Delhi smog: Can artificial rain clean the air for Delhiites to breathe?

Kejriwal wants to use cloud seeding to clean Delhi's pollution

Among the measures announced by Delhi Chief Minister Arvind Kejriwal on Sunday to combat the capital's mounting pollution woes was the possibility of inducing rains artificially through cloud seeding. 

According to media reports, Kejriwal said that the possibility of utilising artificial rains, albeit with the aid of the Centre, was discussed in the emergency meeting conducted on Sunday. (Read more: https://www.business-standard.com/article/current-affairs/delhi-pollution-aap-govt-shuts-schools-bans-construction-for-few-days-116110600302_1.html) 

What is cloud seeding? 

Cloud seeding is the artificial process of inducing rains by treating clouds with chemical agents deployed through aircraft. 

To understand the process, one needs to understand how rainfall occurs in the case of cold and warm cloud seeding, respectively. Cold clouds contain supercooled droplets of water, which are still in the liquid state but are at a temperature below water's freezing point of zero degree Celsius; when these droplets form ice crystals they become too heavy to remain suspended in air and consequently fall, melting along the way to form raindrops. 

By "seeding" the air with chemicals, the water content contained therein can be induced to form ice crystals. 

The most common chemicals or substances used in the process are silver iodide and dry ice for cold cloud seeding. 

Calcium chloride or common salt crystals are used for warm cloud seeding. Warm cloud seeding is the process used in warm-weather regions like India. 

The process of warm cloud seeding is a bit different. According to weathermodification.org, warm cloud seeding involves introducing additional nuclei upon which water vapour may condense and form cloud droplets that are of different sizes, often typically larger than those droplets forming naturally in the cloud. This is intended to promote a process called "collision–coalescence", wherein droplets collide with each other forming larger droplets and eventually becoming rain drops. This process can become ineffective if the droplets are close to or equal in size. The warm cloud seeding agents act to increase the probability of forming larger drops that can collect the smaller similarly-sized droplets and grow to a size where they naturally fall as rain.

Can rain reduce pollution? 

Yes, it can, but the efficiency of the process is subject to many conditions. 

According to a Massachusetts Institute of Technology study, a raindrop can attract "hundreds of tiny aerosol particles to its surface" during its way down. The process, the study said, is called coagulation, and it can clear the air of pollutants like sulfates, soot, and organic particles. 

Cloud seeding in India 

While Maharashtra scrapped its cloud seeding plans in July this year, owing to satisfactory rainfall this monsoon, the state had experimented with the process in August-November last year. 
 

During the 2015 experiment, according to reports, the cloud seeding experiment wasn't very effective because of a paucity of moisture-laden clouds and by the time of the experiment the monsoon was receding. However, the state government believed that starting cloud seeding in June, and continuing till August, would yield better results. 

According to Hindustan Times, before scrapping the plan, Maharashtra had shortlisted Khyati-Weather Modification for spray sorties for about 100 hours at the cost of Rs 30 crore.

Will it work for Delhi? 

We will have to wait and see. However, given the dire straits of the capital's denizens, it might be worth a try. Time will tell if the plan comes to fruition and works.