Cloud chamber

Cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When an alpha or beta particle interacts with the mixture, it ionizes it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight.). When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge. For more detailed track-shape information.

Charles Thomson Rees Wilson (1869–1959), a Scottish physicist, is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911. In Wilson's original chamber the air inside the sealed device was saturated with water vapor, then a diaphragm is used to expand the air inside the chamber (adiabatic expansion). This cools the air and water vapor starts to condense. When an ionizing particle passes through the chamber, water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud. Wilson, along with Arthur Compton, received the Nobel Prize for Physics in 1927 for his work on the cloud chamber. This kind of chamber is also called a Pulsed Chamber, because the conditions for operation are not continuously maintained. Developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. The cloud chamber was the first radioactivity detector.

Structure and Working of Cloud chamber

Simple cloud chamber consists of the parts sealed environment, radioactive source (if you want to), dry ice or a cold plate and some kind of alcohol source (it has to allow easy evaporation)

Lightweight methyl alcohol vapour saturates the chamber. The alcohol falls as it cools down and the cold condenser provides a steep temperature gradient. The result is a supersaturated environment. The alcohol vapour condenses around ion trails left behind by the travelling ionizing particles. The result is cloud formation, seen in the cloud chamber by the presence of droplets falling down to the condenser. As particles pass through the chamber they leave ionization trails and because the alcohol vapour is supersaturated it condenses onto these trails. Since the tracks are emitted radially out from the source, their point of origin can easily be determined.

Just above the cold condenser plate there is an area of the chamber which is sensitive to radioactive tracks. At this height, most of the alcohol has not condensed. This means that the ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation. This sensitive area is increased in height by employing a steep temperature gradient, little convection, and very stable conditions. A strong electric field is often used to draw cloud tracks down to the sensitive region of the chamber and increase the sensitivity of the chamber. While tracks from sources can still be seen without a voltage supply, background tracks are very difficult to observe. In addition, the voltage can also serve to prevent large amounts of "rain" from obscuring the sensitive region of the chamber,caused by condensation forming above the sensitive area of the chamber. This means that ion trails left by radioactive particles are obscured by constant precipitation. The black background makes it easier to observe cloud tracks.

Before tracks can be visible, a tangential light source is needed. This illuminates the white droplets against the black background. Drops should be viewed from a horizontal position. If the chamber is working correctly, tiny droplets should be seen condensing. Often this condensation is not apparent until a shallow pool of alcohol is formed at the condenser plate. The tracks become much more obvious once temperatures and conditions have stabilized in the chamber. This requires the elimination of any significant drift currents (poor chamber sealing)

Cloud seeding system

Most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide). The expansion of liquid propane into a gas has also been used and can produce ice crystals at higher temperatures than silver iodide. The use of hygroscopic materials, such as salt, is increasing in popularity because of some promising research results.

Seeding of clouds requires that they contain supercooled liquid water—that is, liquid water colder than zero degrees Celsius. Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing nucleation. Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase. Unlike seeding with silver iodide, this spontaneous nucleation does not require any existing droplets or particles because it produces extremely high vapor supersaturations near the seeding substance. However, the existing droplets are needed for the ice crystals to grow into large enough particles to precipitate out.

In mid-latitude clouds, the usual seeding strategy has been predicated upon the fact that the equilibrium vapor pressure is lower over ice than over water. When ice particles form in supercooled clouds, this fact allows the ice particles to grow at the expense of liquid droplets. If there is sufficient growth, the particles become heavy enough to fall as snow (or, if melting occurs, rain) 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 (as in the second figure) or by dispersion devices located on the ground (generators, as in first figure, 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 upwards 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.

Effectiveness of Cloud seeding

Referring to the 1903, 1915, 1919 and 1944 and 1947 weather modification experiments, the Australian Federation of Meteorology discounted "rain making." By the 1950s the CSIRO Division of Radiophysics switched to investigating the physics of clouds and had hoped by 1957 to better understand these processes. By the 1960s the dreams of weather making had faded only to be re-ignited post-corporatisation of the Snowy Mountains Scheme in order to achieve "above target" water. This would provide enhanced energy generation and profits to the public agencies who are the principal owners. Cloud seeding has been shown to be effective in altering cloud structure and size and in converting supercooled liquid water to ice particles. The amount of precipitation due to seeding is difficult to quantify. Cloud seeding may also suppress precipitation.

A key challenge is in discerning how much precipitation would have occurred had clouds not been seeded. Overall, there is general expectation that winter cloud seeding over mountains will produce snow, expressed by professional organizations. There is statistical evidence for seasonal precipitation increases of about 10% with winter seeding.

The US government through its National Center for Atmospheric Research has analyzed seeded and unseeded clouds to understand the differences between them, and has conducted seeding research in other countries.

Clouds were seeded during the 2008 Summer Olympics in Beijing using rockets, so that there would be no rain during the opening and closing ceremonies. although others dispute their claims of success.

History of Cloud seeding

Vincent Schaefer (1906–1993) discovered the principle of cloud seeding in July 1946 through a series of serendipitous events. Following ideas generated between himself and Nobel laureate Irving 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., 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 General Electric's Schenectady Research Lab. He was dismayed to find that the deep freezer was not cold enough to produce a "cloud" using breath air. He decided to move the process along by adding a chunk of dry ice just to lower the temperature of his experimental chamber. To his astonishment, as soon as he breathed into the deep freezer, a bluish haze was noted, followed by an eye-popping display of millions of microscopic ice crystals, reflecting the strong light rays from the lamp illuminating a cross-section of the chamber. He instantly realized that he had discovered a way to change supercooled water into ice crystals. The experiment was easily replicated and he explored the temperature gradient to establish the −40˚Climit for liquid water.

