Cloud seeding
Cloud seeding is a weather modification technique that involves introducing substances into clouds to encourage precipitation. This process is primarily used to increase rainfall for agricultural purposes, boost snowfall for winter recreation, disperse fog, and mitigate hail. The substances commonly used for seeding include silver iodide and dry ice for cold clouds, as well as salt crystals for warm clouds. While cloud seeding is relatively inexpensive and straightforward to execute, its effectiveness remains a topic of debate among scientists due to the complex nature of atmospheric processes.
Most cloud seeding operations target cold clouds, where ice crystals can be stimulated to grow larger, eventually falling as precipitation. However, concerns exist regarding the potential redistribution of precipitation, meaning that increased rainfall in one area might lead to reduced rainfall in another. Historical experiments have produced mixed results; for instance, some studies showed no significant increase in rainfall, while only a few demonstrated statistically convincing improvements. Despite ongoing research since World War II, a comprehensive understanding of cloud behavior is still lacking, making predictable large-scale weather modification challenging. In recent years, several U.S. states have initiated cloud seeding programs in response to severe drought conditions, further highlighting its relevance in contemporary water management debates.
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Subject Terms
Cloud seeding
DEFINITION: Practice of introducing agents into clouds for the purpose of intentionally modifying the weather
Because cloud and atmospheric processes are not completely understood, some doubt exists regarding whether cloud seeding actually works. In addition, some environmentalists are concerned that encouraging precipitation in one area may result in less precipitation in another.
Cloud seeding, which is relatively inexpensive and easily conducted, has long been the main technique used in weather modification. The seeding of clouds is undertaken for many purposes, including increasing rainfall for agriculture, increasing snowfall for winter recreational areas, dispersal of fog and clouds, and suppression of hail.
Most cloud seeding is directed at clouds with temperatures below freezing. These cold clouds consist of ice crystals and supercooled droplets (water droplets with subfreezing temperatures). Initially, supercooled droplets far outnumber ice crystals; however, the ice crystals quickly grow larger at the expense of the droplets and, after reaching sufficient size, fall as precipitation. The objective of seeding cold clouds is to stimulate this process in clouds that are deficient in ice crystals. The seeding agent is either silver iodide (AgI) or dry ice—solid carbon dioxide (CO2) with a temperature of −80 degrees Celsius (−112 degrees Fahrenheit). Silver iodide pellets act as nuclei on which water droplets freeze to form precipitation, while dry ice pellets are so cold that they cause the surrounding supercooled droplets to freeze and grow into snowflakes. Warm clouds with temperatures above freezing can also be seeded through injection with salt crystals, which triggers the development of large liquid cloud drops that fall as precipitation.
Clouds must already be present for such weather modification techniques to work, since clouds cannot be generated by seeding. Most cloud seeding is done from aircraft, although silver iodide crystals can be injected into clouds from ground-based generators. One environmental concern is that seeding may only redistribute the supply of precipitation, so that an increase in precipitation in one area might mean a compensating reduction in another.
Since World War II, considerable research has gone into cloud seeding. However, scientists still cannot conclusively answer the question of whether cloud seeding actually works. One of the first large-scale experiments, conducted over south-central Missouri for five years during the 1950s, actually decreased rainfall. Apparently the clouds were overseeded, resulting in too many ice crystals competing for too few water droplets to form precipitation. In the 1970s and early 1980s, the National Oceanic and Atmospheric Administration (NOAA) conducted a major experiment over southern Florida to test the effectiveness of seeding cumulus clouds. The initial results showed that under some conditions seeding increases rainfall, but a second, more statistically rigorous set of experiments failed to confirm the earlier results. Of the many experiments in cloud seeding that have been undertaken, only one, conducted in Israel in the 1960s and 1970s, has yielded statistically convincing confirmation of an increase in precipitation.
The conflicting results of the Florida and Israel studies point out the uncertainties of cloud seeding and the need for more basic knowledge of cloud and atmospheric processes. A major result of the many decades of investigating cloud seeding is the realization that weather events are quite complex and not yet fully understood. Without further basic understanding of atmospheric processes, large-scale weather modification through cloud seeding cannot be carried out with scientifically predictable results.
Several studies conducted in the 2000s and 2010s indicated that cloud seeding over mountainous areas can increase the snowpack by about 5 to 15 percent, which, in turn, increases the amount of snowmelt that can feed into local watersheds. In the early 2020s, several US states— Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming—attempted cloud-seeding programs to alleviate a severe drought that had plagued the region for years.
Bibliography
Ahrens, C. Donald. “Cloud Development and Precipitation.” Essentials of Meteorology: An Invitation to the Atmosphere. 8th ed. Cengage, 2017. Print.
Cotton, William R., and Roger A. Pielke. Human Impacts on Weather and Climate. 2nd ed. New York: Cambridge UP, 2007. Print.
Harvey, Chelsea. "Eight States Are Seeding Clouds to Overcome Megadrought." Scientific American, 16 Mar. 2021, www.scientificamerican.com/article/eight-states-are-seeding-clouds-to-overcome-megadrought/. Accessed 16 July 2024.