In Diwali’s aftermath, the Delhi government may be planning to carry out cloud seeding trials to induce artificial rain over the region amid high levels of air pollution, The Indian Express had recently reported. Two trial runs are already learnt to have been conducted near the airport.But this is not Delhi’s first brush with the experiment. On October 28, 2025, the Delhi government conducted two back-to-back cloud seeding trials, in which a team from IIT-Kanpur flew a small aircraft over parts of Delhi and released silver iodide and salt particles in flares. The experiment, however, was not exactly a success. An Indian Express report from that year found that air quality improvements on Tuesday began hours before the cloud seeding operation, and that pollution levels fluctuated and even rose afterwards. What’s more, Delhi received no rain, while Noida and Greater Noida received only 0.3 mm.How does cloud seeding work? What went wrong last time? And what could be the outcome of a fresh experiment around this time?What is cloud seeding?Cloud seeding refers to introducing substances such as silver iodide or sodium chloride into a particular class of clouds to artificially trigger rain.Cloud seeding experiments began to be carried out around the world after American chemist Vincent Schaefer and his colleagues at General Electric in New York made the first snowstorm in a laboratory freezer in 1946.Also Read | Nearly seven decades ago, a bid to seed clouds in New DelhiGenerally, water vapour condenses around small particles in the air to form the droplets that make up a cloud. These droplets collide and grow; as they get heavy and the cloud gets saturated, it rains.Story continues below this adIn case of cloud seeding, the seed particles are “cloud condensation nuclei (CCN), a particle on which water vapour condensates” or “ice nuclei particles, a particle on which water freezes”, the 2024 document said. CCNs have an affinity for water vapour to form cloud droplets or ice particles, respectively. Ice crystals grow faster than drops, and they enlarge and fall.How is cloud seeding carried out?To induce rain artificially, clouds are usually injected with salts like silver iodide, potassium iodide, or sodium chloride, which is the ‘seed’. These salts are expected to provide additional nuclei around which more cloud droplets can form. They are dispersed into the cloud either using aircraft or through generators on the ground.Rockets, drones or flares can be used for dispersal. In Delhi’s case, flares were deployed, which are pyrotechnic material and burning agent compressed inside a tube and attached to the wings of an aircraft. These flares dispense the seeding material into the clouds.Also read | What is smog and why does it blanket Delhi every winter?Last year, eight flares were used, each weighing around 2 to 2.5 kg. It took about two to two-and-a-half minutes to drop each flare into clouds that had 15 to 20% humidity.Story continues below this adHow exactly does rain from cloud seeding improve air quality?In India, cloud seeding has mostly been tried to deal with drought-like conditions, with varying results. A 2024 document by the Indian Institute of Tropical Meteorology (IITM), Pune mentioned experiments conducted over 100 km inland from the west coast of India over many years (Murthy et al., 2000). They found that certain conditions produced an increase in rainfall of 24% at 4% level.Generally, rain ends up washing away some pollutants, such as PM 2.5 and PM 10. Story continues below this adAn article from the Massachusetts Institute of Technology (MIT) explained the process thus: “As a raindrop falls through the atmosphere, it can attract tens to hundreds of tiny aerosol particles to its surface before hitting the ground. The process by which droplets and aerosols attract is coagulation, a natural phenomenon that can act to clear the air of pollutants like soot, sulfates, and organic particles.”What conditions are needed for cloud seeding?First, cloud cover and clouds of a certain type are necessary. M Rajeevan, former secretary of the Ministry of Earth Sciences, earlier told The Indian Express, “Cloud seeding can only happen if there is a sufficient number of clouds and a particular depth to these clouds. Inside, there needs to be an adequate number of cloud droplets. Cloud seeding is done to increase the radius of the cloud droplets so that they will grow bigger and because of gravity, they will come down as rainfall. But with a clear sky, you can’t do it.”Also Read | Express view on cloud seeding in Delhi: A damp squibIn winter, clouds form over Delhi when a western disturbance moves over the region. These are storms that originate in the Caspian or Mediterranean Sea and bring non-monsoonal rainfall to northwest India.Story continues below this adHowever, they may be inadequate for the purposes of cloud seeding. Sachchida Nand Tripathi, Professor at IIT Kanpur, had said, “In winter, you don’t see the kind of clouds that are needed for seeding, but western disturbances are the way through which clouds form. Even if clouds are there, you need to see what their height is, what their liquid water content is.”What does this tell us?That cloud seeding may not work in Delhi in the post-autumn months. The moisture content of late autumn clouds are insufficient to wash away the particulate matter in Delhi’s air.Even when it does work, there should be “a significant amount of rain so it washes away pollutants,” Gufran Beig, founder project director of the government’s System of Air Quality and Weather Forecasting And Research (SAFAR), earlier said. “It will only be temporary, but if at all it is successful, it will break the flow of pollutants.”Also Read | Cloud seeding can’t be a reliable fix for Delhi’s winter pollution: IIT-Delhi studyApart from concerns about the efficacy and the temporary effect, doubts have been cast over the strategy to target the effect rather than the cause of air pollution — vehicular pollution, industrial emissions, and dust around the year, with stubble burning, weather conditions and fireworks during the winter months worsening the situation in areas in and around Delhi.Story continues below this adA study carried out at the Indian Institute of Technology Delhi concluded that cloud seeding cannot be a “primary or reliable strategy” to manage winter air pollution in Delhi as the capital is climatologically unsuitable for it.Cloud seeding, the study says, “should be viewed, at best, as a potential high-cost, emergency short-term measure, contingent on stringent forecasting criteria”. It underscores that “sustained emission reduction remains the most viable and necessary long-term solution” to the problem of air pollution.