plein-air [Fr.,=open-air], term used for paintings or drawings made directly from nature and infused with a feeling of the open air. Painting outdoors is a relatively recent practice; the impressionists and the painters of the Barbizon school made plein-air painting an important dimension of their landscape work.

lock, air: see air lock.

liquid air, ordinary air that has been liquefied by compression and cooling to extremely low temperatures (see liquefaction). Its commercial preparation involves purification by washing to remove soluble impurities and by passage over calcium oxide (lime) to remove the carbon dioxide; compression, under a pressure of 200 atmospheres, or about 3,000 lb per sq in.; cooling, by passage through pipes immersed in cold water; treatment with sodium hydroxide to remove excess water; and rapid expansion, the expanding air passing back over the pipe from which it has just escaped absorbing so much heat that the air remaining in the pipe becomes liquid. Freshly liquefied air consists of 78.1% nitrogen, 21.0% oxygen, 0.9% argon, and very small amounts of rare gases and hydrogen in solution. Its boiling point is approximately -195°C;. Because of fractional evaporation, its oxygen concentration and its boiling point increase with time. It must be kept in a specially designed container, the Dewar flask, because at ordinary temperatures it absorbs heat rapidly and reverts to the gaseous state. Liquid air is used commercially for freezing other substances and especially as an intermediate step in the production of nitrogen, oxygen, and argon and the other inert gases. As the temperature of liquid air rises, the nitrogen evaporates first at -195.8°C;, the argon next at -185.7°C;, and the oxygen last at -183°C;. See low-temperature physics.

compressed air, air whose volume has been decreased by the application of pressure. Air is compressed by various devices, including the simple hand pump and the reciprocating, rotary, centrifugal, and axial-flow compressors. Compressed air exerts an expansive force that can be controlled and used in various devices including tires, air brakes, caissons, and diving suits. As a source of power it is used to operate pneumatic tools, e.g., pneumatic hammers and drills and spraying equipment. It is widely employed for cleaning dust and dirt out of mechanical equipment. It is used also in mining, tunneling, and the manufacture of explosives, since it is not a fire hazard. Compressed air is in readily available supply and is easily stored and transported.

air-cushion vehicle (ACV), craft designed to travel close to but above ground or water. It is also called a ground-effect machine or Hovercraft. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect). Others are supported by fans that force air down under the vehicle to create lift. In a plenum chamber vehicle the rate of leakage of this air from underneath the vehicle is reduced by placing a skirt around the lower edge of the craft. In an annular jet vehicle the rate of leakage is reduced by directing the air downward and inward from the outer edges of the vehicle. Air propellers, water propellers, or water jets usually provide forward propulsion.

Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles and use much less power than helicopters of the same weight. Air-cushion suspension has also been applied to other forms of transportation, in particular trains, such as the French Aerotrain and the British Hovertrain. A relatively smooth land or water surface, however, is a necessity; most of these vehicles cannot clear waves higher than 3 to 51/2 ft (1-1.67 m).

The first recorded design for an air-cushion vehicle was put forward by Emmanual Swedenborg, a Swedish designer and philosopher, in 1716. The project was short-lived and a craft was never built, for Swedenborg soon realized that to operate such a machine required a source of energy far greater than any available at that time. In the mid-1870s, the British engineer Sir John Thornycroft built a number of model craft to check the air-cushion effects and even filed patents involving air-lubricated hulls, although the technology required to implement the concept did not yet exist. From this time both American and European engineers continued work on the problems of designing a practical craft.

In the early 1950s the British inventor Christopher Cockerell began to experiment with such vehicles, and in 1955 he obtained a patent for a vehicle that was "neither an airplane, nor a boat, nor a wheeled land craft." He had a boat builder produce a two-foot prototype, which he demonstrated to the military in 1956 without arousing interest. Cockerell persevered, and in 1959 a commercially built one-person Hovercraft crossed the English Channel. In 1962 a British vehicle became the first to go into active service on a 19-mi (31-km) ferry run. The maximum size of air-cushion vehicles is now over 100 tons; some of them travel at over 100 mi (160 km) per hr. Although air-cushion vehicles of several thousand tons have been under development for many years, it is in small vehicles, usually called flarecraft, that the greatest current potential market exists; current flarecraft can carry one to eight people at 150 mi (240 km) per hr.

