouds formed by rising warm air) that are mostly warmer than −10 °C	Salt and other hygroscopic (water-attracting) particles	Larger droplets form. They hit and join other droplets, grow and fall as rain [6]

Conditions and limits

• Clouds must already be present.
• Seeding cannot create rain from a clear sky [5].

• WMO says claims to create rain systems, change wind patterns or remove severe weather have no scientific basis [5].

• How strong the evidence is depends on the type.

• Glaciogenic seeding of winter mountain clouds has shown clear cause-and-effect results [5].

• Results from hygroscopic seeding are mixed. A result from one place cannot be copied directly to another (WMO statement, 14 June 2025) [5].

• Proof is hard. It is difficult to show that rain fell because of seeding and would not have fallen anyway [5].

Debates

• Cost: aircraft, radar and chemicals are expensive, and the gains are small.
• Chemical residue: past operations show no significant health or environmental harm from silver iodide [5]. Some people still worry it may build up over time.
• "Rain theft": areas downwind may get less rain. WMO says these downwind effects need more study [5].
• Governance (who decides and who is responsible):
• Questions remain about who should decide on seeding, and who is responsible if floods or droughts follow.
• WMO advises independent expert review before new projects, regular checks on running projects, and reporting to WMO [5].
• WMO neither promotes nor discourages weather modification [5].

Where it occurs

• World: operational seeding programmes run in more than 50 countries [5]. The best results come from seeding winter mountain clouds [5].
• India: early trials. Cloud seeding was tested at Delhi during the monsoon months of 1957–59 [7].
• India: CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment):
• It is run by IITM, Pune (Indian Institute of Tropical Meteorology) under the Ministry of Earth Sciences [4].
• Phase IV (2017–2019) tested seeding in the rain-shadow region of Solapur, Maharashtra [2]. A rain-shadow region is the dry side of a mountain, away from the wind. Solapur lies east of the Western Ghats.
• Rain rose by up to about 46 ± 13% at some places. The average rise was about 18 ± 2.6% over a 100 sq km area downwind of the seeding spot [2].

• It added about 867 million litres of water, with a positive cost–benefit ratio [2].

• India: state programmes. Karnataka and Maharashtra have run their own seeding operations during droughts.

• India: Delhi smog. Trials tested seeding to wash pollutants out of Delhi's winter air with rain. Results depend on whether moist clouds are present, and winter air over Delhi is often dry.
• India: Mission Mausam (Ministry of Earth Sciences; Phase I: 2024–26) [3]:
• It has a weather-modification research area at IITM called the "Weather_MoD" vertical [2].
• Plans include a 16 m tall cloud chamber, hired seeding aircraft, seeding drones built with industry, and work on seeding materials and dispensers [2].

Don't confuse with

• Cloudburst: a natural event that drops about 100 mm or more of rain in one hour over a small area [1]. Cloud seeding is a human intervention, and its effect on rain is small.
• "Artificial rain from a clear sky": this is not possible. Seeding only helps clouds that already exist [5].
• Glaciogenic vs hygroscopic seeding: glaciogenic seeding uses silver iodide or dry ice in cold clouds. Hygroscopic seeding uses salt in warm clouds [6]. Exams often swap the agents.
• Orographic rain: natural rain that falls when moist wind is pushed up a mountain. Solapur gets little of it because it lies in the rain shadow, which is why it was chosen for CAIPEEX seeding [2].

Prelims Hooks

• Glaciogenic seeding = silver iodide or dry ice in cold clouds. Hygroscopic seeding = salt in warm clouds [6].
• Silver iodide works because its crystal shape is similar to ice [6].
• Cloud seeding cannot create rain from a clear sky. Suitable clouds must already be present [5].
• CAIPEEX belongs to IITM, Pune (Ministry of Earth Sciences) [4]. Its Phase IV (2017–19) was in the Solapur rain-shadow area and gave about 18% average rain gain [2].
• Mission Mausam belongs to the Ministry of Earth Sciences, with Phase I in 2024–26 [3]. Its "Weather_MoD" vertical at IITM plans a 16 m cloud chamber [2].
• India's first trials were at Delhi in 1957–59 (monsoon months) [7]. Operational programmes run in 50+ countries worldwide [5].

Mains Points

• Drought tool (GS-III): CAIPEEX gave modest gains that depended on clouds: about 18% average rain gain and about 867 million litres of extra water in Solapur [2]. This can help rain-shadow farmers in drought years. It cannot replace water conservation, watershed management or better crop choices.
• Pollution tool (GS-III): seeding to wash out Delhi's winter smog is limited because winter air is often dry and suitable clouds are rare. Lasting cleaner air needs emissions cut at the source.
• Governance and ethics (GS-III): there are open questions about proof of results, cost, silver iodide residue and "rain theft" from downwind areas [5]. India needs clear rules, independent expert review, regular checks and fair treatment of neighbouring areas, as WMO advises [5]. Mission Mausam's research vertical gives a science base for these rules [2].

Related concepts

• World distribution of rainfall

Read more

• World distribution of rainfall; cloudbursts and cloud seeding