Nothing but blue sky ... and chemical

Every year, China launches thousands of rockets and artillery shells into the sky. They're not part of a set of war games or preparation for a battle with Taiwan, but rather a battle with the weather. Through its Weather Modification Program, the Chinese government hopes to control the fickle forces behind rain. Run by the Weather Modification Department, a division of the Chinese Academy of Meteorological Science, the program employs and trains 32,000 to 35,000 people across China, some of them farmers, who are paid $100 a month to handle anti-aircraft guns and rocket launchers.

The heavy weapons are used to launch pellets containing silver iodide into clouds. Silver iodide is thought to concentrate moisture and cause rain. The process is known as cloud seeding and China has invested heavily in it, using more than 12,000 anti-aircraft guns and rocket launchers in addition to about 30 planes

Chinese research into weather control began in 1958, when the practice was still in its early stages. With a population of more than 1.3 billion, China requires vast amounts of water. Cities like Beijing suffer from terrible smog, and rain can help clear away air pollution. The government is using cloud seeding to try to produce rain for farmers, stave off drought and fill water basins.

So how does it work? Even in areas with very low humidity, water is present in the sky and in clouds. A rainstorm happens after moisture collects around particles in the air, causing it to reach a level of saturation at which point it can no longer hold in that moisture. Cloud seeding essentially helps that process along, providing "nuclei" around which water condenses. These nuclei can be salts, calcium chloride, dry ice or silver iodide, which the Chinese use. Silver iodide is used because its form is similar to ice crystals. Calcium chloride is often used in warm or tropical areas.

The Chinese are not the first to use this technique.Noted atmospheric scientist Bernard Vonnegut (brother of novelist Kurt Vonnegut) is credited with discovering the potential of silver iodide for use in cloud seeding during 1946 while working for the General Electric Corporation in the state of New York. This property is related to a good match in lattice constant between the two types of crystal (the crystallography of ice later played a role in Kurt Vonnegut's novel Cat's Cradle). The first attempt at cloud seeding was in upstate New York during 1946. Vincent Schaefer, a General Electric chemist, caused snow to fall near Schenectady, NY after he dumped six pounds of dry ice into a cloud from a plane.

Silver iodide is mostly used for winter snowfall augmentation over mountains and hail suppression. While not a new technique hygroscopic seeding for enhancement of rainfall in warm clouds is enjoying a revival, based on some positive indications from research in South Africa, Mexico, and elsewhere. The hygroscopic material most commonly used is salt. It is postulated that hygroscopic seeding causes the droplet size spectrum in clouds to become more maritime (bigger drops) and less continental, stimulating rainfall through coalescence.

From March 1967 until July 1972, the US military Operation Popeye cloud seeded silver iodide to extend the monsoon season over North Vietnam, specifically the Ho Chi Minh Trail. The operation resulted in the targeted areas seeing an extension of the monsoon period an average of 30 to 45 days. The 54th Weather Reconnaissance Squadron carried out the operation to "make mud, not war."

One private organization which offered, during the 1970s, to conduct weather modification (cloud seeding from the ground using silver iodide flares) was Irving P. Crick and Associates of Palm Springs, California. They were contracted by the Oklahoma State University in 1972 to conduct such a seeding project to increase warm cloud rainfall in the Lake Carl Blackwell watershed. That lake was, at that time (1972-73), the primary water supply for Stillwater, Oklahoma and was dangerously low. The project did not operate for a long enough time to show statistically any change from natural variations. However, at the same time, seeding operations have been ongoing in California since 1948.

An attempt by the United States military to modify hurricanes in the Atlantic basin using cloud seeding in the 1960s was called Project Stormfury. Only a few hurricanes were tested with cloud seeding because of the strict rules that were set by the scientists of the project. It was unclear whether the project was successful; hurricanes appeared to change in structure slightly, but only temporarily. The fear that cloud seeding could potentially change the course or power of hurricanes and negatively affect people in the storm's path stopped the project.

Two Federal agencies have supported various weather modification research projects, which began in the early 1960s: The United States Bureau of Reclamation (Reclamation; Department of the Interior) and the National Oceanic and Atmospheric Administration (NOAA; Department of Commerce). Reclamation sponsored several cloud seeding research projects under the umbrella of Project Skywater from 1964-1988, and NOAA conducted the Atmospheric Modification Program from 1979-1993. The sponsored projects were carried out in several states and two countries (Thailand and Morocco), studying both winter and summer cloud seeding. More recently, Reclamation sponsored a small cooperative research program with six Western states called the Weather Damage Modification Program, from 2002-2006.

Funding for research has declined in the last two decades. A 2003 study by the United States National Academy of Sciences urges a national research program to clear up remaining questions about weather modification's efficacy and practice.

In Australia, CSIRO conducted major trials between 1947 and the early 1960s:

1947 – 1952: CSIRO scientists dropped dry ice into the tops of cumulus clouds. The method worked reliably with clouds that were very cold, producing rain that would not have otherwise fallen.
1953 – 1956: CSIRO carried out similar trials South Australia, Queensland and other States. Experiments used both ground-based and airborne silver iodide generators.
Late 1950s and early 1960s: Cloud seeding in the Snowy Mountains, on the Cape York Peninsula in Queensland, in the New England district of New South Wales, and in the Warragamba catchment area west of Sydney.
Only the trial conducted in the Snowy Mountains produced statistically significant rainfall increases over the entire experiment.