air, law of the, in the broadest sense, all law connected with the use of the air, including radio and satellite transmissions; more commonly, it refers to laws concerning civil aviation. The development of large-scale air transport after World War I brought with it the need for regulation, both national and international. In 1919 a meeting of the victorious nations of World War I resulted in the International Convention for Air Navigation, commonly called the Paris Convention. The convention recognized the sovereignty of each state over its own air space without prejudice to innocent passage by aircraft of another state. It also provided that each aircraft (like each ship) must have a registered nationality. Rules were adopted as to the airworthiness of aircraft and the certification and licensing of pilots. The United States was among the 33 signatory nations but did not ratify the convention; nevertheless U.S. air laws were modeled on it. The Warsaw Convention on International Carriage by Air (1929) determined that the owner or operator of the carrier is liable for any injury, death, or property damage. World War II emphasized the need for sounder regulation of international air transport and for uniformity of equipment, laws, and regulation. An international civil aviation conference of 52 nations, not including the USSR, met in Chicago in 1944. There was much discussion of the "five freedoms of the air"—freedom to fly across the territory of a state without landing; freedom to land for nontraffic purposes; the right to disembark in a foreign country traffic from the country of registry of the aircraft; the right to pick up in a foreign country traffic destined for the country of registry; and the right to carry traffic between two foreign countries. The first two were accepted, but the fifth was bitterly opposed; only the first two were included in the International Air Services Transit Agreement, which was generally signed. The convention set up a provisional body that in 1947 became the International Civil Aviation Organization, affiliated with the United Nations. There have been several general conferences since the Chicago Convention and many bilateral agreements have been concluded by parties to it. In the United States, deregulation of the airline industry in the late 1970s led to the eventual dissolution of the Civil Aeronautics Board. Since 1984, U.S. air laws have been administered by the Federal Aviation Administration, a division of the Department of Transportation. The successful launching of satellites necessitated the development of space law.

air transportation: see aviation.

air traffic control, the system by which airplanes are safely routed into and out of major airports. Air traffic control in the United States is centered in a number of regional control centers that route airplanes along established airways to airport traffic control centers. There Instrument Landing Systems and Microwave Landing Systems enable planes to land safely in almost any weather conditions. Air traffic controllers, who are responsible for maintaining safe distances between planes, are employees of the U.S. Dept. of Transportation. Air traffic control is made possible by special transponders installed in every commercial and many private aircraft, which automatically transmit information on a plane's altitude and speed to the ground controller. The distortion that often affects voice transmissions can be eliminated by the use of cockpit datalinks; collision avoidance systems provide further safety margins. Knowing the course, speed, and altitude of every plane in the sector, the controller can contact each in turn to give landing or course instructions. Modern air traffic control has contributed to making air travel far safer than highway travel, and on a passenger-mile basis safer even than rail travel.

air pollution, contamination of the air by noxious gases and minute particles of solid and liquid matter (particulates) in concentrations that endanger health. The major sources of air pollution are transportation engines, power and heat generation, industrial processes, and the burning of solid waste.

Sources of Air Pollution

The combustion of gasoline and other hydrocarbon fuels in automobiles, trucks, and jet airplanes produces several primary pollutants: nitrogen oxides, gaseous hydrocarbons, and carbon monoxide, as well as large quantities of particulates, chiefly lead. In the presence of sunlight, nitrogen oxides combine with hydrocarbons to form a secondary class of pollutants, the photochemical oxidants, among them ozone and the eye-stinging peroxyacetylnitrate (PAN). Nitrogen oxides also react with oxygen in the air to form nitrogen dioxide, a foul-smelling brown gas. In urban areas like Los Angeles where transportation is the main cause of air pollution, nitrogen dioxide tints the air, blending with other contaminants and the atmospheric water vapor to produce brown smog. Although the use of catalytic converters has reduced smog-producing compounds in motor vehicle exhaust emissions, recent studies have shown that in so doing the converters produce nitrous oxide, which contributes substantially to global warming.

In cities, air may be severely polluted not only by transportation but also by the burning of fossil fuels (oil and coal) in generating stations, factories, office buildings, and homes and by the incineration of garbage. The massive combustion produces tons of ash, soot, and other particulates responsible for the gray smog of cities like New York and Chicago, along with enormous quantities of sulfur oxides (which also may be result from burning coal and oil). These oxides rust iron, damage building stone, decompose nylon, tarnish silver, and kill plants. Air pollution from cities also affects rural areas for many miles downwind.

Every industrial process exhibits its own pattern of air pollution. Petroleum refineries are responsible for extensive hydrocarbon and particulate pollution. Iron and steel mills, metal smelters, pulp and paper mills, chemical plants, cement and asphalt plants—all discharge vast amounts of various particulates. Uninsulated high-voltage power lines ionize the adjacent air, forming ozone and other hazardous pollutants. Airborne pollutants from other sources include insecticides, herbicides, radioactive fallout, and dust from fertilizers, mining operations, and livestock feedlots.

Effects on Health and the Environment

Like photochemical pollutants, sulfur oxides contribute to the incidence of respiratory diseases. Acid rain, a form of precipitation that contains high levels of sulfuric or nitric acids, can contaminate drinking water and vegetation, damage aquatic life, and erode buildings. When a weather condition known as a temperature inversion prevents dispersal of smog, inhabitants of the area, especially children and the elderly and chronically ill, are warned to stay indoors and avoid physical stress. The dramatic and debilitating effects of severe air pollution episodes in cities throughout the world—such as the London smog of 1952 that resulted in 4,000 deaths—have alerted governments to the necessity for crisis procedures. Even everyday levels of air pollution may insidiously affect health and behavior. Indoor air pollution is a problem in developed countries, where efficient insulation keeps pollutants inside the structure. In less developed nations, the lack of running water and indoor sanitation can encourage respiratory infections. Carbon monoxide, for example, by driving oxygen out of the bloodstream, causes apathy, fatigue, headache, disorientation, and decreased muscular coordination and visual acuity.

