In order to address the increasing pollution rates, on 28 October, a cloud seeding trial was held in Delhi. The silver iodide (AgI) chemicals were scattered in the clouds on a plane. Depending on the release of the chemicals, it can take 15 minutes to 4 hours before the water droplets or ice crystals form hence the delay in the onset of rain. In case the trial is successful, the process will be conducted on a regular basis.
The quality of air in Delhi has again become dangerous, and the Air Quality Index (AQI) is over 300, which may be regarded as very poor. As a response, the Delhi government carried out the first cloud seeding trial today (28 October 2025). The method operates under the principle of causing artificial rains, which are caused through spraying chemicals into the clouds to aid in cleaning the pollutants. However, what is the reason that it takes 15 minutes to 4 hours before the rain falls after seeding?
What Is Cloud Seeding?
Cloud seeding is a weather modification process that is 80 years old. It is a process where some chemicals are injected into clouds in order to induce precipitation in rain or a snowfall. Of course, rain is formed as the small particles (nuclei) such as dust or salt aid the process of condensing water vapor into droplets or ice. Nevertheless, there are clouds that do not have sufficient amounts of these nuclei and are dry. Cloud seeding adds unnatural nuclei to help in quickening the rain-making process.
Scientific fact:
Seeding can be of two main types, glaciogenic (when dealing with cold clouds) and hygroscopic (when dealing with warm clouds). Silver iodide (AgI) is mostly employed in the city of Delhi since it enhances the formation of ice crystals.
Statistics:
Cloud seeding is able to augment rain by 5-15 percent globally. As an example, a 10 percent growth has been noted in the Reno area in the United States.
Why Cloud Seeding in Delhi?
Delhi is one of the most contaminated cities in this world. In the winter season, the burning of stubble and vehicle exhausts produce fine particulate matter (PM2.5) into the atmosphere. These particles can be washed using artificial rain through cloud seeding.
In May 2025 the Delhi Cabinet agreed to fund [?]3.21 crore of five cloud seeding trials. In September, an MoU was signed with the IIT Kanpur to execute the project.
Today’s Trial (28 October):
The planting was done between 12: 30 pm and 1 pm by air. Previous attempts were delayed because of the unavailability of appropriate clouds. According to the Environment Minister Manjinder Singh Sirsa, it may start raining soon. The operation targeted certain regions of the Delhi-NCR and a total of five trials were to be undertaken, an estimated cost of about [?]64 lakh per trial.
The Complete Cloud Seeding Process (6 Steps)
- Cloud Selection: The radar and satellite image make scientists see the appropriate clouds that have supercooled water (below 0degC). The altitude of the clouds was selected to 2-5 km in Delhi.
- Fact: The optimal temperature of the cloud to conduct AgI activity is in the -5degC to -20degC.
- Seeding: The aircraft transports the agi crystals (0.1-1 micron) through the clouds. These are ice forming nuclei.
- Statistics: 100-500 grams of AgI are required per kilometre; in the Delhi experiment, 10-15 kg/aeroplane was used.
- Nucleation: The AgI crystals bind themselves to water droplets creating tiny ice embryos which multiply quickly at approximately -10degC.
- Fact: One AgI particle has the capability of producing 100-1,000 ice crystals.
- Growth and Coalescence: The moisture is absorbed by the ice crystals and becomes bigger. They later fall into rain in cold clouds. The droplets combine in warm clouds because of salt particles.
- Data: Crystals may increase by 10-100 microns/min.
- Precipitation: The particles once heavy enough drop. It takes 10-30 minutes as they move 2-4 km between the base of the clouds and ground.
- Rain Distribution: The rain usually covers 10-50 square kilometres, and 5-10 mm of rain is the target of Delhi.
Why Does Rain Take 15 Minutes to 4 Hours After Seeding?
It is not that rain occurs immediately as the physical mechanism of droplet and crystal formation does not happen immediately. Cloud seeding is not magic as such.
15–30 minutes (Initial Phase):
Formation of nuclei starts in 15 minutes. When the clouds are mature, then drizzle can begin.
Fact: AgI spreads all over the cloud in 5-10 minutes of releasing.
30 minutes to 1 hour (Growth Phase):
Crystals also require between 20-40 minutes to develop and start falling as air moves through them.
Data: during experiments of CAIPEEX in India, the initial raindrops were observed 25-45 minutes after the seeding.
1–4 hours (Full Rainfall Phase):
Large raindrops are formed, melt, and spread in 1-2 hours. It can take as long as 4 hours under low wind or when clouds are higher than normal.
Reality: Israeli tests revealed that the mean rainfall followed 45-90 minutes (and even 3-4 hours) when the wind speeds were 10-20 km/h.
WMO Data: The results of 70% of the cases according to the World Meteorological Organization fall within the 20-60 minutes and 20-30 minutes, respectively, based on cloud thickness (1-3 km) and humidity (80-100%).
In the case of the trial of Delhi, light rain will be received in 1-2 hours.
Effectiveness and Benefits
- Rain Increase: Indicatively, Idaho (USA) registered an increase of 10-12 percent and CAIPEEX project in India registered 10-20 percent.
- Cost-Benefit: Rs 500- 1,000 per millimetre of rain. The five trials in Delhi would result in 50-100 mm of rain, which would drop AQI by 100-150 indexes.
- Benefits:
- Less pollution (PM2.5 level might decrease by 30-50 percent)
- Improved crop health
- Drought mitigation
Are There Any Risks?
The amount of silver iodide is usually safe, according to the WHO recommendations (less than 0.1 grams per square kilometre). Nevertheless, it might cause soil buildup in the long run in case it is overused. In 70 years of the worldwide economy, no serious health effects have been noted. Nevertheless, there is a failure of trials (20-30%) in the absence of appropriate clouds.
