Cloud Seeding: Using Silver Iodide to Increase Precipitation

Cloud Seeding

What is Cloud Seeding?

Cloud seeding is a weather modification technique that aims to change the amount or type of precipitation that falls from clouds by dispersing certain substances into the air that act as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intended result of cloud seeding is to increase precipitation (rain or snow depending on temperature), but hail and fog suppression are also targeted sometimes.

How Does It Work?

Cloud Seeding  with substances like silver iodide, dry ice, or salt allows additional water droplets or ice crystals to form which makes the clouds more likely to precipitate. In cold clouds (below freezing point), cloud seeders disperse silver iodide flares or generators on the ground or from airplanes. The silver iodide particles provide sites for water molecule attachment in super-cooled clouds, allowing ice crystals to multiply from a few to billions. When the ice crystals get heavy enough, they fall as snow to the ground below.

In warm clouds (above freezing point), salt flares or salt shells fired from ground-based generators or dropped from airplanes seed the clouds. The salt particles serve as sites for water droplet attachment, causing more numerous and larger raindrops to fall faster from the cloud base. This makes precipitation more likely below clouds that would otherwise produce little or no rain on their own.

Effectiveness of Cloud Seeding

Decades of experimental research in the U.S., Australia, South Africa, and other countries have demonstrated that properly conducted cloud seeding can increase winter snowpack and precipitation amounts from 5-15%, depending on location and weather conditions. Snowpack increases are especially important for water supply, as melted snow provides water for reservoirs during summer dry seasons in many regions.

Australian studies of cloud seeding programs operating over 20-30 years have confirmed statistically significant increases in rainfall on the order of 10-15% in seeded areas compared to unseeded control regions. Many snowpack augmentation projects in the western U.S. have also shown 6-15% increases in seasonal snow totals for targeted mountain watersheds using ground-based silver iodide generators.

Cold Clouds vs Warm Clouds

Winter orographic cloud seeding programs target cold clouds capable of producing snowfall on mountains or high terrain. Cold cloud seeding aims to trigger more ice crystal formation and snow growth for additional snowfall. Summer cloud seeding over warm ocean clouds along coastlines or targeted at thunderstorms tries to produce heavier raindrops that fall faster from the cloud base.

Warm cloud seeding has been more challenging to prove effective compared to cold cloud programs. The microphysical processes driving precipitation in warm clouds are more complex than cold clouds which favor ice nucleation induced by seeding. Subtropical and tropical cloud systems also contain natural mechanisms for precipitation already. However, meta-analyses of multiple seeding experiments indicate rainfall increases of 5-15% are reasonably attributable to proper warm cloud seeding.

Potential Downsides

While cloud seeding holds promise for augmenting water supplies, some concerns exist. Effects of silver iodide or other seeding agents on human health and the environment requires more study. Modeling challenges also remain in verifying seeded precipitation separately from natural variability. Ethical issues have also been raised regarding the impacts of weather modification on communities downstream or downwind. Additionally, cloud seeding requires adequate cloud cover and proper conditions to be effective, limiting its applications. Overall though, when properly conducted and monitored, cloud seeding appears able to positively influence precipitation amounts without causing harm.

Regulation of Cloud Seeding Activities

Most countries and U.S. states that conduct cloud seeding regulate the practice through a permitting system. Operators must obtain licenses from weather modification boards to perform seeding and supply detailed plans and records of activities. Monitoring programs using control regions aim to scientifically verify precipitation increases. Cross-border or trans-country seeding also brings international issues since clouds and precipitation do not stop at political boundaries.

Overall, as climate change threatens regional water supplies, and rain-producing weather patterns shift, cloud seeding may play a greater role in augmenting water resources – if the technology and governance can keep pace. Continued research refining seeding methods and verification will help optimize this weather modification tool for maximizing positive impacts on precipitation worldwide in the coming decades.

Get more insights, On Cloud Seeding