Air pollution may possibly harm populations in ways so subtle or slow that they have not yet been detected. For that reason research is now under way to assess the long-term effects of chronic exposure to low levels of air pollution—what most people experience—as well as to determine how air pollutants interact with one another in the body and with physical factors such as nutrition, stress, alcohol, cigarette smoking, and common medicines. Another subject of investigation is the relation of air pollution to cancer, birth defects, and genetic mutations.

A recently discovered result of air pollution are seasonal "holes" in the ozone layer in the atmosphere above Antarctica and the Arctic, coupled with growing evidence of global ozone depletion. This can increase the amount of ultraviolet radiation reaching the earth, where it damages crops and plants and can lead to skin cancer and cataracts. This depletion has been caused largely by the emission of chlorofluorocarbons (CFCs) from refrigerators, air conditioners, and aerosols. The Montreal Protocol of 1987 required that developed nations signing the accord not exceed 1986 CFC levels. Several more meetings were held from 1990 to 1997 to adopt agreements to accelerate the phasing out of ozone-depleting substances.

Solutions to Air Pollution

To combat pollution in the United States, the Clean Air Act Amendments of 1970 gave the Environmental Protection Agency (EPA) the authority to establish and enforce air pollution standards and to set emission standards for new factories and extremely hazardous industrial pollutants. The states were required to meet "ambient air quality standards" by regulating the emissions of various pollutants from existing stationary sources, such as power plants and incinerators, in part by the installation of smokestack scrubbers, electrostatic precipitators, and other filters. Auto manufacturers were mandated to install exhaust controls or develop less polluting engines. The Clean Air Act, as amended in 1977, authorized the EPA to impose stricter pollution standards and higher penalties for failure to comply with air quality standards.

In 1990 when the act was reauthorized it required most cities to meet existing smog reduction regulations by the year 2005. The 1990 amendments also expanded the scope and strength of the regulations for controlling industrial pollution. The result has been limited progress in reducing the quantities of sulfur dioxide, carbon monoxide, nitrogen oxide, ozone, particulate matter, and lead in the air. The EPA also regulated hazardous air pollutants, which in 1992 included mercury, beryllium, asbestos, vinylchloride, benzene, radioactive substances, and inorganic arsenic.

The most satisfactory long-term solutions to air pollution may well be the elimination of fossil fuels and the ultimate replacement of the internal-combustion engine. To these ends efforts have begun in the United States, Japan, and Europe to develop alternative energy sources (see energy, sources of), as well as different kinds of transportation engines, perhaps powered by electricity or steam. A system of pollution allowances based on trading emission rights has been established in the United States in an attempt to use the free market to reward pollution reductions, and the international sale of surplus emission rights is permitted under the Kyoto Protocol (see below). Other proposed solutions include raising electricity and gasoline rates to better reflect environmental costs and to discourage waste and inefficiency, and mechanical controls on coal-fired utility plants.

In 1992, 150 nations signed a treaty on global warming at the UN-sponsored summit on the environment in Rio de Janeiro. A UN Conference on Climate Change, held in Kyoto, Japan, in 1997, produced an international agreement to combat global warming by sharply reducing emissions of industrial gases. Although the United States abandoned the treaty in 2001, saying it was counter to U.S. interests, most other nations agreed that year on the details necessary to make the protocol a binding international treaty, and the necessary ratifications brought the treaty into force in 2005.

See environmentalism; pollution.

air plant: see epiphyte.

air navigation, science and technology of determining the position of an aircraft with respect to the surface of the earth and accurately maintaining a desired course (see navigation).

Visual and Instrument Flight

The simplest and least sophisticated way to keep track of position, course, and speed is to use pilotage, a method in which landmarks are noted and compared with an aeronautical chart. Whether these landmarks are observed visually or on radar, this technique of air navigation is usually called flying under visual flight regulations (VFR). These establish the minimum weather conditions under which pilotage is permissible.

Pilotage is not satisfactory for long trips, especially over water or terrain lacking distinctive features. In these cases, or when weather conditions do not permit navigation by visual reference, planes must fly according to instrument flight regulations (IFR), which require that the aircraft be equipped with the necessary position-finding instruments and that the pilot be trained in operating those instruments. Also required under IFR is the filing of a flight plan with air traffic control authorities at the departure point. The aircraft is then cleared for a given course and a given altitude. Air traffic controllers monitor the craft until it reaches its destination.

Aircraft Instruments

Light aircraft, flown by pilotage, typically have a simple set of navigational instruments, including an airspeed indicator (see pitot static system), an aneroid altimeter, and a magnetic compass. For supersonic and hypersonic aircraft the airspeed indicator is altered to show the airspeed as a Mach number, which is the ratio of the speed of an aircraft to the speed of sound. Advanced aircraft also use electronic systems to give the pilot highly accurate positional information for use during landing. The Instrument Landing System enables an airplane to navigate through clouds or darkness to an airport's runway; the Microwave Landing System, installed in U.S. airports beginning in 1988, is capable of landing the plane automatically, although the pilot always has the option of overriding manually.

