Cloud seeding, a weather modification technique, has emerged as an innovative attempt to mitigate the persistent problem of air pollution. The idea of inducing artificial rain to cleanse the air of suspended particulate matter (PM) has recently gained renewed attention, particularly in India, where Delhi has faced alarming levels of air pollution during the post-monsoon season. The recent cloud seeding trials conducted over the city mark the first such attempt in nearly five decades, aiming to settle smog and PM through induced rainfall.
Understanding this science of cloud seeding, its history and outcomes in Delhi, provides crucial insights into both its potential and limitations as a pollution control strategy.
The Concept of Cloud Seeding
Clouds are composed of tiny droplets of water that form when water vapour condenses around tiny particles, such as dust or salt suspended in the atmosphere. Without these particles, called condensation or ice nuclei, raindrops or snowflakes could not form, and precipitation does not occur. Cloud seeding enhances a cloud’s ability to produce rain or snow by introducing additional artificial nuclei, generally silver iodide or other salts into suitable subfreezing or moisture-bearing clouds. These nuclei act as a base for water droplets or ice crystals to form, which then combine and fall as precipitation, thereby increasing rainfall or snowfall.
Globally, the process has been shown to increase snowfall or rainfall by around 10 to 15 per cent in long-term projects such as those conducted in Nevada, the Snowy Range and Sierra Madre Range of Wyoming, the Bridger Range of Western Montana, and the Snowy Mountains of Australia. Both ground-based generators and aircraft are used to release silver iodide solution into the clouds when storm systems with adequate moisture are present. This method is environmentally safe and is not known to cause harm to humans or wildlife. However, the presence of moisture-laden clouds is essential for any seeding to succeed, as the process could not be carried out during dry weather or when storm systems are absent.
Mechanism and Requirements for Cloud Seeding
The Indian Institute of Tropical Meteorology (IITM), Pune, in its 2024 document, described cloud seeding as the modification of suitable clouds with ‘seed’ particles to enhance rainfall.
In natural cloud formation, water vapour condenses on small particles to form droplets that grow and eventually fall as rain when they become heavy enough. In artificial induction, salts, such as silver iodide, potassium iodide, or sodium chloride act as ‘cloud condensation nuclei’ or ‘ice nuclei’ to facilitate the amalgamation of water droplets. These salts are dispersed using aircraft, drones, rockets, or ground-based generators.
In the Delhi experiment, flares were deployed, which were pyrotechnic material with burning agent compressed inside a tube. These flares were attached to the wings of an aircraft. These flares released salt-based particles into clouds. Each flare weighed about 2 to 2.5 kg and was dropped into clouds with a humidity level of 15 to 20 per cent. However, for successful rainfall induction, certain meteorological conditions are critical. There must be sufficient cloud cover, specific cloud depth with adequate moisture, relative humidity above 50 to 60 per cent with cloud temperatures around –5 °C or lower in the case of ‘cold clouds’ (or –20 °C to 7 °C in some protocols). Additionally, favourable wind patterns, vertical up-drafts and cloud thickness of at least 1 km are required. These detailed requirements emphasise the ‘highly conditional’ nature of the technique.
Historical Development of Cloud Seeding
Cloud seeding, as a scientific concept, dates to the 1940s. General Electric scientists William Schaefer and Bernard Vonnegut discovered that dry ice could form ice crystals in their lab freezer and then they decided to extend the experiment to natural clouds. Their efforts successfully produced snow over Pittsfield, Massachusetts, US, leading to the launch of Project Cirrus by the US government. Initially, there were ambitions to control hurricanes through weather modification, though this objective was never realised. In the 1950s and 1960s, cloud seeding became popular globally as a tool for weather modification. The Soviets conducted seeding over Leningrad to ensure clear skies during May Day parades and China adopted similar methods for the 2008 Olympics opening ceremony. The US introduced Project Skywater, using aircraft to disperse silver iodide over the Rockies.
In India, the first experiments began in 1952 under S. K. Banerji, the first Director-General of the Indian Meteorological Department (IMD). Salt and silver iodide were released through hydrogen balloons in Kolkata, later replaced by ground-based rockets. The results were inconsistent; while increased rainfall was observed on seeded days, it could not be conclusively linked to the seeding. A similar attempt in Delhi, in 1962, had further failed. From the 1970s onwards, aircraft-based seeding gained prominence and states facing drought experimented with the method. However, issues like high costs, uncertain results, and lack of systematic evaluation limited its widespread adoption.
