Delhi winter unsuitable for cloud seeding, reveals study amid pollution woes

It said the exercise was not feasible, citing a lack of sufficient moisture and saturation during peak winter, particularly in December and January

Delhi’s atmosphere in the winter months, when pollution levels are highest and necessitate interventions, is climatologically unsuitable for “consistent” and “effective” cloud seeding, the Indian Institute of Technology (IIT), Delhi, said in a study on Friday.

Smog on a winter morning. (RAJ K RAJ /HT PHOTO)

It said the exercise was not feasible, citing a lack of sufficient moisture and saturation during peak winter, particularly in December and January, unless there is impetus from a western disturbance. Even then, viable “windows of opportunity” are rare and confined to specific anomalous events, the report said.

“There is a fundamental lack of sufficient moisture and saturation during the peak pollution months... coinciding precisely when intervention is most needed. Even on days identified as potentially promising (e.g., cloudy WD days without rain), a multi-criteria Moisture Suitability Index (MSI) indicates they frequently lack the necessary combination of moisture depth, saturation, and atmospheric lift required for successful seeding...” the report said.

The report went on to mention that cloud seeding cannot be recommended “as a primary or reliable strategy for Delhi’s winter air pollution management.”

“It should be viewed, at best, as a potential high-cost, emergency short-term measure, contingent on stringent forecasting criteria...” the report added.

On the same day, Delhi’s environment minister Manjinder Singh Sirsa admitted cloud seeding was not a permanent solution, but only a temporary measure during high-pollution events. Reacting to the study at a press conference, Sirsa said the government was not looking at cloud seeding as a permanent solution. “We know it is not. It is only to tackle and bring down alarming levels of pollution,” he said.

The study, carried out by PhD students from IIT Delhi’s Centre for Atmospheric Sciences (CAS), highlighted the complexities arising from Delhi’s high-aerosol environment.

It said high aerosol loading—the presence of tiny solid particles or liquid droplets suspended in the air—is associated with increased cloud cover, particularly during rainy conditions. However, favourable microphysical conditions, such as a low cloud base and a high water content in the air, often coincide with naturally occurring precipitation, limiting the potential added benefit of seeding.

“The vertical separation between the shallow aerosol layer (below 2 km) and typical seedable cloud layers (2-5 km) also presents significant operational targeting challenges,” the report added.

Shahzad Gani, assistant professor at CAS and one of the mentors involved in the study, said the analysis clearly shows that the meteorological conditions required for cloud seeding—adequate moisture, rain-bearing clouds, and favourable thermodynamic structure—are rarely present during the most polluted periods.

“Even when rain does occur, air quality typically rebounds within a day or two because emissions continue,” he said.

The report said heavy natural rainfall was highly effective (>80-95% washout for PM2.5, PM10, NOX), while light rain offered minimal impact. Overall, the analysis only identified 92 days over the past decade, from 2011 to 2021, when the moisture and cloud cover was found comparable to naturally occurring moderate-to-heavy rainfall day, when seeding would have been possible.

“Importantly, even after significant washout, air quality improvements are short-lived, with pollutant concentrations typically recovering to pre-event levels within one to five days due to persistent emissions,” it said.

Professor Sagnik Dey, head, CAS, said the study points to the fact that cloud seeding is not an effective and sustainable solution for Delhi. “This is based on the atmospheric conditions prevailing at this time of the year and the fast recovery rate of air pollutants due to high emission strength,” he said.

Further, the study said the downside of cloud seeding was an increase in ozone emission.

“Ozone concentrations often increase post-rainfall. While dry WDs (western disturbance) provide some limited ventilation, significant concerns remain regarding the environmental or health impacts of seeding agents like (silver iodide) AgI, high operational costs, and scientific uncertainties. Given these constraints, cloud seeding cannot be recommended as a primary or reliable strategy for Delhi’s winter air pollution management,” the report said.

The Delhi government has already held three cloud seeding trials in the capital so far, with no rainfall. A trial was first held on October 23, followed by two trials on October 28. IIT Kanpur, the expert institute executing the project on behalf of the Delhi government, had said that while a moisture content of around 50% was required for seeding, it was only around 15-20% during all three trials.

IIT Kanpur director Manindra Agrawal had subsequently admitted the trial was not completely successful due to unfavourable meteorological conditions. He had also said it was only a stopgap to Delhi’s pollution problem.

“This is an SOS solution. When you have a crisis situation, very high pollution, it is one of the methods one can attempt in order to bring down pollution. It is not a permanent solution. The permanent solution, of course, is to control the sources of pollution,” he said on October 28.

The Capital faces an annual problem of air pollution, which increases manifold during the winter months of October to February due to high emission rates from multiple sources, including vehicular traffic, industrial activities, construction dust, agricultural biomass burning, and residential heating, which combine with unfavourable meteorological conditions to create prolonged episodes of severe pollution.