Other navigational aids include the radio altimeter, a radar device that indicates the distance of the plane from the ground; the ground-speed indicator, which operates by measuring the Doppler shift in a radio wave reflected from the ground; and, in commercial airliners, the flight management computer, which can display altitude, speed, course, wind conditions, and route information, as well as monitor the airplane's progress through the airway. Other similar systems use inertial devices such as free-swinging pendulums and gyroscopes as references in determining position. These automated and semiautomated procedures free the pilot from many of the activities previously necessary for navigation and thus allow the pilot to concentrate on actually flying the aircraft. Another device which is useful in this way is the automatic pilot, which interprets data on direction, speed, attitude, and altitude to maintain an aircraft in straight, level flight on a given course at a given speed.

Airways and Radio Ranges

Basic to air traffic control are special air routes called airways. Airways are defined on charts and are provided with radio ranges, devices that allow the pilot whose craft has a suitable receiver to determine the plane's bearing and distance from a fixed location. The most common beacon is a very high frequency omnidirectional radio beacon, which emits a signal that varies according to the direction in which it is transmitted. Using a special receiver, an air navigator can obtain an accurate bearing on the transmitter and, using distance-measuring equipment (DME), distance from it as well.

The system of radio ranges around the United States is often called the VORTAC system. For long distances other electronic navigation systems have been developed: Omega, accurate to about two miles (3 km); Loran-C, accurate to within .25 mi (.4 km) but available only in the United States; and the Global Positioning System (GPS), a network of 24 satellites that is accurate to within a few yards and is making radio ranging obsolete.

air mass, large body of air within the earth's atmosphere in which temperature and humidity, although varying at different heights, remain similar throughout the body at any one height. Air masses form over parts of the earth's surface called source regions, which are large bodies of water or landmasses with relatively uniform topography, often ranging hundreds of thousands of square miles in area. When a body of air remains over a source region for days or weeks, it reaches an equilibrium with the surface. Radiation and convection exchanges between the surface and the air determines the air mass characteristics. Air masses formed over oceans generally contain more moisture than continental ones; air masses formed in polar latitudes are colder than those from the tropics. As an air mass moves away from its source region, it brings its particular weather conditions to areas over which it travels. At the same time, its characteristic properties are slowly modified by exposure to new environments. The boundaries between air masses, called fronts, are, typically, zones of rapid transition from cold to warm or from dry to moist air. Turbulence at the boundary often breeds low-pressure storms.

air lock, compartment connecting two different environments, usually at different pressures, that enables personnel to transfer from one environment to the other. Space capsules have air locks to enable astronauts to move between the pressurized cabin and the near vacuum of space. A more common example is the air lock between the outer atmosphere and the working chamber of a caisson. By its means access can be gained to the working chamber without loss of pressure. It is also used at the head of tunnel excavations underwater. There is a door at each end. When the outer door of the air lock is opened, men or material may be admitted into the compartment. After the outer door is closed, compressed air is admitted to raise pressure in the air lock to the level of the pressure in the working chamber, and the inner door can be opened. The reverse of this procedure takes place on leaving the working chamber. Great care must be exercised in passing workmen through an air lock, so that the change of atmospheric pressure takes place gradually. Too sudden a change of pressure may cause decompression sickness.

air forces, those portions of a nation's military organization employing heavier-than-air aircraft for reconnaissance, support of ground troops, aerial combat, and bombing of enemy lines of communication and targets of industrial and military importance.

Early Military Use of Aircraft

The history of air forces begins with the use of balloons by French forces in Italy in 1859 and by Union forces in the U.S. Civil War. Balloons thereafter proved useful as a means of observation, but air forces in the modern sense date from World War I, when the offensive capabilities of the airplane were first demonstrated. The somewhat tentative use of scout planes at the beginning of the war was followed by the creation of small forces of fighter planes that engaged in aerial combat and bombing raids. Although Germany took the lead in air strategy, the Allies soon closed the gap. Indeed, throughout World War I, such development and counterdevelopment accounted for the rapid advance of military aeronautics. The use of aircraft for reconnaissance, which made control of the skies important to military operations, resulted in the development of aerial combat, which led to formation flying, dogfights, and the bombing of enemy lines of communication and munitions depots.

Evolution of the Modern Air Force

As the effectiveness of aircraft as a tactical weapon increased, consideration was given to the establishment of air forces independent of a nation's ground forces. After the war a few allied strategists, including Giulio Douhet and others, such as Gen. William Mitchell of the United States, fought for the intensive development of airpower and pleaded for large air forces, arguing that future wars would be won by strategic bombardment of an enemy's industrial centers, thereby destroying the economic means of conducting a war. In the 1920s and 1930s the French, British, and Italians used airplanes for reconnaissance and strategic bombing in colonial wars in Africa, the Middle East, and India. These experiences, combined with the rapid and extensive advances in aeronautical technique that followed World War I, resulted in a much broader application of airpower in World War II.

