How Rain and Cloud Seeding Shape Air Pollution

Summary

Rain is often connected with renewal, growth, and, importantly, cleansing. The concept of cleansing becomes quite literal in the context of air pollution. Have you ever wondered why the air feels fresh on a rainy morning but stuffy during winter? That’s because rain helps wash pollutants from the atmosphere, sometimes assisted by thunderstorms and lightning.

But what exactly is removed from the air, and how? Do all pollutants get washed out? And once they’re washed away, where do they go, and do they create new hygiene issues lumping on the roadside? More importantly, how long does this “clean air” effect actually last?

As Delhi prepares for its first-ever cloud seeding attempt to tame/combat pollution, we explore if such technological remedies offer lasting results. With a case study of China’s extensive investments and large-scale cloud seeding projects, we analyze the viability of artificial rain as a tool for improving air quality.

How Rain and Cloud Seeding Shape Air Pollution

The blog discusses the correlation between rain and air quality and the growing interest in replicating rain through technology.

Science Behind Rain Cleaning:

In-Cloud Scavenging (Rain Formation Phase)

During cloud formation, water vapor present in the air turns into tiny droplets. To form droplets, the water vapour needs something to stick to like dust, smoke, or soot present in the air. These pollutant particles act like tiny "seeds" that help raindrops form. 

Hence, even before the rain falls, some amount of pollution is already trapped inside the raindrops forming in the cloud.

Below-Cloud Scavenging (Washout While Falling)

As rain falls through the air, the raindrops bump into tiny pollutant particles like dust, soot, and also absorb some gases such as sulfur dioxide (SO₂) and nitrogen dioxide (NO₂). 

This way, the rain either traps particles on its surface while falling or absorbs gases into the liquid drop. In this way, the pollutants get washed out of the atmosphere.

Pollutants That Rain Can Remove

Rain is an effective short-term way of removing pollutants from the air like particulate matter (PM_2.5 and PM₁₀), sulfur dioxide (SO₂), and, to some extent, nitrogen oxides (NOx). The pollutants get captured either physically when the raindrops fall or get dissolved in the rainwater, also called wet deposition. That’s the reason why the air feels noticeably fresh after rain, especially in polluted cities where this effect can be closely felt. Larger particles, such as dust, soot, and pollen, are also efficiently washed out during heavy rainfall.

Still, rain doesn’t soak everything. Pollutants like ozone (O₃), carbon monoxide (CO), and many volatile organic compounds (VOCs) are either not water-soluble or are chemically stable and resistant to breakdown by water. In fact, ozone levels can stay high even after rain, and CO concentrations are barely affected, as found in some studies of Ozone Resistance. Which explains why the “fresh air” felt after rain is mostly temporary and incomplete.

Though the pollutants get washed away by rain, they still persist in the environment, degrading key areas. These pollutants get collected in drains, ditches, and stormwater systems, where urban runoff carries them into rivers, lakes, and groundwater. This lasting contamination degrades natural ecosystems, threatens human health, and damages infrastructure even long after the rain stops.

Effect Of Rain On AQI

Rain often brings a noticeable drop in pollution levels, making air feel fresher and easier to breathe, though the fresh air is usually short-lived. Mostly in urban areas, the effect lasts a couple of hours to a day, depending on the intensity of rainfall and how quickly pollution sources resume. Constant emissions from vehicles, industries, and construction rapidly cancel the cleansing that rain provides.

Factors such as wind speed, temperature, and humidity also influence how long clean air lasts. For instance, a strong storm with steady winds may offer a longer break from pollution, while light showers may have minimal impact. To keep the air consistently clean, the focus should pivot more towards reducing the emissions themselves. For example: According to real-time CPCB data, RK Puram recorded an AQI of 126 (Moderate) on May 13, which jumped to 253 (Poor) by May 15.

Is Cloud Seeding The Next Logical Step?

Rain does offer quick relief from air pollution, even if its effects are short-lived and dependent on weather conditions. Despite this, several cities and governments have been prompted to explore artificial ways of introducing rain during peak pollution levels. As even temporary relief can be of value during extreme pollution levels.

Cloud seeding can be done using ground-based generators or aircraft. Ground generators are often remotely operated and placed in strategic locations, whereas aircraft directly release seeding agents into targeted clouds. Mostly, a compound named silver iodide (AgI), dry ice (solid CO₂), or other salts, which helps in formation of ice crystals, are used. Silver iodide, which is naturally available in the environment in low concentrations, is not known to be harmful to humans or wildlife.

