Artificial rain to fix pollution remains a nebulous science

There is no evidence so far that cloud seeding will be successful in non-monsoon months; also, ‘not all clouds are seedable, not all clouds will make rain’

Updated - November 12, 2023 11:13 am IST

Published - November 11, 2023 08:14 pm IST - NEW DELHI

People visit the Lodhi Garden, in New Delhi. Rain led to a rapid improvement in Delhi’s air quality morning and cleared the haze that had been lingering for over 10 days.

People visit the Lodhi Garden, in New Delhi. Rain led to a rapid improvement in Delhi’s air quality morning and cleared the haze that had been lingering for over 10 days. | Photo Credit: PTI

The Delhi government’s attempt to alleviate severe pollution in Delhi by resorting to artificial rain or cloud seeding is an experimental endeavour with no evidence so far that it will be successful in non-monsoon months. Though cloud seeding has been attempted since the 1940s and was pioneered by the United States military, having clouds deliver enough rain over a desired region continues to be a nebulous endeavour.

With air quality ‘severely’ bad, the Delhi government met experts from the Indian Institute of Technology (IIT)-Kanpur earlier this week, who plan to submit a proposal to the government, who in turn will submit it to the Supreme Court. The plan, the full details of which are not known, is to fly aircraft into rain clouds — which are anticipated in north India around November 20 — and litter them with a proprietary salt solution. Much like seeds sprayed onto fertile soil, the hope is that these salts will interact with existing water droplets in the clouds, make them bigger and heavier, and they will eventually condense as rain.

In principle, this works, but many factors must align for it to succeed, as the most exhaustive analysis of cloud seeding in India has found. Such an experiment in Solapur, Maharashtra over two monsoons in 2018 and 2019 , the results of which were published in the Bulletin of the American Meteorological Society in October, concluded that seeding resulted in 18% more rainfall or about 8.67 mm more rainfall (or 867 million litres of water) than in the absence of it. Solapur was chosen because it is a rain-shadow region that normally receives little monsoon rain. Even so, the research team, consisting largely of atmospheric-research scientists from the Indian Institute of Tropical Meteorology, Pune, could find 276 “convective clouds”. These are clouds with a minimum of 0.5 gm of water per sq. cm. of cloud.

Thara Prabhakaran, the lead scientist for the experiment, said that differences in cloud shapes and their existing water content significantly influenced the success with inducing artificial rain. The ₹200-crore CAIPEX Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX Phase-4), as it was called, even tracked individual clouds to see the extent to which they were influenced by seeding. “Not all clouds are seedable, not all clouds will make rain. There are clouds that will get dissipated on seeding. Overall, however, there was a net increase of 18% [rainfall] in a 100 sq. km. area. Our experiment was very controlled and we selected clouds based on their potential to make rain,” Dr. Prabhakaran told The Hindu.

Conducting the experiment during pre-monsoon or post monsoon (as is the case in Delhi) could have different outcomes as the number of convective clouds were fewer and relatively lower in the atmosphere (and therefore having less water), Dr. Prabhakaran said. Some of the rain from the seeded clouds also drifted several kilometres beyond the marked area, she added.

Clouds were also ‘warm’ and ‘cold’, and that required different chemicals to stimulate water production in them. In Solapur, for instance, the clouds were warmer and were doused with calcium chloride. Colder clouds could be best served with silver iodide, as was the case when China carried out cloud-seeding around the Beijing Olympics to manipulate weather. “Western disturbances have clouds that have many dry layers. It could happen that water from the cloud will fall into the dryer layers and evaporate, never reaching the ground. Clouds are chaotic and clouding is a complex phenomenon, and we are still studying it. We need a lot of planning and strategy to execute a successful cloud seeding operation,” Dr. Prabhakaran said.

The researchers at IIT-Kanpur, who are gearing up for the cloud seeding exercise, say that the 18% improvement in rainfall observed in the CAIPEX experiment was an encouraging reason to attempt cloud seeding to allay Delhi’s pollution woes. IIT-Kanpur has its own aircraft, had developed its own salt solution (the combination has not been disclosed), and Beijing’s success at controlling pollution with cloud seeding were good reasons to experiment in Delhi.

“It is certainly worth trying in Delhi, given the extremely high levels of pollution. Whenever we have clouds available, we will perform it,” Sachchida Nand Tripathi, Professor, Department of Civil Engineering and Sustainable Engineering, IIT-Kanpur, said. “Even a few millimetres of rain can go a long way in clearing the air,” he added.

On Friday, a mild drizzle from a western disturbance in several parts of Delhi-National Capital Region (NCR) cleared the air and improved air quality by roughly 100 points on the air quality index.

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