The CAIPEEX Experiment in India
A significant step towards scientific evaluation of cloud seeding in India came through the research project Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX), initiated in 2009 by IITM Pune. Conducted over several phases between 2009 and 2019, it aimed to quantify the benefits of cloud seeding by studying cloud physics and droplet formation. The experiments over Solapur, a drought-prone region in Maharashtra, used calcium chloride instead of silver iodide. Aircraft fired flares into selected clouds, while others were left unseeded for comparison. The results were notable: an additional 867 million litres of water were generated, with a 46 per cent increase in rainfall over seeded locations and an 18 per cent increase over a 100-square-kilometre downwind area. These findings led to the development of comprehensive protocols and guidelines for future weather-modification operations in India, highlighting possible applications, such as targeted rainfall enhancement, fog suppression, marine cloud brightening, and hail prevention.
Cloud Seeding in Delhi: the 2025 Trials
In September 2025, the Delhi government signed a Memorandum of Understanding (MoU) with Indian Institute of Technology (IIT), Kanpur, to address its severe winter air pollution through cloud seeding. The project, estimated at a cost of Rs 3.5 crore, aimed to induce rainfall and improve air quality. The Cessna 206H aircraft operated from the Hindon airbase, and flew over Delhi covering areas, such as Burari, Mayur Vihar, Khera, and Karol Bagh. The plan was to conduct several trials between late October 2025 and February 2026, depending on the weather conditions. On October 28, 2025, two flights were carried out, with flares dispersed in phases. However, the results were disappointing. The clouds lacked sufficient moisture, and no significant rainfall occurred. However, some parts of Delhi reported light drizzles and a marginal improvement in air quality.
Experts attributed to failure primarily to the absence of monsoon-type clouds, which are essential for seeding success. In 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. However, they may be inadequate for the purposes of cloud seeding. With relative humidity at only 15 to 20 per cent, far below the required 50 to 60 per cent, the clouds could not sustain the condensation process needed for rainfall. The IMD noted that October to December is generally a dry season with calm winds and limited cloud formation, making seeding efforts largely ineffective. Consequently, several scheduled trials have been put on hold.
The Observed Impact and Insights
Although the Delhi experiment did not yield the expected rain, it was not without value. Observations showed a temporary reduction in PM concentration following the trials. Data indicated a 15–18 per cent improvement in PM2.5 levels and 17–19 per cent reduction in PM10 levels shortly after seeding, demonstrating that even limited drizzle or increased atmospheric moisture could have a cleansing effect. However, the improvement was short-lived as pollutant levels rose again the following day when no trials were conducted. This underscored that the benefits of cloud seeding, particularly for air quality, are transient and would require multiple successive operations for a lasting effect.
The experiment further provided valuable data on Delhi’s atmospheric conditions, helping researchers understand the challenges of conducting cloud seeding during the winter-transition period. It emphasised the critical role of humidity, temperature, cloud structure in determining outcomes. While the attempt did not succeed in producing significant rainfall, it highlighted the need for precise timing satellite validation, Doppler radar tracking, and high-resolution meteorological modelling for future trials. IIT Kanpur expressed its intent to continue studies to improve the process during subsequent seasons.
Cloud Seeding and Air Pollution Mitigation
The rationale behind using cloud seeding to combat air pollution lies in the natural cleansing effect of rain. As raindrops fall through the atmosphere, they attract and capture aerosol particles, a process known as coagulation, which helps remove pollutants, such as soot, sulphates, and organic matter. Even short-duration rainfall could provide temporary relief from smog by settling PM. According to experts, while cloud seeding could not address the root causes of air pollution such as vehicular emissions, industrial activities, or stubble burning, it could serve as a short-term mitigation measure during severe pollution episodes. Gufran Beig, founder project director of the System of Air Quality and Weather Forecasting and Research (SAFAR) project, has noted that while the effect would be temporary, any successful rainfall could help break the persistent flow of pollutants.
Nevertheless, there are limitations and concerns. The process is highly condition-dependent and could not guarantee rain on demand. Inappropriate timing or lack of suitable clouds could lead to failure, as observed in Delhi. Furthermore, repeated or large-scale use may lead to possible chemical accumulation in the atmosphere and ecological impacts, though substances like silver iodide are generally considered safe at low concentrations. Ethical and geopolitical concerns also emerge, such as the potential diversion of rainfall from neighbouring regions or inadvertent changes in rainfall patterns. Focusing on cloud seeding as a pollution-control strategy may distract from essential, long-term reforms in air-quality management and sustainable urban planning.
Way forward
Cloud seeding represents a fusion of atmospheric science and environmental management offering both promise and complexity. Its long global history and the results from experiments like CAIPEEX demonstrate its potential for enhancing precipitation under suitable conditions. In the context of Delhi’s air pollution crisis, the 2025 trials underscore both the challenges and scientific insights gained. While the absence of conducive clouds limited its success, the effort provided short-lived improvements in air quality. As researchers refine their understanding of India’s atmospheric patterns and as technology evolves, future applications of cloud seeding, executed under optimal conditions, may well contribute to a more sustainable approach to managing both drought and pollution. For now, it remains a reminder that even advanced scientific interventions must align closely with nature’s own rhythms to succeed.
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