During World War II

During the 1930s, Germany devoted great efforts to air armament and the early days of World War II seemed to uphold Hitler's boasts of the effectiveness of the Luftwaffe (air force) under Hermann Goering. This was especially true of tactical air support for the ground troops, which was a crucial part of Germany's successful form of mechanized warfare, the blitzkrieg. The first great air battle in history was the Battle of Britain, in which the British Royal Air Force defeated the German Luftwaffe (1940) over Britain. In the Pacific, Japan entered the war with a stunning air attack launched from aircraft carriers on Pearl Harbor.

The subsequent development of airpower greatly altered the nature of warfare, and the use of aircraft over both land and sea played a major role in nearly all of the important engagements of World War II. Airplanes were used for strategic and tactical bombing, attacking of naval and merchant ships, transportation of personnel and cargo, mining of harbors and shipping lanes, antisubmarine patrols, photographic reconnaissance, and support of ground, naval, and amphibious operations. Throughout the war, the British and U.S. air forces conducted massive strategic bombing of Germany, but postwar bombing surveys showed it was not decisive in the Allied victory. In the Pacific, U.S. carrier-based aircraft by the end of 1944 had destroyed the Japanese fleet and air force. In the last months of the war, Japan itself was subjected to intense strategic bombardment, ending with the dropping of atomic bombs on Hiroshima and Nagasaki. Other major developments of World War II included improved techniques of flying and aircraft design and an accumulation of geographical and technological knowledge essential to modern aviation. By the end of the war, the importance of airpower was accepted by all.

Postwar Use of Airpower

Since World War II, the increased role of helicopters has been a major development, allowing for increased air support of ground troops. In the Korean War air forces of the United Nations Command effectively enveloped the North Korean army and later cut supply arteries to Chinese Communist troops so that an armistice could be negotiated. Similar ground-air tactics were employed by the United States in Vietnam, while the North Vietnamese made effective use of Soviet-built ground-to-air missiles and tactical air support. The Persian Gulf War, which saw the introduction of stealth fighter planes (see stealth technology), was the first unambiguously decisive airpower victory in warfare, but even there the conflict was only ended after the ground forces attacked. Airpower was also used fairly effectively, although with less than immediate results, by the North Atlantic Treaty Organization to force the capitulation of the Yugoslavia during the Kosovo crisis in 1999. Fighting in Afghanistan (2001) saw precision-guided smart weapons become the predominant ordnance, but these were often targeted most effectively when the air forces worked in conjunction with spotters on the ground.

The development of nuclear weapons, jet propulsion, the guided missile, and satellites has widened the concept of airpower and the role of air forces. The U.S. Air Force (see Air Force, United States Department of the) now refers to aerospace power (instead of airpower) and considers space a crucial military theater. Air forces also have come to assume a primary strategic role in deterring war by employing in readiness a second-strike retaliatory force (see nuclear strategy) consisting of both aircraft and missiles. In the United States this mission was carried by the Strategic Air Command, which has been replaced by the interservice Strategic Command.

air embolism: see embolus.

air conditioning, mechanical process for controlling the humidity, temperature, cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and regulated to maintain the temperature-humidity ratio that is most comfortable and healthful. In the process, dust, soot, and pollen are filtered out, and the air may be sterilized, as is sometimes done in hospitals and public places.

Most air-conditioning units operate by ducting air across the colder, heat-absorbing side of a refrigeration apparatus and directing it back into the air-conditioned space (see refrigeration). The refrigeration apparatus is controlled by some form of thermostat. In water-cooled air-conditioning units, the waste heat is carried away by a flow of water. For recirculation in water-cooled units, a cooling tower is used. This apparatus maintains a constant level of water in the system and replaces water lost by evaporation. The development of small self-contained systems has greatly expanded the use of air conditioning in homes. A portable or window-mounted air conditioner is usually adequate for one room.

Often domestic heating systems are converted to provide complete air conditioning for a home. Usually, this is done by combining a heating device and a cooling device in one unit. In regions where the outside temperature does not fall too low, heat pumps have become popular. A heat pump is a reversible device that does mechanical work to extract heat from a cooler place and deliver heat to a warmer place. The heat delivered to the warmer place is, approximately, the sum of the original heat and the work done. Greater temperature differences between the warm and cold regions require greater amounts of work. In warm weather the heat pump acts like a traditional air conditioner, removing heat from the indoors and delivering heat to the outdoors. In cool weather, it removes heat from the outdoors and delivers heat to the indoors. The efficiency of a heat pump as a heating device depends upon the outdoor temperature. At 50°F; (10°C;) a heat pump is more efficient than a traditional heating system. Below 32°F; (0°C;) it is less efficient and requires augmenting with conventional heaters.

In the construction of office buildings in the United States, air-conditioning systems are commonly included as integral parts of the structure. First used c.1900 in the textile industry, air conditioning found little use outside factories until the late 1920s. It is of great importance in chemical, pharmaceutical, and other industrial plants where air contamination, humidity, and temperature affect manufacturing processes.

air cargo: see aviation.

air brake: see brake.

air bladder, in fish: see swim bladder.

air bag: see automobile.

air: see atmosphere; liquid air; ventilation.

