Explained | Delhi cloud-seeding trials: Artificial cloud-modification technique

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 making use of an aircraft -- which act as "seeds" and 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 under zero degree Celsius, particles of silver iodide are added to a cloud, which accumulate water and ice. Being heavy, the fused particles fall, melting along the way as temperatures turn warm closer to the ground, says the report aimed at addressing commonly-thought questions of the public, administrators and policymakers.

In warm clouds, where the temperature is above zero degree Celsius, a chemical solution, such as sodium chloride (NaCl) or potassium chloride (KCl), is used as a "seeding agent" to promote fusing of water droplets and improve 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 on Earth.

First attempts of 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. The instance is the first documentation of clouds made artificially in a laboratory.

Atmospheric scientist Bernard Vonnegut took artificial rain-making attempts forward 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 on the ground along with artificial rain or snow -- can present an environmental hazard as "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 gas that contributes 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-per cent 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 places across India to identify a suitable location to conduct cloud seeding.

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

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

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