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Aperture

In optics, the aperture of an optical system (including a system consisted of a single lens) is a hole or an opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image of the aperture and focal length of an optical system determine the cone angle of a bundle of rays that comes to a focus in the image plane.

An optical system typically has many openings or structures that limit ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that primarily determines the cone of rays that an optical system accepts (see entrance pupil). As a result, it also determines the ray cone angle and brightness at the image point (see exit pupil). The aperture stop generally depends on the object point location; on-axis object points at different object planes may have different aperture stops, and even object points at different lateral locations at the same object plane may have different aperture stops (vignetted). In practice, many object systems are designed to have a single aperture stop at designed working distance and field of view.

In some contexts, especially in photography and astronomy, aperture refers to the opening diameter of the aperture stop through which light can pass. For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 cm aperture. The aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a linear measure (for example, in inches or millimetres) or as the dimensionless ratio between that measure and the focal length. In other photography, it is usually given as a ratio.

A usual expectation is that the term aperture refers to the opening of the aperture stop, but in reality, the term aperture and the aperture stop are mixed in use. Sometimes even stops that are not the aperture stop of an optical system are also called apertures. Contexts need to clarify these terms.

The word aperture is also used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy, for example, a photometric aperture around a star usually corresponds to a circular window around the image of a star within which the light intensity is assumed.

Application
The aperture stop is an important element in most optical designs. Its most obvious feature is that it limits the amount of light that can reach the image/film plane. This can be either unavoidable due to the practical limit of the aperture stop size, or deliberate to prevent saturation of a detector or overexposure of film. In both cases, the size of the aperture stop determines the amount of light admitted by an optical system. The aperture stop also affects other optical system properties:
 * The opening size of the stop is one factor that affects DOF (depth of field). A smaller stop (larger f number) produces a longer DOF because it only allows a smaller angle of the cone of light reaching the image plane so the spread of the image of an object point is reduced. A longer DOF allows objects at a wide range of distances from the viewer to all be in focus at the same time.
 * The stop limits the effect of optical aberrations by limiting light such that the light does not reach edges of optics where aberrations are usually stronger than the optics centers. If the opening of the stop (called the aperture) is too large, then the image will be distorted by stronger aberrations. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger aperture and therefore greater light collecting ability.
 * The stop determines whether the image will be vignetted. Larger stops can cause the light intensity reaching the film or detector to fall off toward the edges of the picture, especially when, for off-axis points, a different stop becomes the aperture stop by virtue of cutting off more light than did the stop that was the aperture stop on the optic axis.
 * The stop location determines the telecentricity. If the aperture stop of a lens is located at the front focal plane of the lens, then it becomes image-space telecentricity, i.e., the lateral size of the image is insensitive to the image plane location. If the stop is at the back focal plane of the lens, then it becomes object-space telecentricity where the image size is insensitive to the object plane location. The telecentricity helps precise two-dimensional measurements because measurement systems with the telecentricity are insensitive to axial position errors of samples or the sensor.

In addition to an aperture stop, a photographic lens may have one or more field stops, which limit the system's field of view. When the field of view is limited by a field stop in the lens (rather than at the film or sensor) vignetting results; this is only a problem if the resulting field of view is less than was desired.

In astronomy, the opening diameter of the aperture stop (called the aperture) is a critical parameter in the design of a telescope. Generally, one would want the aperture to be as large as possible, to collect the maximum amount of light from the distant objects being imaged. The size of the aperture is limited, however, in practice by considerations of its manufacturing cost and time and its weight, as well as prevention of aberrations (as mentioned above).

Apertures are also used in laser energy control, close aperture z-scan technique, diffractions/patterns, and beam cleaning. Laser applications include spatial filters, Q-switching, high intensity x-ray control.

In light microscopy, the word aperture may be used with reference to either the condenser (that changes the angle of light onto the specimen field), field iris (that changes the area of illumination on specimens) or possibly objective lens (forms primary images). See Optical microscope.

In photography
The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's or image sensor's degree of exposure to light. Typically, a fast shutter will require a larger aperture to ensure sufficient light exposure, and a slow shutter will require a smaller aperture to avoid excessive exposure.

A device called a diaphragm usually serves as the aperture stop and controls the aperture (the opening of the aperture stop). The diaphragm functions much like the iris of the eye – it controls the effective diameter of the lens opening (called pupil in the eyes). Reducing the aperture size (increasing the f-number) provides less light to sensor and also increases the depth of field (by limiting the angle of cone of image light reaching the sensor), which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the f-number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter (the diameter of the entrance pupil). A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture which allows more light to reach the film or image sensor. The photography term "one f-stop" refers to a factor of $\sqrt{2}$ (approx. 1.41) change in f-number which corresponds to a $\sqrt{2}$ change in aperture diameter, which in turn corresponds to a factor of 2 change in light intensity (by a factor 2 change in the aperture area).