Wright-Patterson Air Force Base, U.S. military installation, 8,023 acres (3,247 hectares), W Ohio, NE of Dayton; est. 1917. One of the largest airport installations in the world, it is the air force's main research and development base, and the headquarters of the Air Force Logistics Command (national center for defense activities). The Aerospace Medical Laboratory and the Air Force Museum are also on the base.

Vandenberg Air Force Base, U.S. military installation, 3,456 acres (1,399 hectares), SW Calif., near Lompoc; chief Pacific coast launch site for military satellites. Commercial satellites are launched from the base as well, and it is also a site for test-firing intercontinental ballistic missiles.

United States Air Force Academy, at Colorado Springs, Colo.; for training young men and women to be officers in the U.S. air force; authorized in 1954 by Congress. Temporary quarters were opened at the former Lowry Air Force Base at Denver in 1955, and the permanent campus opened in 1958. The academy began admitting women in 1975 and they now make up some 13% of the cadet corps.

Candidates must be between 17 and 22 years old and meet special physical and educational qualifications. An applicant must obtain a nomination to be considered for an appointment to the academy. The sources of nomination are the President of the United States; the Vice President; U.S. Senators and Representatives; and the representatives of the District of Columbia and the U.S. territories. Special appointment categories include children of deceased and disabled veterans or of career military personnel, foreign students, regular U.S. air force, U.S. air force reserve, honor graduates of military and naval schools and ROTC, and children of Medal of Honor recipients. Approximately 4,000 cadets are enrolled in the four-year course of instruction that leads to a bachelor's degree and a position of second lieutenant. The curriculum includes standard academic subjects as well as military training, flight instruction, and athletics. Cadets receive free tuition and room and board and a monthly allotment to pay for supplies, clothing, and personal expenses.

Strategic Air Command (SAC), former command of the U.S. air force (see Air Force, United States Department of the) charged with organizing, training, equipping, administering, and preparing strategic air forces for combat; it was headquartered at Offutt Air Force Base. From 1946 to 1992, SAC controlled most U.S. strategic nuclear weapons. Its bombers and guided missiles played a key role in the nuclear strategy of the cold war.

SAC was abolished in 1992 as part of the reorganization of the Department of Defense, and the Strategic Command was created. The Space Command was merged into the Strategic Command in 2002. The interservice Strategic Command, also based at Offutt, now coordinates nuclear plans for both the U.S. air force and navy and oversees all U.S. nuclear forces, conducts reconnaissance for strategic targets, oversees the radar and satellites that detect ballistic missile launches, and protects military computers and networks.

Offutt Air Force Base, U.S. military installation, 1,907 acres (772 hectares), E Neb., S of Omaha; est. 1896 as Fort Crook, an army base. Converted to an airbase in the early 1900s and renamed in 1924, it is the headquarters of the Strategic Command, the successor to the Strategic Air Command. The Nebraska Museum of Aerospace History is on the base.

Malmstrom Air Force Base, U.S. military installation, 3,573 acres (1,446 hectares), W central Mont., E of Great Falls; est. 1942. During World War II, it was the takeoff point for Soviet-bound lend-lease materiel; after the war it was a training base for crews in the Berlin Airlift. The now-dissolved Strategic Air Command (SAC) assumed command in 1954; SAC's first Minuteman missile wing was established there in 1961. The missile complex adjoining the base is one of the largest in the world.

Langley Air Force Base, U.S. military installation, 3,195 acres (1,293 hectares), SE Va., N of Hampton; est. 1917 and named for aviation pioneer Samuel P. Langley. The facility, the oldest continuously active air force base in the United States, is the headquarters of the Air Combat Command and has air-defense missile units. NASA's Langley Research Center is adjacent.

Lackland Air Force Base, U.S. military installation, c.6,835 acres (2,766 hectares), S Tex., W of San Antonio; est. 1941. It is a major air force training center.

Edwards Air Force Base, U.S. military installation, 301,000 acres (121,805 hectares), S Calif., NE of Lancaster; est. 1933. It is one of the largest air force bases in the United States and has the world's longest runway. The base houses the Air Force Flight Test Center, which researches and develops aerospace weapons and rocket-propulsion systems, and the National Aeronautical and Space Administration's Flight Research Center. The base is also the proving ground for military aircraft. It has been the landing point for a number of space shuttle missions; on Sept. 5, 1983, the Challenger made the first nighttime landing of a spacecraft there.

Andrews Air Force Base, U.S. military installation, 4,279 acres (1,732 hectares), central Md., est. 1943. It is the chief military airport of Washington, D.C., as well as the headquarters for the air force's high-priority airlift command.

