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How does cloud seeding attempt to produce rain?

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Illustration of various geoengineering techniques to address climate change, including carbon capture and storage methods like artificial trees, industrial scrubbers, and deep ocean CO2 storage, as well as solar reflectance strategies such as sulfur aerosols, reflective crops, cloud whitening, and orbital mirrors. The scene shows these interventions applied to land, sea, and atmosphere, with labeled features explaining each process. — Geoengineering Various geoengineering proposals designed to increase solar reflectance or capture and store carbon.

Cloud seeding attempts to produce or increase precipitation by introducing specific substances into clouds to act as nuclei, facilitating the formation of raindrops or ice crystals. The method depends on the temperature and state of the cloud:12

Supercooled Clouds: These clouds contain liquid water droplets at temperatures below 0°C. Because nature may be deficient in natural ice nuclei, these clouds can remain stable without yielding rain. By introducing silver iodide or dry ice, scientists provide nuclei that cause water droplets to freeze into ice crystals. These crystals grow as more droplets attach to them, eventually becoming heavy enough to fall as precipitation.123
Warm Clouds: In clouds with temperatures above 0°C, seeding agents like sodium chloride or calcium chloride particles are used. These act as condensation nuclei, encouraging the formation of giant cloud droplets that grow through coalescence and fall as rain.123

While these techniques can sometimes increase localized precipitation, their overall effectiveness remains a subject of scientific debate.24