Has cloud seeding been effective? | Explained

The story so far: For the first time in nearly 50 years, Delhi conducted two cloud seeding trials with the Indian Institute of Technology Kanpur (IITK) last week. The aim was to induce rain over Delhi to settle the build of smog and particulate matter that had deteriorated the air quality.

What is cloud seeding?

Cloud seeding involves spraying a salt mixture into clouds. The science is that such seeding, which is done by aircraft fitted with flares that fire the salt mixture into clouds, can induce ice or water vapour within the clouds to form water droplets. When lots of such droplets coalesce, they can pour down as rain.

What has been its history?

Cloud seeding has been around for at least three quarters of a century with mixed success. Beginning in the 1940s, General Electric scientists William Schaefer and Bernard Vonnegut chanced upon the principle of using dry ice to form ice crystals in their lab freezer. They then decided to experiment on real clouds. It was reported that they successfully made it snow over Pittsfield in Massachusetts, U.S. This got the U.S. government excited and a formal programme called Project Cirrus was born. While creating rain was certainly on the back of their minds, the big excitement was the prospect of taming hurricanes, which did not pan out well. In the 1950s and 60s, the use of cloud seeding as a weather modification tool became popular. The Soviets seeded clouds over Leningrad to protect May Day parades — years before China used cloud seeding for clear skies ahead of the inaugural ceremony of the Olympics in 2008. The U.S. launched Project Skywater, dumping silver iodide from planes over the Rockies.

What has India’s experience been?

Nearly coincident with Project Cirrus, S.K. Banerji, the first Indian Director General of the India Meteorological Department (IMD), oversaw the first cloud seeding experiments in Kolkata by releasing salt and silver iodide in hydrogen balloons in 1952. Most of these were administered as rockets that were fired from the ground. And while these experiments seemed to suggest that on the days when seeding was done, there was more rain compared to days when there was no seeding, it wasn’t verifiable if the rain was due to natural sources or from the seeding. There was even an attempt to conduct such seeding in Delhi in 1962 but it failed.

It’s only from the 1970s that researchers properly started to use planes and fly to the top of the clouds to spray salt solutions. They also studied cloud physics, condensation, what kind of clouds gave rain, which ones didn’t, and so on. Several States, when grappling with drought, have experimented with cloud seeding. The results have been sporadic and there was never any systematic way to tell how much rain could reasonably be expected if a certain amount of salt mixture was scattered. There was also less clarity on where exactly one could expect rain. The cost-benefit also was not clear, given that hiring aircraft, pilots, technical personnel and making salt mixtures was expensive.

What was the CAIPEEX?

Initiated by the Pune-based Indian Institute of Tropical Meteorology in 2009, Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) was a systematic scientific investigation to quantify if there were any benefits from cloud seeding. For that it actually studied the interior world of clouds, its physics, and water droplet formation for nearly a decade after which from 2017-2019 they physically identified, using radar and other instruments, clouds that were suitable for seeding.

This experiment was conducted over a drought prone region called Solapur, Maharashtra, and hence a natural test ground to measure enhancement (if there was any). Once the clouds were identified they flew aircraft and fired flares of calcium chloride (no silver iodide used) into some clouds and left others ‘unseeded.’ Their overall finding was that Solapur got an extra 867 million litres of water — which is considerable. In terms of rainfall measured on the ground: seeded clouds gave an average 46% more rain at the seeded locations relative to the unseeded ones.

Over a 100 square km area downwind, there was 18% more rain in the seed versus unseeded.

What happened in Delhi?

There were two flights on October 28 when IIT Kanpur flew its own plane and flared clouds. The results were disappointing with no rainfall triggered, though researchers at IITK said that some parts of Delhi reported a ‘light drizzle’ and a ‘small improvement’ in air quality. The drawback was the quality of clouds. The CAIPEEX demonstrated that only monsoon clouds which had a certain quantity of moisture could hope to yield sufficient water. Such clouds are absent in the post-monsoon over Delhi.

For seven years, there have been various proposals for seeding over Delhi that have been discouraged by scientists due to the winter atmospheric characteristics. IIT Kanpur has however said that it will continue ‘trials’ during this season.