Natural rainfall occurs when supercooled cold water interacts with particles of dust salt or sand and forms ice crystals. These ice crystals then form a nucleus; more water droplets can then attach themselves to this nucleus and increase the size of the droplet (in colder air these droplets are snowflakes). When this droplet or snow flake reaches a critical size it falls as snow or rain.
The idea of cloudseeding started in 1946, when Dr. Vincent J. Schaefer (who was working at the General Electric Laboratory in New York), was researching how to create artificial clouds in a chilled chamber. Schaefer added dry ice to one experiment, to cool the chamber further. The water vapour in the chamber formed a cloud around this dry ice; the ice crystals in the dry ice had created a nucleus around which droplets of water could now form. This process is known as the cold rain process.
Cloud seeding increases the number of the nuclei available so to take greater advantage of the moisture in the cloud, forming raindrops that would not normally have formed.
The warm rain process involves clouds in tropical regions that never reach freezing point. In these cases, raindrops form around a hygroscopic nucleus, which is a particle like salt or dust that attracts water. Small droplets collide and amalgamate until they form a droplet large enough to fall. Another type of cloudseeding, dynamic cloud seeding, aims to boost vertical air currents; this encourages more water to pass through the clouds, which in turn leads to more rain.
To encourage the warm rain process, calcium chloride is commonly used to provide the nucleus for raindrop formation. For the cold rain process, silver-iodide (introduced either via the air or the ground) can be used as a nuclei; its structure is very similar to ice crystals. Dry ice can be introduced from the air (at -80°C) into clouds; this lowers the air temperature so that some of the supercooled water droplets can be converted into ice crystals.
Common salt or fine water droplets can also be used to encourage coalescence. Most of the methods that are used to limit the development of hail use cloud seeding that employs ice nucleants, or use silver oxide. Hail damage can in theory be reduced by 25% by using cloud seeding; there have been no quantifiable results demonstrating this however.
CSIRO conducted cloudseeding trials in Australia between 1947 and the early 1960s. Only the trial conducted in the Snowy Mountains during the late 1950’s and the early 1960’s produced any statistically significant rainfall increases. The Queensland government of Australia announced in December 2006 $7.6 million in funding for "warm cloud" seeding research.
Clouds were seeded during the 2008 Summer Olympics in Beijing using rockets, to guarantee there would be no rain during the opening and closing ceremonies. In the United States, cloud seeding is used to increase rainfall in areas experiencing drought, to reduce the size of hailstones that form in thunderstorms, and also to reduce the amount of fog in and around airports. It is also sometimes used by ski resorts to induce snowfall. After the Chernobyl disaster, Soviet military pilots seeded clouds over the Belorussian SSR to remove radioactive particles from clouds heading toward Moscow. Cloud seeding is used on a national scale in Mali and Niger.
Diagrams explaining cloud seeding: http://bit.ly/Wcb3ap; http://bit.ly/ZsiszE
The image shows a proposed cloud-seeding ship with Flettner rotors that spin about their vertical axis and act as powerful computer-controlled sails. Seawater sprays from the tops of the rotors to seed clouds. According to theory and computer models, seeding marine stratocumulus clouds by spraying them with an ultrafine saltwater mist from ships could significantly enhance cloud droplet number concentration. The clouds, containing more particles, would cast enough sunlight back into space to at least partially offset the warming effects of CO2 from burning fossil fuels (http://bit.ly/VFV8ia).
Image: ©John MacNeil Illustration