Artificial rain is an idea that has found political currency in the national capital region to scrub the air of hazardous pollutants. Employed as a costly drought relief measure since the 1950s across the country to address human distress, Delhi-NCR tried it out to reduce atmospheric pollution. However, three back-to-back attempts to make the skies open up in Delhi’s outskirts by coaxing sparse clouds failed.
The Delhi government and the Indian Institute of Technology, Kanpur had teamed up to produce artificial rain using IIT-K’s aircraft. However, quick fix solutions without addressing the underlying causes of pollution often fail to deliver relief.
Stages of seeding
At the heart of the concept of artificial rain is a process called cloud seeding. It is a technique of weather modification, which catalyses the natural process of cloud precipitation. The most important condition for its success is that the target clouds must hold sufficient levels of moisture. These clouds are injected with chemical particles, which act as a kind of seed around which water droplets and ice crystals can form. These formations are called the nuclei.
Once a sufficient volume of droplets or ice crystals forms inside a cloud, they start growing and combine to become heavy enough to fall as rain or snow. Inducing the cloud to dump the water that it might otherwise carry away or hold back is part of the process.
Scientists begin by making cloud and moisture assessment in the region where artificial rain is desired. Apart from the presence of clouds and moisture in them, parameters like proper height, atmospheric temperature and density of clouds come into play before the seeding target is selected.
As for the seeding agents, they include chemical substances such as silver iodide, solid carbon dioxide or dry ice, salt particles or sodium chloride, rock salt and iodised salt. These materials help create the nuclei for condensation or freezing. These seeding agents are then released into the clouds by aircraft, rockets, or ground-based generators, each one of which has cost implications. For Delhi, the IIT-Kanpur team decided to use a modified aircraft to carry flares that burned the chemicals at a particular altitude.
There are two types of seeding, hygroscopic seeding (using salts) at the base of warm clouds; and glaciogenic seeding (using silver iodide particles) in cold clouds. IIT-Kanpur attempted glaciogenic seeding in Delhi.
The case of Delhi
For decades, the NCR and its adjoining areas have been reeling under humongous air pollution. It attains life-threatening levels during the winter when winds drop, temperature inversion traps pollutants, stubble-burning in neighbouring states adds to smoke, and dust gets stirred up from construction sites. Vehicular emissions add to the noxious cocktail.
As a counter-measure, the Delhi government sought to experiment with artificial rain. It hoped that artificial rain, even for a short time, could help wash away some of the fine particulate matter, like PM2.5 and PM10, from the air, improve visibility, and give some relief from high pollution.
Experts believe that for a city like Delhi, even if a little rain can be induced to help wash away particulate matter, it can contribute to public health, visibility and overall ease of living. Globally, cloud seeding has been attempted in several countries, like the US, China, and UAE, mostly for enhancing rainfall, snowpack, controlling hail, clearing fog, etc., and the Delhi government wants to build on this theme.
The government signed a memorandum of understanding with IIT-Kanpur to carry out several — initially five, possibly up to nine — experimental cloud-seeding flights. It sanctioned `3.21 crore for the initial phase of up to five trials. Each aircraft sortie carried out so far lasted 90 minutes, covering roughly 100 sq km in and around Delhi, using a modified Cessna 206H that carried flare-type seeding equipment. The target areas were mainly in the northwest and outer Delhi and Noida.
The experiment
The IIT-Kanpur team chose the day of operation when the meteorological data showed favourable cloud movement over Delhi and sufficient moisture concentration, and dropped seeding chemicals into the clouds. The expectation was that rainfall could begin within 15 minutes to four hours after seeding, but there was not even a drop.
Scientists believe the key reason for the failure was low moisture content in the targeted clouds. The IIT-Kanpur team noted humidity levels as low as 10-15% in the seeded clouds, well below what is generally considered favourable for the success of such an exercise.
Another IIT-Delhi report analysed data from 2011 to 2021 to conclude that Delhi's winter 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. It found December-January coinciding with both the most severe pollution episodes and the driest climatological conditions.
Western Disturbance
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) indicated they frequently lack the necessary combination of moisture depth, saturation and atmospheric lift required for successful seeding, the report said.
Even if successful, “induced rainfall would likely provide only a brief respite (typically one to three days) before pollution levels rebound, the report said. Given the high operational costs, the scientific uncertainties inherent in aerosol-laden environments, and the absence of any moderation in underlying emission sources, cloud seeding cannot be recommended as a primary or strategic measure for Delhi's pollution management, the authors argued.
Coming back to the recent experiment, despite there being no significant rain, some reduction in the levels of particulate matter was observed in Delhi's trial zones. For example, before seeding, PM2.5 levels were 221, 230, and 229 micrograms per cubic metre in areas like Mayur Vihar, Karol Bagh, and Burari in Delhi, respectively. Post-seeding, they dropped to 207, 206, and 203. Similar small drops for PM10 were noted. Experts, however, say that since rainfall did not take place, the decline in the levels of particulate matter or pollution in general may have been caused by other factors, like wind settling and timing of measurement.
Learning curve
For his part, IIT-Kanpur Director Manindra Agrawal said though the cloud seeding trials did not produce rain, they provided valuable data and scientific insights for future operations. "It helped us understand the relationship between the amount of seeding material used, the moisture content of clouds, and their ground-level impact. This will help us fine-tune future operations," he said.
"We need at least 30 to 50% cloud moisture. Since that was not available, we decided to wait until conditions improve," he said, adding the trials will resume once the situation improves.
