What is cloud seeding?
Cloud seeding is a form of weather modification. It can be used to disperse fog, suppress hail, or control winds, but is most often used to increase precipitation. In order to understand the process, however, a basic understanding of clouds and how precipitation is formed is needed.
As warm air rises from the Earth, it begins to cool and forms tiny droplets of water that condense into cloud droplets. Cloud droplets are formed around particles of dust, salt, or soil (called cloud condensation nuclei) that are always present in the atmosphere. These cloud droplets group together into clouds, which can form precipitation in one of two ways. In warm temperatures, the droplets in the clouds merge with many other droplets and become heavy enough to fall to the Earth as rain. (It takes millions of cloud droplets to form a single raindrop.) In colder temperatures, the droplets of water form ice crystals. Other droplets freeze onto these ice crystals, which grow larger and heavier until they fall to the ground as rain, snow, or hail.
Cloud seeding is actually a very complex process. In the simplest terms, it introduces other particles into a cloud to serve as cloud condensation nuclei and aid in the formation of precipitation. There are three types of cloud seeding: static mode, dynamic mode, and hygroscopic seeding.
Static mode cloud seeding seeks to increase rainfall by adding ice crystals (usually in the form of silver iodide or dry ice) to cold clouds. Dynamic mode cloud seeding increases rainfall by enhancing “vertical air currents in clouds and thereby vertically process more water through the clouds.” Basically, in this method of seeding, a much larger number of ice crystals are added to the cloud than in the static mode. In hygroscopic seeding, salt crystals are released into a cloud. These particles grow until they are large enough to cause precipitation to form. Clouds can be seeded from above with the help of airplanes that drop pyrotechnics, or from the ground by using artillery or ground-to-air rockets
There are three cloud seeding methods: static, dynamic and hygroscopic.
- Static cloud seeding involves spreading a chemical like silver iodide into clouds. The silver iodide provides a crystal around which moisture can condense. The moisture is already present in the clouds, but silver iodide essentially makes rain clouds more effective at dispensing their water.
- Dynamic cloud seeding aims to boost vertical air currents, which encourages more water to pass through the clouds, translating into more rain [source: Cotton]. Up to 100 times more ice crystals are used in dynamic cloud seeding than in the static method. The process is considered more complex than static clouding seeding because it depends on a sequence of events working properly. Dr. William R. Cotton, a professor of atmospheric science at Colorado State University, and other researchers break down dynamic cloud seeding into 11 separate stages. An unexpected outcome in one stage could ruin the entire process, making the technique less dependable than static cloud seeding.
- Hygroscopic cloud seeding disperses salts through flares or explosives in the lower portions of clouds. The salts grow in size as water joins with them. In his report on cloud seeding, Cotton says that hygroscopic cloud seeding holds much promise, but requires further research.
Cloud seeding is a process where states or other agencies attempt to manipulate weather in order to increase precipitation. This practices dates back several decades, with varying degrees of success. For example, winter precipitation with cloud seeding increased precipitation 5 to 20 percent in continental regions.
There is debate among scientists whether cloud seeding works. According to the March/April 2007 issue of “Southwest Hydrology,” most professional organizations believe there is a 5 to 15 percent increase in precipitation over targeted areas. But in 2003 the National Academy of Sciences stated there was no evidence that cloud seeding works. The determination of cloud seeding’s effectiveness blurs because clouds chosen to seed typically look like they would cause rain anyway.
Cloud seeding experimentation began in earnest with Vincent Schaefer’s 1946 work on cloud modification. He seeded clouds in the Berkshire Mountains with dry ice to stimulate precipitation. The work has continued, even broadening to international scientists. There are currently over 150 weather modification programs in over 37 countries around the world. Most of those are cloud-seeding programs. Eleven U.S. states participate in these programs.
The effectiveness and full environmental issues related to cloud seeding are still in question. The May 2005 “Journal of Applied Meteorology” references varied viewpoints of this argument. Though some seeding has shown statistically in some areas, there is no direct scientific evidence to support this. Some scientists has suggested that pollution itself acts as cloud seeding by performing the same function as clean aerosols. The Journal also stated that recent documentation has shown that pollution aerosols decrease precipitation.