Air Force, United States Department of the, military department within the U.S. Dept. of Defense (see Defense, United States Department of). The Air Force traces its roots to the founding of the Aeronautical Division of the Army Signal Corps (1907), variously renamed before becoming a separate service under the National Security Act of 1947. In 1949 the National Security Act Amendments made the Air Force a military department within the newly organized Department of Defense. The chain of command goes directly from the President to the Secretary of Defense to the Secretary of the Air Force. The Air Force played an important role in World War I (see William Mitchell; Eddie Rickenbacker) and World War II (see H. H. Arnold; atomic bomb; James Harold Doolittle). After World War II, the Air Force quickly grew in importance, becoming the cornerstone of President Eisenhower's defense policy. The Air Force played a major part in the Korean War, the Vietnam War, and numerous cold war confrontations (see Berlin airlift, Cuban Missile Crisis). Its control of long-range, land-based guided missiles and the strategic bombers gives the Air Force monopolies on two major components of U.S. nuclear strategy. It has the leading role in the military exploration of space and uses aircraft and satellites to collect photo, video, and signal intelligence.

Colourless, toxic gas (NO), formed from nitrogen and oxygen by the action of electric sparks or high temperatures or, more conveniently, by the action of dilute nitric acid on copper or mercury. First prepared circa 1620 by Jan B. Helmont, it was first studied in 1772 by Joseph Priestley, who called it “nitrous air.” An industrial procedure for the manufacture of hydroxylamine is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst. The formation of nitric oxide from nitric acid and mercury is applied in a volumetric method of analysis for nitric acid or its salts. The gas is synthesized via enzyme-catalyzed reactions in humans and other animals, where it serves as a signaling molecule. Among its numerous biological roles, it causes dilation of blood vessels and as such is an important regulator of blood pressure. Nitric oxide is one of the components of air pollution generated by internal-combustion engines.

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Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula CO₂. Constituting about 0.03percnt of air by volume, it is produced when carbon-containing materials burn completely, and it is a product of fermentation and animal respiration. Plants use CO₂ in photosynthesis to make carbohydrates. CO₂ in Earth's atmosphere keeps some of the Sun's energy from radiating back into space (see greenhouse effect). In water, CO₂ forms a solution of a weak acid, carbonic acid (H₂CO₃). The reaction of CO₂ and ammonia is the first step in synthesizing urea. An important industrial material, CO₂ is recovered from sources including flue gases, limekilns, and the process that prepares hydrogen for synthesis of ammonia. It is used as a refrigerant, a chemical intermediate, and an inert atmosphere; in fire extinguishers, foam rubber and plastics, carbonated beverages (see carbonation), and aerosol sprays; in water treatment, welding, and cloud seeding; and for promoting plant growth in greenhouses. Under pressure it becomes a liquid, the form most often used in industry. If the liquid is allowed to expand, it cools and partially freezes to the solid form, dry ice.

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or hovercraft

Vehicle supported above the surface of land or water by an air cushion, produced by downwardly directed fans, enclosed within a flexible skirt beneath the hull. The concept was first proposed by John Thornycroft in the 1870s, but a working model was not produced until 1955, when Christopher Cockerell solved the problem of keeping the air cushion from escaping from under the vehicle, and formed Hovercraft Ltd. to manufacture prototypes. Problems with skirt design and engine maintenance have restricted the vehicle's commercial application; today hovercraft are used mainly as ferries.

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Control of temperature, humidity, purity, and motion of air in an enclosed space, independent of outside conditions. In a self-contained air-conditioning unit, air is heated in a boiler unit or cooled by being blown across a refrigerant-filled coil and then distributed to a controlled indoor environment. Central air-conditioning in a large building generally consists of a main plant located on the roof or mechanical floor and intermittently spaced air-handling units, or fans that deliver air through ducts to zones within the building. The air then returns to the central air-conditioning machinery through spaces called plenums to be recooled (or reheated) and recirculated. Alternate systems of cooling use chilled water, with water cooled by a refrigerant at a central location and circulated by pumps to units with fans that circulate air locally.

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Military operations conducted by airplanes, helicopters, or other aircraft against aircraft or targets on the ground and in the water. Air warfare did not become important until World War I (1914–18). The British, French, German, Russian, and Italian armed forces had flying units, including biplanes armed with machine guns for “dogfights” with enemy fighter aircraft. Zeppelins and larger airplanes carried out bombing raids. The 1920s and '30s saw the development of the monoplane, the all-metal fuselage, and the aircraft carrier. During World War II (1939–45), the Battle of Britain was the first fought exclusively in the air, the Battle of the Coral Sea was the first between carrier-based aircraft, and the atomic bombings of Hiroshima and Nagasaki were the first use of nuclear-armed bombers. In the jet age, air power has continued to be used in strategic bombing of an enemy's home territory (as in the Vietnam War, 1965–74), destroying enemy air forces (as in the Arab-Israeli wars), attacking and defending carrier-based naval fleets (as in the Falkland Islands War, 1982), and supporting ground forces (as in the Persian Gulf War, 1990–91).

