In the 8 hours prior to and during the opening ceremony of the 2008 Summer Olympics, the Beijing Municipal Meteorological Bureau shot 1,104 rockets into the air to prevent rain during the ceremony. Rain was forecast, the humidity was 90%, and Beijing usually receives 1-4 inches during similar weather patterns. Despite the threatening clouds, it did not rain over the stadium. Instead, Baoding City, upwind of Beijing, received 4 inches of rain.
Seeding clouds to cause or prevent rain is no longer a new technology, but the results are still uncertain. When there is rain after a cloud seeding operation, it can't be proved that the cloud seeding caused it. However, modern cities and agriculture are also more vulnerable to storms and drought, so large scale cloud seeding is now common. The amounts of chemicals used are much less than in salting and snow removal during an average snowfall. What is there to lose?
All clouds consist of water vapor, but whether it will rain depends on whether water can condense around nuclei and whether the resulting droplets are heavy enough to fall. Cloud seeding works by adding salt, dry ice, or silver iodide on which cloud water vapor can condense. These substances can be shot up into the cloud with rockets or dropped from an airplane. All cloud seeding promotes rainfall or snowfall. To prevent rainfall in an area or reduce its intensity, cloud seeding tries to trigger rainfall before the rain event reaches that area. To prevent hail in an area, cloud seeding tries to trigger rainfall before the cloud grows into a supercell. Cloud seeding may also disperse fog by forcing the water vapor to condense into droplets heavy enough to fall.
There are three types of cloud seeding, static, dynamic and hygroscopic. Static seeding adds nuclei to the cloud. Supercooled water vapor can condense around these nuclei to form ice crystals, which melt into raindrops as they fall. Dynamic seeding tries to promote cloud development from cumulus into cumulonimbus by encouraging vertical air circulation. This also draws surrounding moist air into the cloud. Hygroscopic seeding, also called warm cloud seeding, is what the Beijing Municipal Meteorological Bureau used during the Olympics. It imitates the hygroscopic action of industrial smokestacks by seeding the bottoms of rainclouds, broadening water droplets and making them heavier.
In areas with low rainfall, cloud seeding could cause rain to fall in one area at the expense of downwind areas. Cloud seeding cannot work at all if there are no clouds. This makes cloud seeding the next legal battlefield over water rights in arid and drought regions.
Chinese weather modification employs 40,000 people, with 7,000 cannon and 4,687 rocket launchers, and is used before all major events and public holidays. During the Beijing Olympics closing ceremonies, Beijing once again fired 241 rockets into the cloud base to keep away the rain. This time, airplanes were also used to cover a larger area. Once again, it did not rain.