What is cloud seeding? Who started the technology?
What is cloud seeding? Who started the technology?
Cloud seeding is the process where substances like dry ice and silver iodide are put into clouds in an attempt to make precipitation fall. Cloud seeding has also been used to dissipate fog and weaken some storms.
Cloud seeding can be used to enhance precipitation and has been used to prevent areas from destruction from drought.
In 1891 Louis Gathmann suggested shooting liquid carbon dioxide into rain clouds to cause them to rain. During the 1930s, the Bergeron–Findeisen process theorized that supercooled water droplets present while ice crystals are released into rain clouds would cause rain. While researching aircraft icing, General Electric (GE)’s Vincent Schaefer and Irving Langmuir confirmed the theory. Schaefer discovered the principle of cloud seeding in July 1946 through a series of serendipitous events. Following ideas generated between him and Nobel laureate Langmuir while climbing Mt Washington in New Hampshire, Schaefer, Langmuir’s research associate, created a way of experimenting with supercooled clouds using a deep freeze unit of potential agents to stimulate ice crystal growth, i.e., table salt, talcum powder, soils, dust, and various chemical agents with minor effect. Then one hot and humid July 14, 1946, he wanted to try a few experiments at GE’s Schenectady Research Lab
Cloud seeding first began in the mid-1940s when Dr. Vincent J. Schaefer was studying cloud formation for General Electric. Cloud seeding has been subsequently used to enhance precipitation, dissipate fog, modify hurricanes, and decrease lightning and hail in thunderstorms. Cloud seeding can occur naturally but is more often done by humans.
Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. Its effectiveness is debated; some studies have suggested that it is “difficult to show clearly that cloud seeding has a very large effect”. The usual objective is to increase precipitation (rain or snow), either for its own sake or to prevent precipitation from occurring in days afterward.
The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice (solid carbon dioxide). Liquid propane, which expands into a gas, has also been used. This can produce ice crystals at higher temperatures than silver iodide. After promising research, the use of hygroscopic materials, such as table salt, is becoming more popular.
When cloud seeding, increased snowfall takes place when temperatures within the clouds are between −4 and 19 °F (−20 and −7 °C). Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation.
In mid-altitude clouds, the usual seeding strategy has been based on the fact that the equilibrium vapor pressure is lower over ice than over water. The formation of ice particles in supercooled clouds allows those particles to grow at the expense of liquid droplets. If sufficient growth takes place, the particles become heavy enough to fall as precipitation from clouds that otherwise would produce no precipitation. This process is known as “static” seeding.
Seeding of warm-season or tropical cumulonimbus (convective) clouds seeks to exploit the latent heat released by freezing. This strategy of “dynamic” seeding assumes that the additional latent heat adds buoyancy, strengthens updrafts, ensures more low-level convergence, and ultimately causes rapid growth of properly selected clouds.
Cloud seeding chemicals may be dispersed by aircraft or by dispersion devices located on the ground (generators or canisters fired from anti-aircraft guns or rockets). For release by aircraft, silver iodide flares are ignited and dispersed as an aircraft flies through the inflow of a cloud. When released by devices on the ground, the fine particles are carried downwind and upward by air currents after release. An electronic mechanism was tested in 2010, when infrared laser pulses were directed to the air above Berlin by researchers from the University of Geneva. The experimenters posited that the pulses would encourage atmospheric sulfur dioxide and nitrogen dioxide to form particles that would then act as seeds.
Natural cloud seeding occurs when fallstreaks composed of ice crystals from cirrus clouds fall into cumulus or stratus clouds lower in the atmosphere. The ice crystals act like seeds to start the formation of larger crystals which lead to raindrop formation. Silver iodide has been used in cloud seeding because it has a similar physical structure to natural ice crystals. It is often used in combination with dry ice, which is very cold. The silver iodide lowers the temperature in the cloud and provides more ice crystal seeds, but doesn’t freeze all of the liquid water, either. Ice crystals grow from the liquid freezing onto them and then the ice lumps together. The ice crystals then fall and create snow, and if they melt before reaching the ground, produce rain.
Cloud seeding isn’t just used for enhancing precipitation. It can also be used to evaporate fog and clouds. At airports, when the fog is very cold, adding large quantities of dry ice causes the fog to dissipate because it turns all the liquid to ice, and the ice settles to the ground, leaving clear air. The dry ice cools the liquid, supercooled water droplets even further and the droplets turn into ice crystals. However, this only works with fog that is very cold, but not ice.
Hail and lighting can be very damaging. Adding large amounts of dry ice to thunderstorms has the potential to suppress the formation of hail. Since hail can cause intracloud lightning, suppression of hail causes suppression of lightning, too. Russian scientists claim this method of hail suppression works very well, while US scientists say the experiments are inconclusive from experiments conducted in the early to mid-1900s.
Hurricane modification was only tried a couple times in the 1960s. The first time that hurricanes were seeded, the hurricane slightly weakened and the eye of the hurricane became much larger. The second time, the hurricane weakened dramatically. It is still unknown whether these hurricanes would have weakened naturally or if the seeding had something to do with it. However, it is difficult to do experiments with seeding of hurricanes now because of the potential for changes in path or strength which could result in significant damage and legal liabilities.