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Development and operation of aircraft. In 1783 the balloon became the first aircraft to carry humans. Production of a successful glider in 1891 and refinement of the internal-combustion engine led to the first successful engine-powered airplane flight by Wilbur and Orville Wright in 1903. World War I accelerated the expansion of aviation, and in the 1920s the first small airlines began carrying mail and passengers. World War II was another period of innovation in aircraft size, speed, and range. In the late 1940s the jet engine made possible the subsequent development of commercial airlines throughout the world. Seealso airship; helicopter; seaplane.

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Site and installations for the takeoff and landing of aircraft. Early airports were open, grass-covered fields, called landing fields, that allowed a pilot to head directly into the wind to aid a plane's lift on takeoff and to decrease its speed on landing. In the 1930s heavier airplanes required paved runway surfaces. Larger planes needed longer runways, which today can reach 15,000 ft (4,500 m) to accommodate the largest jet aircraft. Air traffic is regulated from control towers and regional centres. Passenger and cargo terminals include baggage-movement and passenger-transit operations.

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or barometric pressure

Force per unit area exerted by the air above the surface of the Earth. Standard sea-level pressure, by definition, equals 1 atmosphere (atm), or 29.92 in. (760 mm) of mercury, 14.70 lbs per square in., or 101.35 kilopascals, but pressure varies with elevation and temperature. It is usually measured with a mercury barometer (hence the term barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere above it. It may also be measured using an aneroid barometer, in which the action of atmospheric pressure in bending a metallic surface is made to move a pointer.

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Any plant that grows upon or is attached to another plant or object merely for physical support. Epiphytes are found mostly in the tropics and are also known as air plants because they have no attachment to the ground or other obvious nutrient source. They obtain water and minerals from rain and from debris on the supporting plants. Orchids, ferns, and members of the pineapple family are common tropical epiphytes. Lichens, mosses, liverworts, and algae are epiphytes of temperate regions.

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In meteorology, a large body of air having nearly uniform conditions of temperature and humidity at any given altitude. Such a mass has distinct boundaries and may extend hundreds or thousands of miles horizontally and sometimes as high as the top of the troposphere. An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface long enough to acquire its temperature and moisture properties. The Earth's major air masses all originate in polar or subtropical latitudes. The middle latitudes constitute essentially a zone of modification, interaction, and mixing of the polar and tropical air masses.

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Military organization that has the primary responsibility for conducting air warfare. The air force must gain control of the air, support ground forces (e.g., by attacking enemy ground forces), and accomplish strategic-bombing objectives. Its basic weapons platforms are fighters, bombers, attack aircraft, and early warning and control aircraft. Since the mid-20th century, some air forces have also been responsible for land-based nuclear missiles as well as nuclear-armed bombers. The army and naval branches of a state's armed forces may also operate aircraft.

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Machine for increasing the pressure of a gas by mechanically decreasing its volume. Air is the most frequently compressed gas, but natural gas, oxygen, nitrogen, and other industrially important gases are also frequently compressed. There are three general types of compressors. Positive-displacement compressors are usually of the reciprocating piston type (see piston and cylinder), useful for supplying small amounts of a gas at relatively high pressures. Centrifugal compressors are particularly suited for compressing large volumes of gas to moderate pressures. Axial compressors are used for jet aircraft engines and gas turbines.

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Either of two kinds of braking systems. The first, used by trains, trucks, and buses, operates by a piston driven by compressed air from reservoirs connected to brake cylinders (see piston and cylinder). When air pressure in the brake pipe is reduced, air is automatically admitted into the brake cylinder. The first practical air brake for railroads was invented in the 1860s by George Westinghouse. The second type, used by aircraft and race cars, consists of a flap or surface that can be mechanically projected into the airstream to increase the resistance of the vehicle to air and lower its speed.

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Mixture of gases constituting the earth's atmosphere. Some gases occur in steady concentrations. The most important are molecular nitrogen (N₂), 78percnt by volume, and molecular oxygen (O₂), 21percnt. Small amounts of argon (Ar; 1.9percnt), neon (Ne), helium (He), methane (CH₄), krypton (Kr), hydrogen (H₂), nitrous oxide (N₂O), and xenon (Xe) are also present in almost constant proportions. Other gases occur in variable concentrations: water vapour (H₂O), ozone (O₃), carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Air also contains trace amounts of ammonia and hydrogen sulfide. The variable constituents are important for maintaining life. Water vapour is the source for all forms of precipitation and is an important absorber and emitter of infrared radiation. Carbon dioxide is necessary for photosynthesis and is also an important absorber and emitter of infrared radiation. Ozone in the stratosphere (see ozone layer) is an effective absorber of ultraviolet radiation from the Sun but at ground-level is a corrosive pollutant and a major constituent of smog.

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Institution for the training of commissioned officers for the U.S. Air Force, located in Colorado Springs, Colorado. Created by an act of Congress in 1954, it opened in 1955. Graduates receive a bachelor's degree and a second lieutenant's commission. Most physically qualified graduates go on to Air Force pilot-training schools. Candidates may come from the ranks of the U.S. Army or Air Force, may be children of deceased veterans of the armed forces, or may be nominated by U.S. senators or representatives or by the president or vice president. All applicants must take a competitive entrance examination.

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