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Even as Delhi-NCR witnessed light showers on Thursday night (November 9), the Delhi government had announced earlier this week that it was considering cloud seeding or ‘artificial rain’ to wash away pollutants in the air.
The now mooted proposal has been attempted previously in India but only in the monsoon season – when clouds with moisture are present – and pre-monsoon months. Besides, it has only been done before in the country with the purpose of bringing rainfall to drought-prone areas, and not to mitigate pollution.
What exactly is the mechanism and how is it expected to help bring down the concentration of pollutants?
Water vapour condenses around small particles 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.
With cloud seeding, 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.
“Seeding accelerates cloud microphysical processes. You need sufficiently large droplets that can reach the surface of the earth and not evaporate on the way,” said Sachchida Nand Tripathi, Professor at IIT Kanpur and Steering Committee member, National Clean Air Programme.
Tripathi added: “The substance that is dispersed into the cloud needs to have cloud condensation nuclei and ice nuclei and these two come from two different salts.” The cloud condensation nuclei help form cloud droplets, and ice nuclei help to form ice crystals. Ice crystals grow faster than drops, and they become large and fall.
Firstly, cloud cover and clouds of a certain type are necessary.
M Rajeevan, former secretary of the Ministry of Earth Sciences, said, “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.
With a stable atmosphere in winter, clouds are expected to form when a western disturbance disturbs this stability of the atmosphere.
“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,” Tripathi said.
While the possibility of cloud formation can be determined in advance through radars, other conditions will have to be studied on the day seeding is likely to be done.
Seeding has mostly been attempted during the monsoon in India, in places such as Karnataka, Maharashtra and Tamil Nadu.
A more recent experiment, the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX-IV) that took place in the monsoon seasons of 2018 and 2019, was conducted in drought-prone Solapur in Maharashtra. It pointed to a relative enhancement of 18 per cent in rainfall.
“With that experiment, we have tried to understand more about cloud seeding, but still there are a lot of difficulties. Cloud microphysics is more complicated than we think. We can get some advantage from it in the monsoon season if there are enough clouds. When you seed, all clouds won’t rain, and even without seeding clouds can rain. It is still a very complex and uncertain field of research,” said Rajeevan, who was involved with the Solapur experiment.
IIT Kanpur attempted it in April and May of 2018, which are the pre-monsoon months, on their campus. It said five out of six trials resulted in rain.
In 2018, cloud seeding was floated as a proposal in Delhi but it didn’t happen, with the many permissions that were required coming in the way, along with the absence of seeding equipment on IIT Kanpur’s own aircraft.
Thara Prabhakaran, a scientist at the Indian Institute of Tropical Meteorology (IITM), who specialises in cloud microphysics and was also part of the experiment in Solapur, said, “In our experiment in Solapur, we have done statistical and physical experiments by registering what is happening inside the cloud before and after seeding and documenting the processes leading to precipitation.”
“Rain forms in the cloud, but whether it reaches the surface or not is determined by several factors… it can even evaporate on its way down to the surface since we have tropical conditions. There are many uncertainties. In winter, it has not been tried. In winter, cloud systems are different. There will have to be further research to investigate this possibility,” he said.
Is it expected to help with pollution levels?
In India so far, cloud seeding has not been tried with the purpose of reducing pollution, but only been tried to deal with drought-like conditions.
Prabhakaran said, “There are a few cases where China tried weather management options. In India, we don’t have investigations done in this aspect (impact of cloud seeding on pollution). Our conditions are different, and we will need a dedicated study on this. Clouds and their processes are very complex, these are non-linear processes. If you do something, it is not known exactly that it is going to give you this much rain or not. Clouds are also able to rain naturally, so how do you separate natural rain from seeded rainfall?”
Gufran Beig, founder project director SAFAR, and chair professor, National Institute of Advanced Studies, said, “This is being done for the first time for air pollution. 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.”