In an ideal weather setup, a small amount of silver iodide is either burned from the ground or released from an aircraft. When it reaches moisture-laden clouds, it acts as a base for precipitate formation, aiding the development of rain or snow. Cloud seeding is typically done during the winter season, when moving storms provide the necessary cloud conditions. It cannot be performed during dry weather, since some amount of existing cloud moisture is essential.

Upsides and Downsides:

Though there are some success stories surrounding cloud seeding, such as Beijing’s 2008 Olympics, where efforts were made to induce rain earlier to clear skies. However, scientific evidence remains uncertain, The American Meteorological Society in 2010 stated that evidence is uncertain whether cloud seeding is reliable to achieve desired results. Cloud seeding drives rainfall by encouraging ice crystals to grow and fall, though it doesn't assure rain, it helps in building more clouds which increases the rain potential.

Upsides of Cloud Seeding

• Cost-Effectiveness: Boosting rainfall generally costs between $200,000 and $1 million per year, much lower than desalination or major dam infrastructure.

• Potential for Rain Enhancement: It can add 50–125 mm of rain, acting as a fix during requirement.

• Flexible Applications: It’s used for multiple purposes: agriculture, urban water supply, air quality improvement during events, hailstorm suppression.

Downsides of Cloud Seeding

• Scientific uncertainty: According to the American Meteorological Society (2010), there is no concrete evidence that it consistently delivers results.

• Weather-Dependent: It requires moisture-rich clouds and won't work during dry weather or conditions with clear skies.

• Limited Control: It increases the chances of rain but doesn’t guarantee rain; results may vary based on cloud type, altitude, and temperature, humidity.

Delhi’s Interest In Cloud Seeding: 

In recent times, Delhi, as part of efforts to control persistent smog, is planning to conduct its first artificial rain trial. The project costs around ₹3.21 crore, approved by the Delhi cabinet, which aims to induce rainfall to wash out pollutants like PM_2.5 and PM₁₀, providing temporary relief from hazardous air quality. The operation is jointly being implemented by IIT-Kanpur and IMD Pune.

Delhi's first cloud seeding trial is planned likely around August, however its timing timeline remains dependent on suitable weather conditions. The real-time data will be collected and then analyzed to assess the trial's efficiency. Experts have pointed out that while cloud seeding may help reduce pollution short-term, global estimates suggest only 5–15% increase in rain yield. Again, the success of the trial depends on favorable conditions, which may shift the operation if weather changes.

Case Study

China has invested remarkably in artificial rain technology as a multipurpose tool in its weather modification and environmental plan. Originally adopted to support agriculture and mitigate drought, the technology is also used to improve weather conditions for major public events, such as the 2008 Beijing Olympics and the 2014 APEC Summit.

One notable application of cloud seeding has been for air quality improvement. Prior to a major public celebration in 2021, a two-hour seeding operation was conducted using silver iodide rockets. The resulting artificial rain was linked to a more than two-thirds reduction in PM2.5 pollution, shifting air quality from “moderate” to “good” by international standards.

China plans to expand its weather modification program to cover 5.5 million square kilometers by 2025, deploying technologies for rain, snow, and hail suppression. In regions like Xinjiang, weather modification technologies have helped reduce hail damage by up to 70% annually.

Conclusion

Rain offers a much needed momentary breath of fresh air, a pause from the pollution we face daily. Though artificial rain through cloud seeding might appear as the logical next step, it still remains a reactive measure, one that can assist, but not replace, the fundamentals of air quality management. The real mitigation strategy lies in stronger coordination, better monitoring, and public awareness, which helps address the root of the problem, not just its symptoms.

In an era fueled by technological advancements, we must not overlook the fundamentals of such the entire air pollution problem: science, policy, and the people. Platforms like Indian Air Quality Network (IAQN) can play a vital role in bridging these elements, turning knowledge into networks and awareness into action. The challenge of clean air is not impossible to overcome, it just needs better alignment between will, tools, and understanding to get there.

A Collective Responsibility

In this modern age, air quality is everyone's responsibility. Each action we take can contribute to a healthier planet. Planting trees, choosing public transportation, or even supporting local clean air initiatives can make a difference. Awareness is the first step toward change.

Join us in advocating for cleaner air. Together, we can help nature regain its voice.

Let’s work together to ensure our world thrives in harmony with nature.