Within the month, Schaefer's colleague, the noted atmospheric scientist Dr. Bernard Vonnegut (brother of novelist Kurt Vonnegut) is credited with discovering another method for "seeding" supercooled cloud water. Vonnegut accomplished his discovery at the desk, looking up information in a basic chemistry text and then tinkering with silver and iodide chemicals to produce silver iodide. Together with Dr. Vonnegut, Professor Henry Chessin, SUNY Albany, a crystallographer, co-authored a publication in Science Magazine [17] and received a patent in 1975. Both methods were adopted for use in cloud seeding during 1946 while working for the General Electric Corporation in the state of New York. Schaefer's altered a cloud's heat budget, Vonnegut's altered formative crystal structure – an ingenious property 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.) The first attempt to modify natural clouds in the field through "cloud seeding" began during a flight that began in upstate New York on 13 November 1946. Schaefer was able to cause snow to fall near Mount Greylock in western Massachusetts, after he dumped six pounds of dry ice into the target cloud from a plane after a 60-mile easterly chase from the Schenectady County Airport.

Dry ice and silver iodide agents are effective in changing the physical chemistry of supercooled clouds, thus useful in augmentation of winter snowfall over mountains and under certain conditions, and lightning and hail suppression. While not a new technique, hygroscopic seeding for enhancement of rainfall in warm clouds is enjoying a revival, based on some positive indications from research in South Africa, Mexico, and elsewhere. The hygroscopic material most commonly used is salt. It is postulated that hygroscopic seeding causes the droplet size spectrum in clouds to become more maritime (bigger drops) and less continental, stimulating rainfall through coalescence. From March 1967 until July 1972, the U.S. military's Operation Popeye cloud-seeded silver iodide to extend the monsoon season over North Vietnam, specifically the Ho Chi Minh Trail. The operation resulted in the targeted areas seeing an extension of the monsoon period an average of 30 to 45 days. The 54th Weather Reconnaissance Squadron carried out the operation to "make mud, not war.

In 1969 at the Woodstock Festival, various people claimed to have witnessed clouds being seeded by the U.S. military. This was said to be the cause of the rain which lasted throughout most of the festival.

Cloud seeding In United States

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 snowfall. Eleven western states and one Canadian province (Alberta) have ongoing weather modification operational programs . 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.

A number of commercial companies, such as Aero Systems Incorporated , Atmospherics Incorporated , North American Weather Consultants , Weather Modification Incorporated , Weather Enhancement Technologies International , Seeding Operations and Atmospheric Research (SOAR) , offer weather modification services centered on cloud seeding. The USAF proposed its use on the battlefield in 1996, although the U.S. signed an international treaty in 1978 banning the use of weather modification for hostile purposes.

During the sixties, Irving P. Krick & Associates operated a successful cloud seeding operation in the area around Calgary, Alberta. This utilized both aircraft and ground-based generators that pumped silver iodide into the atmosphere in an attempt to reduce the threat of hail damage. Ralph Langeman, Lynn Garrison, and Stan McLeod, all ex-members of the RCAF's 403 Squadron, attending the University of Alberta, spent their summers flying hail suppression. A number of surplus Harvard aircraft were fitted with racks under each wing containing 32 railroad fuzees that were impregnated with silver iodide. These could be ignited individually or all at once, depending upon the threat. In coordination with ground units, the aircraft would lay a plume of silver iodide in front of approaching cumulo-nimbus clouds with noticeable effect. Large, active CBs were reduced to nothing. Heavy hail storms were reduced in intensity. This effective program was funded by farmer contributions and government grants.

Cloud seeding In Europe

Cloud seeding was begun in France during the 1950s with the intent of reducing hail damage to crops. The ANELFA project consists of local agencies acting within a non-profit organization. A similar project in Spain is managed by the Consorcio por la Lucha Antigranizo de Aragon. The success of the French program was supported by insurance data; that of the Spanish program in studies conducted by the Spanish Agricultural Ministry.

Soviet military pilots seeded clouds over the Byelorussian SSR after the Chernobyl disaster to remove radioactive particles from clouds heading toward Moscow. 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. Rain drenched the summit anyway. In Moscow, the Russian Airforce tried seeding clouds with bags of cement on June 17, 2008. One of the bags did not pulverize and went through the roof of a house. In October 2009, the Mayor of Moscow promised a "winter without snow" for the city after revealing efforts by the Russian Air Force to seed the clouds upwind from Moscow throughout the winter.

Cloud seeding In Asia

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. About 24 countries currently practice weather modification operationally. China used cloud seeding in Beijing just before the 2008 Olympic Games in order to clear the air of pollution, but there are disputes regarding the Chinese claims. In February 2009, China also blasted iodide sticks over Beijing to artificially induce snowfall after four months of drought, and blasted iodide sticks over other areas of northern China to increase snowfall. The snowfall in Beijing lasted for approximately three days and led to the closure of 12 main roads around Beijing. At the end of October 2009 Beijing claimed it had its earliest snowfall since 1987 due to cloud seeding.

In Southeast Asia, open burning produces haze that pollutes the regional environment. Cloud-seeding has been used to improve the air quality by encouraging rainfall. In India, cloud seeding operations were conducted during the years 2003 and 2004 through U.S. based Weather Modification Inc. in state of Maharashtra. In 2008, there are plans for 12 districts of state of Andhra Pradesh.