The use of cloud seeding is very controversial. Some people say it isn’t worth the resources, because even the most dramatic rainfall increases only amount to 5-20%. If it is done incorrectly, it can even reduce the amount of rain that might have fallen by dissipating clouds. If it does work, some people say it will alter energy transport across the world and have catastrophic repercussions in the future. Some countries employ cloud seeding to try and maintain a level of precipitation high enough to support them, but think nothing of the countries downstream who would have gotten the rainfall had the clouds not been seeded. Even at the local level, seeding clouds in one place could result in a reduction of rainfall downwind and negatively impact their crops, resulting in potential lawsuits. At the 2008 Summer Olympic Games, Beijing employed cloud seeding to clear the air of pollutants and to keep it from raining over the Bird’s Nest stadium while the games were going on. Occurrences like this may become more frequent if climate change alters precipitation patterns and we try to adapt to it with cloud seeding.
Whether cloud seeding is effective in producing a statistically significant increase in precipitation is still a matter of academic debate, with contrasting results depending on the study in question, and contrasting opinion among experts.
A study conducted by the National Academy of Sciences failed to find statistically significant support for the effectiveness of cloud seeding. Based on the report’s findings, Stanford University ecologist Rob Jackson said: “I think you can squeeze out a little more snow or rain in some places under some conditions, but that’s quite different from a program claiming to reliably increase precipitation.” Data similar to that of the NAS study was acquired in a separate study conducted by the Wyoming Weather Modification Pilot Project. However, whereas the NAS study concluded that “it is difficult to show clearly that cloud seeding has a very large effect,” the WWMPP study concluded that “seeding could augment the snowpack by a maximum of 3% over an entire season.”
In 2003 the US National Research Council (NRC) released a report stating, “…science is unable to say with assurance which, if any, seeding techniques produce positive effects. In the 55 years following the first cloud-seeding demonstrations, substantial progress has been made in understanding the natural processes that account for our daily weather. Yet scientifically acceptable proof for significant seeding effects has not been achieved”.
A 2010 Tel Aviv University study claimed that the common practice of cloud seeding to improve rainfall, with materials such as silver iodide and frozen carbon dioxide, seems to have little if any impact on the amount of precipitation. A 2011 study suggested that airplanes may produce ice particles by freezing cloud droplets that cool as they flow around the tips of propellers, over wings or over jet aircraft, and thereby unintentionally seed clouds. This could have potentially serious consequences for particular hail stone formation.
However, Jeff Tilley, director of weather modification at the Desert Research Institute in Reno, claimed in 2016 that new technology and research has produced reliable results that make cloud seeding a dependable and affordable water supply practice for many regions. Moreover, in 1998 the American Meteorological Society held that “precipitation from supercooled orographic clouds (clouds that develop over mountains) has been seasonally increased by about 10%.”
Despite the mixed scientific results, cloud seeding was attempted during the 2008 Summer Olympics in Beijing to coax rain showers out of clouds before they reached the Olympic city in order to prevent rain during the opening and closing ceremonies. Whether this attempt was successful is a matter of dispute, with Roelof Bruintjes, who leads the National Center for Atmospheric Research’s weather-modification group, remarking that “we cannot make clouds or chase clouds away.”
Impact on environment and health
With an NFPA 704 health hazard rating of 2, silver iodide can cause temporary incapacitation or possible residual injury to humans and other mammals with intense or chronic exposure. However, there have been several detailed ecological studies that showed negligible environmental and health impacts. The toxicity of silver and silver compounds (from silver iodide) was shown to be of low order in some studies. These findings likely result from the minute amounts of silver generated by cloud seeding, which are about one percent of industry emissions into the atmosphere in many parts of the world, or individual exposure from tooth fillings.
Accumulations in the soil, vegetation, and surface runoff have not been large enough to measure above natural background. A 1995 environmental assessment in the Sierra Nevada of Californiaand a 2004 independent panel of experts in Australia confirmed these earlier findings.
“In 1978, an estimated 3,000 tonnes of silver were released into the US environment. This led the US Health Services and EPA to conduct studies regarding the potential for environmental and human health hazards related to silver. These agencies and other state agencies applied the Clean Water Act of 1977 and 1987 to establish regulations on this type of pollution.”
Cloud seeding over Kosciuszko National Park—a biosphere reserve—is problematic in that several rapid changes of environmental legislation were made to enable the trial. Environmentalists are concerned about the uptake of elemental silver in a highly sensitive environment affecting the pygmy possum among other species as well as recent high level algal blooms in once pristine glacial lakes. Research 50 years ago and analysis by the former Snowy Mountains Authority led to the cessation of the cloud seeding program in the 1950s with non-definitive results. Formerly, cloud seeding was rejected in Australia on environmental grounds because of concerns about the protected species, the pygmy possum. Since silver iodide and not elemental silver is the cloud seeding material, the claims of negative environmental impact are disputed by peer-reviewed research as summarized by the international Weather Modification Association.
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