IIT's 'suitable' theory on why Delhi did not get artificial rain

Delhi’s winter atmosphere is climatologically unsuitable for consistent cloud seeding due to a fundamental lack of sufficient moisture and saturation, particularly during the peak pollution months of December and January, according to an IIT-Delhi report.

The report, based on a comprehensive analysis integrating climatological data (2011-2021) by IIT’s Centre for Atmospheric Sciences, comes against the backdrop of the Delhi government conducting two cloud-seeding trials in Burari, north Karol Bagh and Mayur Vihar in collaboration with IIT-Kanpur, but there was no rain. The institute had earlier conducted successful trials in Kanpur in 2017-18, but this was the first such experiment in the Delhi-NCR region.

“While cloud seeding is theoretically feasible under specific atmospheric conditions, during Delhi’s winter, its practical utility as a consistent and reliable air-quality intervention is constrained. The necessary atmospheric conditions are rare and frequently coincide with natural rainfall, limiting the potential marginal gain,” the report said.

“Even when successful, induced rainfall would likely provide only a brief respite (typically one to three days) before pollution levels rebound. Given the high operational costs, the scientific uncertainties inherent in aerosol-laden environments, and the absence of any impact on underlying emission sources, cloud seeding cannot be recommended as a primary or strategic measure for Delhi’s pollution management.

“At best, it could serve as a high-cost, tactical intervention during declared air-quality emergencies, contingent upon a forecast meeting stringent MSI-based suitability criteria. Ultimately, the study underscores that sustained emission reduction remains the best viable and durable solution to Delhi’s chronic air pollution crisis,” it added.

The report noted that decadal analysis (2011-2021) indicates that the core winter months of December and January coincide with both the most severe pollution episodes and the driest climatological conditions.

“There is a fundamental lack of sufficient moisture and saturation during the peak pollution months (December-January), coinciding precisely when intervention is most needed. While Western Disturbances (WDs) are the primary drivers of potential seeding conditions, viable ‘windows of opportunity’ are rare, confined to specific anomalous events.

“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,” it said.

The study highlights complexities arising from Delhi’s high aerosol environment.

High aerosol loading (characterised by high Aerosol Optical Depth) is associated with increased cloud cover and higher liquid/ice water content, particularly during rainy conditions.

However, favourable microphysical conditions (low cloud base, high water content) 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.

Thermally, glaciogenic seeding appears potentially viable during core winter, but operational feasibility seems restricted to existing rainy conditions.

Regarding pollution removal, the analysis confirms that heavy natural rainfall is highly effective (>80-95 per cent washout for PM2.5, PM10, NOX), while light rain offers minimal impact. 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.

Ozone concentrations often increase post-rainfall. While dry WDs provide some limited ventilation, significant concerns remain regarding the environmental/health impacts of seeding agents like 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. It should be viewed, at best, as a potential high-cost, emergency short-term measure, contingent on stringent forecasting criteria.

“The study underscores that sustained emission reduction remains the most viable and necessary long-term solution,” the report said.