Cloud seeding
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Image:Cessna 210 Hagelflieger Detail.jpg Cloud seeding, also known as weather modification, is the attempt to change the amount or type of precipitation that falls out of clouds, or their structure, by dispersing substances into the air which serve as cloud condensation nuclei and aid in the formation of precipitation. The most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide).
Seeding of clouds requires that they contain supercooled water--that is, liquid water colder than zero Celsius. Introduction of a substance, such as silver iodide, that has a crystalline structure similar to that of ice will induce freezing (heterogeneous nucleation).
In mid-latitude clouds, the usual seeding strategy has been based upon the vapor pressure being lower over water than over ice. When ice particles form in supercooled clouds, they grow at the expense of liquid droplets and become heavy enough to fall as rain from clouds that otherwise would produce none.
Seeding of tropical cumuli sought to exploit the latent heat released by freezing as well. This strategy of "dynamic seeding" assumed that the additional latent heat would add buoyancy, strengthen the updrafts, ensure more low-level convergence, and ultimately cause explosive growth of properly selected cumuli.
Cloud seeding chemicals may be dispersed by aircraft or by dispersion devices located on the ground. For release by aircraft, silver iodide flares are ignited and dispersed as an aircraft flies through a cloud. When released by devices on the ground, the fine particles are pulled further up into the air by the air currents after release.
While cloud seeding has shown to be effective in reducing the amount of cloud cover, it is more controversial whether cloud seeding increases the amount of precipitation from a cloud. Part of the problem is that it is presently impossible to know how much precipitation would have occurred had the cloud not been "seeded".
The National Center for Atmospheric Research (NCAR), an institution in Boulder, Colorado, has made some statistical analysis of seeded and unseeded clouds trying to understand the differences between them. They have conducted seeding research in several countries that include Mexico, South Africa, Thailand, Italy, and Argentina.
[edit] History
One scientist associated with the invention of cloud seeding is Irving Langmuir. The first attempt at cloud seeding was in Massachusetts in 1946. A plane "seeded" a cloud with crushed dry ice and snow began falling out of that cloud. Noted atmospheric scientist Bernard Vonnegut (brother of novelist Kurt Vonnegut) is credited with discovering the potential of silver iodide for use in cloud seeding. 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). It is important to mention that silver iodide is mostly used for hail suppression while the new technique called hygroscopic seeding is used for the enhancement of rainfall in warm clouds.
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.
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.
[edit] Modern uses
The largest cloud seeding system in the world is that of the People's Republic of China, which believes that it increases the amount of rain over several increasingly arid regions, including its capital city, Beijing, by firing silver iodide rockets into the sky where rain is desired. There is even political strife caused by neighboring regions which accuse each other of "stealing rain" using cloud seeding[1].
In the United States, cloud seeding is used to increase precipitation in areas experiencing drought, to reduce the size of hailstones that form in thunderstorms, and to reduce the amount of fog in and around airports. Cloud seeding is also occasionally used by major ski resorts to induce snow fall. In January 2006, an $8.8 million cloud seeding project began in Wyoming to examine the effects of cloud seeding on snowfall over Wyoming's Medicine Bow, Sierra Madre, and Wind River mountain ranges. [2]
A number of commercial companies, such as Aero Systems Incorporated [3], Atmospherics Incorporated [4], North American Weather Consultants [5] and Weather Modification Incorporated [6] offer weather modification service centered around cloud seeding, and the USAF proposed its use on the battlefield in 1996.
In Australia, CSIRO’s activities in Tasmania in the 1960s were successful. Seeding over the Hydro-Electricity Commission catchment area on the Central Plateau achieved rainfall increases as high as 30% in autumn. The Tasmanian experiments were so successful that the Commission has regularly undertaken seeding ever since in mountainous parts of the State.
Beginning in Winter 2004, Snowy Hydro Limited is conducting a six-year research project of winter cloud seeding to assess the feasibility of increasing snow precipitation in the Snowy Mountains in Australia. This project was discussed at a summit in Narrabri, NSW on 1st December 2006. The summit met with the intention of outlining a proposal for a 5 year trial, focussing on Northern NSW. The various implications of such a widespread trial were discussed, drawing on the combined knowledge of several worldwide experts, including representatives from the Tasmanian Hydro Cloud Seeding Project.
At the July 2006 G8 Summit, President Putin commented that air force jets had been deployed to seed incoming clouds so they rained over Finland. However, not much seemed to come of this as there was torrential rain at the St Petersburg site during the summit meeting [7]
In Southeast Asia, open burning produces haze that pollutes the regional environmental. Cloud-seeding has been used to improve the air quality to encourage rainfall.
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