Delhi air pollution: Why cloud seeding in winter has a specially low chance of success, according to science

Let’s unpack step by step from the scientific and meteorological perspective of cloud seeding to diffuse clouds of confusion. Cloud seeding is a form of engineering of the natural weather where substances like silver iodide, sodium chloride, or similar kinds are introduced into a particular class of clouds to artificially or forcefully trigger rain. The underlying physics is straightforward: these chemicals act as cloud condensation nuclei or ice nuclei, around which water vapor condenses or ice crystals form and may produce rain.

Crucially, seeding only works when injected in a particular class of clouds, known as hygroscopic / warm-rain clouds. This class of clouds is considered good for seeding because they have abundant liquid water and respond quickly when extra nuclei are introduced. In contrast, seeding is ineffective in other clouds because their microphysical structure resists external perturbation and hence rain.

The limitation

Analysis of decades of climatological records reveals that, unlike the more visible cloud types, hygroscopic or warm-rain clouds, the ones suitable for seeding, are rare and form only under specific atmospheric conditions. These clouds typically appear during the pre-monsoon period, as monsoon dynamics begin to build, and remain common through the monsoon itself.

Statistical analyses indicate their probability of occurrence is about 51% during pre- and post-monsoon months, but plummets to just 5–10% in winter, precisely when air pollution peaks (November to January) and the demand for cleansing rain is highest. So, it’s seasonally misaligned with the air pollution menace.

It is understandable (although this author disagrees) as a solution to drought because you need rain on any day of a larger window without waiting for the Air Quality Index (AQI) to drop. To put it simply, we are betting on a razor-thin probability. Here is why: If cloud conditions are ideal, the global success rate of seeding is roughly 50%; the likelihood of such a cloud forming in the critical winter months is below 10%; and we need those specific clouds on the very days when the AQI spikes, not randomly. The stakes, scientific, operational, and financial, are significant. The rest is left to the reader’s judgment.

Not a solution to pollution

Proponents of the October 28 cloud-seeding experiment in Delhi claim that the AQI improved from “Very Poor” to “Poor” in several pockets, a selective success, at best. But we seem to have nurtured an illusion: we worry when AQI crosses 400 and celebrate when it dips below 300, ignoring the fact that in both cases, PM2.5 concentrations, the lead pollutant driving AQI in Indian cities, remain alarmingly above national standards. Clinically, the initial exposure to fine particles inflicts the greatest harm; further increments merely deepen the injury.

Scientifically, we must thank nature that the drizzle, if it indeed resulted from seeding, did not coincide with cooler temperatures. Substantial rainfall is required to effectively wash out particulates; light drizzles under cooler conditions often do the opposite. This counterintuitive effect, known as secondary aerosol formation, allows fine particles to attach to micro-droplets, coagulate, and multiply, leading to a rebound in PM levels. Delhi has witnessed this phenomenon repeatedly and may yet again when December’s western disturbances bring shallow rain. Even if the operation is deemed “successful” in triggering rainfall, such interventions can only yield short-lived relief. The fundamental reality remains: emissions are produced continuously and swiftly re-accumulate into concentrations through weather interactions. Within a day, the system resets, and we find ourselves back at square one.

The modest success of the October 28 experiment, if at all, has been attributed to insufficient moisture in the target clouds, a condition that expert agencies were expected to assess and ensure beforehand. Whether their technical judgment was sought or what opinion was offered remains unclear. Yet, there can be a sense of relief that the long-awaited experiment finally took place, for two reasons. First, it fulfilled a scientific curiosity that had lingered for years. Second, the post-event analysis will likely settle the debate once and for all on the scientific sanctity of such interventions — much like how the case of smoke towers, first tested in Delhi in 2021 with similar enthusiasm, has gradually found its quiet conclusion.

Air pollution in Delhi remains an unresolved, recurring challenge. As winter 2025 sets in, we are confronted once again with the same fundamental question: what is truly polluting Delhi? Crop residue burning is a short, seasonal episode; festival fireworks last only a few days. Yet, the discourse continues to orbit around these transient factors, overlooking the deeper, systemic drivers. It’s time to shift from a city-centric mindset to an airshed approach, one that considers the climatological “family area” responsible for the transboundary movement of pollutants, a proposition promoted by NARFI (National Resource Framework of India) of NIAS, a project supported by the office of Principal Scientific Advisor to the Government of India. Air knows no administrative boundaries, and city-only solutions risk compromising the larger airshed simply because their implementation seems complex.

Mitigation must be anchored in coordinated year-round actions targeting persistent sources. A clear starting point is accelerating the transition away from fossil fuels, where the emerging electric vehicle revolution offers real promise. According to SAFAR’s latest estimates, vehicular emissions contribute over 40% of Delhi’s PM2.5 load, and unfortunately, rapid growth in vehicle numbers is offsetting the technological advances.

The challenges are enormous, but complex problems demand innovative solutions, and each innovation requires rigorous scientific evaluation to sustain credibility and impact. Above all, we must stop normalising toxic air. This is not merely a winter problem, not just a Delhi problem, and certainly not a poor person’s problem. It is everyone’s problem, corroding our health, eroding our productivity, and weakening our collective dignity.

The author is Chair Professor, National Institute of Advanced Studies, IISC-Campus, and Founder Project Director, SAFAR