Expert Explains: ‘Not all clouds are seedable. Not every condition will work’

Moisture incursion [into the clouds] is vital – for example, due to the cyclonic circulations above the Arabian Sea, weather systems above Delhi are currently suitable for an attempt at cloud-seeding. So, to attempt cloud-seeding, there have to be clouds to begin with – and secondly, they should be the right kind of clouds. The seeding of the clouds can be carried out by various methods.

What are the main findings of the studies on cloud-seeding carried out by IITM Pune?

We have conducted detailed research that looks into the scientific aspects — specific enhancements, whether we can increase precipitation in water-limited areas. We need several samples to see whether cloud-seeding will work, and like medical trials, we have done cloud-seeding in different environments where we have found suitable clouds.

From our randomised study — some of the clouds were seeded and others were not — we found an 80% enhancement in rainfall. The clouds were also staying for a longer time. These findings are consistent with findings on aerosol-cloud interaction.

Clouds are modified by aerosols. Hygroscopic aerosols absorb water vapour from the atmosphere and grow in size and mass. But there are also hydrophobic aerosols like black soot, which do not support cloud formation and can actually suppress precipitation.

It is very important to identify the right kind of clouds and the right seeding material. Not all clouds are seedable. Not every condition will work.

Can cloud-seeding be used for pollution control, as is being attempted in Delhi?

Using cloud-seeding for pollution control is totally different from using the process to induce rain. Numerous aerosols are already present in the atmosphere, and we still do not know enough about the process.

We have to experiment more and document more. We need depth of information regarding what is happening in the cloud, how long it has rained, and what the overall impact has been.

We cannot think about the operational aspect right now. We first need the basic things to be fixed, like what kind of conditions and what kind of clouds are best suited for artificial rain. There are many questions and a lot of research needs to be done.

How environmentally safe is this process? Are there any major concerns?

Typically, in warm clouds we have used salts such as sodium chloride. Silver iodide is toxic. Silver is toxic for aquatic life even in minute quantities, and clouds are small entities… To know whether seeding material falls to the surface after precipitation, we need systematic studies.

Ground-based experiments carried out in the United States have detected silver at nearby locations after cloud-seeding trials. There is absolutely no information on the long-term environmental impact in India, and that is something we need to study carefully.

What role do anthropogenic emissions play in cloud formation or rainfall?

A lot of particulates are released into the atmosphere through anthropogenic emissions, including biomass burning, which can suppress cloud formation.

We need to sensitise people to the fact that the environment belongs to everyone — how can we clean the atmosphere otherwise? We cannot think of a long-term solution through a single approach; we have to explore different options and ways to mitigate the problem. Everyone has to work together, including citizens.

Dr Thara Prabhakaran is Scientist G and Project Director (CAIPEEX/ PDTC) at IITM Pune. CAIPEEX, or Cloud Aerosol Interaction and Precipitation Enhancement Experiment, is IITM’s long-term program to study cloud-aerosol interactions and rainfall enhancement.