Delhi Cloud-Seeding Trials: Understanding the Artificial Cloud Modification Technique

PTI, 28 October 2025, New Delhi: Here is an explainer on the technique that artificially modifies a cloud so that it produces more rainfall, as Delhi saw its first cloud-seeding trial on Tuesday in an attempt to address air pollution.

Particles are added to a cloud—typically using an aircraft—which act as “seeds” around which water vapour condenses, explains a 2023 report by researchers from the Indian Institute of Tropical Meteorology (IITM), Pune.

In cold clouds, where the temperature is below zero degrees Celsius, particles of silver iodide are added to a cloud, where they accumulate water and ice. Being heavy, the fused particles fall, melting along the way as temperatures rise closer to the ground, says the report aimed at addressing commonly-held questions from the public, administrators, and policymakers.

In warm clouds, where the temperature is above zero degrees Celsius, a chemical solution, such as sodium chloride (NaCl) or potassium chloride (KCl), is used as a “seeding agent” to promote the fusing of water droplets and improve the efficiency of rain formation, the researchers explain.

A cloud is naturally formed when air is saturated with water vapours. Unable to hold water in the vapour state, particles start coming together and condense into visible droplets of water or ice crystals, thereby forming clouds. Rainfall or snowfall occurs when the droplets or crystals grow large and heavy enough to fall to Earth.

The first attempts at artificial rain or snowfall are said to have been made in 1946, when American chemist and meteorologist Vincent Schaefer conducted experiments to understand the physics of precipitation.

Schaefer added dry ice to a chilled chamber and saw that a cloud instantly formed around the ice particles. This event is the first documented instance of clouds being artificially created in a laboratory.

Atmospheric scientist Bernard Vonnegut advanced artificial rain-making attempts in 1947, when using silver iodide crystals produced better results in cloud seeding compared to dry ice.

Studies from the United States suggest that cold cloud seeding in orographic clouds—over mountainous areas where natural lifting processes in the air help form clouds—can enhance snowfall, the IITM report says.

However, globally, there is limited evidence on the effectiveness of cloud seeding, which poses a challenge in evaluating the technique for its effects, according to a 2024 report by the US Government Accountability Office.

Further, seeding agents—which fall to the ground along with artificial rain or snow—can present an environmental hazard. Residual silver (from silver iodide) discovered in places near cloud-seeding projects is considered toxic, write researchers in a January 2025 study published in the Advances in Agricultural Technology and Plant Sciences journal.

"Dry ice can also be a source of greenhouse gases that contribute to global warming, as it is basically (solid) carbon dioxide," they say.

The IITM report points to cloud-seeding experiments conducted by the institute in the 1970s, which suggested a 17-percent enhancement in rainfall, even as no definite conclusions could be drawn on the efficacy of the technique.

In recent decades, experiments and observations have been conducted to survey how aerosol and cloud droplet particles behave in different places across India to identify a suitable location for conducting cloud seeding.

The IITM researchers also highlighted precautions for properly conducting cloud-seeding experiments, including being informed of weather conditions and imminent severe weather.

Flying restrictions and permissions need to be obtained in advance for safety and security, they added.

Information on clouds before, during, and after seeding, aircraft availability, and observations regarding which cloud can be selected for seeding are among the requirements for conducting trials, the researchers said.