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Context

• Cloud seeding has only been done earlier to bring rainfall to drought-prone areas, not to mitigate pollution. What exactly is the mechanism and how is it expected to help reduce the concentration of pollutants?

About

What is cloud seeding?

• Water vapor condenses around small particles to form the droplets that make up a cloud.
• These droplets collide and grow; as they get heavy and the cloud gets saturated, it rains.
• With cloud seeding, clouds are usually injected with salts like silver iodide, potassium iodide, or sodium chloride, the ‘seed’.
• These salts are expected to provide additional nuclei around which more cloud droplets can form.
• They are dispersed into the cloud either using aircraft or through generators on the ground.
• Seeding accelerates cloud microphysical processes. You need sufficiently large droplets that can reach the surface of the earth and not evaporate on the way.
• The substance that is dispersed into the cloud needs to have cloud condensation nuclei and ice nuclei and these two come from two different salts.
• The cloud condensation nuclei help form cloud droplets, and ice nuclei help to form ice crystals. Ice crystals grow faster than drops, and they become large and fall.

What are the conditions required for cloud seeding to be done?

• Firstly, cloud cover and clouds of a certain type are necessary.
• Cloud seeding can only happen if there is a sufficient number of clouds and a particular depth to these clouds. Inside, there needs to be an adequate number of cloud droplets.
• Cloud seeding is done to increase the radius of the cloud droplets so that they will grow bigger and because of gravity, they will come down as rainfall. But with a clear sky, you can’t do it.
• In winter, clouds form over Delhi when a western disturbance moves over the region.
• These are storms that originate in the Caspian or Mediterranean Sea and bring non-monsoonal rainfall to northwest India.
• With a stable atmosphere in winter, clouds are expected to form when a western disturbance disturbs this stability of the atmosphere.
• In winter, you don’t see the kind of clouds that are needed for seeding, but western disturbances are the way through which clouds form.
• Even if clouds are there, you need to see what their height is, and what their liquid water content is.
• While the possibility of cloud formation can be determined in advance through radars, other conditions will have to be studied on the day seeding is likely to be done.
• With that experiment, we have tried to understand more about cloud seeding, but still, there are a lot of difficulties.
• Cloud microphysics is more complicated than we think. We can get some advantage from it in the monsoon season if there are enough clouds.
• When you seed, all clouds won’t rain, and even without seeding clouds can rain. It is still a very complex and uncertain field of research,” said Rajeevan, who was involved with the Solapur experiment.

Has cloud seeding been done before in India, and has it been successful?

• Seeding has mostly been attempted during the monsoon in India, in places such as Karnataka, Maharashtra and Tamil Nadu.
• A more recent experiment, the fourth phase of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX-IV) that took place in the monsoon seasons of 2018 and 2019, was conducted in drought-prone Solapur in Maharashtra.
• It pointed to a relative enhancement of 18 per cent in rainfall.
• In 2018, cloud seeding was floated as a proposal in Delhi but it didn’t happen, with the many permissions that were required coming in the way, along with the absence of seeding equipment on IIT Kanpur’s own aircraft.
• Thara Prabhakaran, a scientist at the Indian Institute of Tropical Meteorology (IITM), who specialises in cloud microphysics and was also part of the experiment in Solapur, said, “In our experiment in Solapur, we have done statistical and physical experiments by registering what is happening inside the cloud before and after seeding and documenting the processes leading to precipitation.”
• Rain forms in the cloud, but whether it reaches the surface or not is determined by several factors.
• it can even evaporate on its way down to the surface since we have tropical conditions.
• There are many uncertainties. In winter, it has not been tried.
• In winter, cloud systems are different. There will have to be further research to investigate this possibility.

How does cloud seeding work?

• Cloud seeding does not create new clouds.
• It increases the amount of rainfall by 10-15% from existing clouds by adding tiny particles called ice nuclei to the clouds.
• Inside the seeded clouds, water vapor freezes onto the particles. These heavier frozen particles fall towards the ground as rain.
• Cloud-seeding materials are released via ground-based and/or airborne systems.
• The most common cloud-seeding materials are silver iodide and dry ice.

Why is cloud seeding so attractive?

• Cloud seeding is a highly portable and flexible technology. It does not require the construction of large, permanent, and costly structures, such as dams or water conveyance systems.
• Projects can be mobilized quickly and operations can be regulated.
• Studies have indicated no significant environmental impacts.
• Further, the benefit/cost ratios associated with most cloud seeding projects are typically very favorable.

Cloud Seeding Techniques:

Hygroscopic Cloud Seeding:

• Objective: Accelerate droplet coalescence in liquid clouds.
• Seeding agents act as efficient cloud condensation nuclei (CCN) or Giant Cloud Condensation Nuclei (GCCN).
• Strengthens condensation and collision–coalescence process, increasing precipitation efficiency.

Glaciogenic Cloud Seeding:

• Centers on prompting the generation of ice in clouds that exist in a supercooled state.
• Involves dispersing efficient ice nuclei (e.g., silver iodide, dry ice) into the cloud.
• Enhances ice particle production, leading to increased rainfall.

PRACTICE QUESTION Q. Cloud seeding has been proposed as a technology to augment precipitation and address water scarcity issues. Examine the scientific principles behind cloud seeding and its potential applications in the context of water resource management.