What conditions are needed for cloud seeding, and why have earlier proposals for it come under criticism? We explain.
At around 3 pm on Tuesday (October 28), a small aircraft known as the Cessna 206H conducted a cloud seeding trial in Delhi amid the recent spike in air pollution. The aircraft took off from an airstrip at IIT-Kanpur and landed in Meerut. It then flew over Delhi and covered areas including Burari, Mayur Vihar, and north Karol Bagh for the experiment.
Delhi Environment Manjinder Singh Sirsa said in a video posted on X that this was likely the first major attempt in India to reduce pollution “scientifically” in this manner. He said light rains were expected within four hours of the experiment. If successful, and if weather conditions permit it, similar experiments will continue in Delhi throughout the winter season, till February.
What is cloud seeding?
A 2024 document by the Indian Institute of Tropical Meteorology (IITM), Pune, defined it as a technique “used to modify suitable clouds with ‘seed’ particles to increase rainfall.” Globally, it was first tested in the 1940s. Generally, 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. 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.
But 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.
The IITM noted that 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.
What conditions are required for cloud seeding?
Firstly, 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.”
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. 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.” Several monitoring tools are being used to understand cloud characteristics, including their moisture content.
Finally, what is the rationale for using cloud seeding to improve air quality?
In India, cloud seeding has mostly been tried to deal with drought-like conditions, with varying results. The IITM document 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. An earlier 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.”
Gufran Beig, founder project director of the government’s System of Air Quality and Weather Forecasting And Research (SAFAR), earlier said, “There should be a significant amount of rain so it washes away pollutants. It will only be temporary, but if at all it is successful, it will break the flow of pollutants.”
Apart from concerns about the efficacy, 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.
Source: Indian Express, 28/10/25
