National capital’s winter not suitable for effective cloud seeding: IIT-Delhi

A new report from the Indian Institute of Technology (IIT)-Delhi has found that Delhi’s winter atmosphere is not suitable for consistent and effective cloud seeding to reduce air pollution. The study, conducted at IIT-Delhi’s Centre for Atmospheric Sciences, concludes that low moisture, weak atmospheric lift and high aerosol levels make cloud seeding unreliable as an air-quality intervention.

The report, titled “Can Cloud Seeding Help Tackle Delhi’s Air Pollution?”, analysed climatological and air quality data from 2011 to 2021. It assessed cloud moisture, aerosol-cloud interactions and the ability of rainfall to wash out pollutants such as PM2.5, PM10 and nitrogen oxides.

Researchers found that Delhi’s winter months - December and January - have the lowest atmospheric moisture levels, with total column water vapour often below 9 millimeters. These months also coincide with the city’s most severe pollution episodes. The study’s Moisture Suitability Index (MSI) showed that the combination of moisture, saturation and vertical lift required for successful seeding rarely occurs.

The report states that western disturbances occasionally create favorable conditions, but such events are limited. Even on potentially suitable days, cloud systems often lack the depth and water content needed for seeding to generate additional rainfall. “Delhi’s winter atmosphere is climatologically unsuitable for consistent and effective cloud seeding,” the study concludes.

The researchers also examined the city’s high aerosol load, which influences cloud formation. While aerosols can increase cloud cover and water content, they can also inhibit natural precipitation. The report notes that effective conditions for seeding generally coincide with days that already produce rain, reducing the potential benefits of artificial intervention.

An analysis of 10 years of weather data showed only a small number of “seedable” days each winter —days with sufficient cloud cover and moisture but no rainfall. Most of these occur during western disturbance events, making them rare opportunities rather than regular possibilities.

The study also evaluated the effect of rainfall on air quality. It found that heavy rain can remove more than 80 per cent of particulate matter and nitrogen oxides from the air, while light rain has little impact. However, pollution levels typically return to pre-rainfall concentrations within one to five days due to ongoing emissions. Ozone levels often increase after rainfall events.

The report highlights other concerns, including the potential environmental and health risks of silver iodide, commonly used in seeding, as well as high operational costs of such missions. It refers to similar experiments in other countries where seeding produced limited or short-lived results.

Given these findings, the study concludes that cloud seeding should not be viewed as a primary solution to Delhi’s air pollution. Instead, it could serve only as an emergency, short-term measure during rare favorable conditions. Long-term improvement, it says, depends on reducing emissions from transport, industry, construction and biomass burning.