Aperture priority is a semi-automatic shooting mode used in cameras. It permits the photographer to select an aperture setting and let the camera decide the shutter speed and sometimes also ISO sensitivity for the correct exposure. This is also referred to as Aperture Priority Auto Exposure, A mode, AV mode (aperture-value mode), or semi-auto mode.

Typical ranges of apertures used in photography are about –  or  –, covering six stops, which may be divided into wide, middle, and narrow of two stops each, roughly (using round numbers)  – ,  – , and  –  or (for a slower lens)  – ,  – , and  –. These are not sharp divisions, and ranges for specific lenses vary.

Maximum and minimum apertures
The specifications for a given lens typically include the maximum and minimum aperture (opening) sizes, for example, –. In this case, is currently the maximum aperture (the widest opening on a full-frame format for practical use ), and  is the minimum aperture (the smallest opening). The maximum aperture tends to be of most interest and is always included when describing a lens. This value is also known as the lens "speed", as it affects the exposure time. As the aperture area is proportional to the light admitted by a lens or an optical system, the aperture diameter is proportional to the square root of the light admitted, and thus inversely proportional to the square root of required exposure time, such that an aperture of allows for exposure times one quarter that of. ( is 4 times larger than in the aperture area.)

Lenses with apertures opening or wider are referred to as "fast" lenses, although the specific point has changed over time (for example, in the early 20th century aperture openings wider than  were considered fast. The fastest lenses for the common 35 mm film format in general production have apertures of  or, with more at  and , and many at  or slower;  is unusual, though sees some use. When comparing "fast" lenses, the image format used must be considered. Lenses designed for a small format such as half frame or APS-C need to project a much smaller image circle than a lens used for large format photography. Thus the optical elements built into the lens can be far smaller and cheaper.

In exceptional circumstances lenses can have even wider apertures with f-numbers smaller than 1.0; see lens speed: fast lenses for a detailed list. For instance, both the current Leica Noctilux-M 50mm ASPH and a 1960s-era Canon 50mm rangefinder lens have a maximum aperture of. Cheaper alternatives began appearing in the early 2010s, such as the Cosina Voigtländer Nokton (several in the $10.5 mm$ range) and  ($29 mm$) Super Nokton manual focus lenses in the for the Micro Four-Thirds System, and the Venus Optics (Laowa) Argus $35 mm$.

Professional lenses for some movie cameras have f-numbers as small as. Stanley Kubrick's film Barry Lyndon has scenes shot by candlelight with a NASA/Zeiss 50mm f/0.7, the fastest lens in film history. Beyond the expense, these lenses have limited application due to the correspondingly shallower depth of field (DOF) – the scene must either be shallow, shot from a distance, or will be significantly defocused, though this may be the desired effect.

Zoom lenses typically have a maximum relative aperture (minimum f-number) of to  through their range. High-end lenses will have a constant aperture, such as or, which means that the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable maximum relative aperture since it is harder and more expensive to keep the maximum relative aperture proportional to the focal length at long focal lengths; to  is an example of a common variable aperture range in a consumer zoom lens.

By contrast, the minimum aperture does not depend on the focal length – it is limited by how narrowly the aperture closes, not the lens design – and is instead generally chosen based on practicality: very small apertures have lower sharpness due to diffraction at aperture edges, while the added depth of field is not generally useful, and thus there is generally little benefit in using such apertures. Accordingly, DSLR lens typically have minimum aperture of, , or , while large format may go down to , as reflected in the name of Group f/64. Depth of field is a significant concern in macro photography, however, and there one sees smaller apertures. For example, the Canon MP-E 65mm can have effective aperture (due to magnification) as small as. The pinhole optic for Lensbaby creative lenses has an aperture of just.

Aperture area
The amount of light captured by an optical system is proportional to the area of the entrance pupil that is the object space-side image of the aperture of the system, equal to:

 * $$\mathrm{Area} = \pi \left({D \over 2}\right)^2 = \pi \left({f \over 2N}\right)^2 $$

Where the two equivalent forms are related via the f-number N = f / D, with focal length f and entrance pupil diameter D.

The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene. In that case, the relative focal-plane illuminance, however, would depend only on the f-number N, so it is less in the camera with the larger format, longer focal length, and higher f-number. This assumes both lenses have identical transmissivity.

Aperture control
Though as early as 1933 Torkel Korling had invented and patented for the Graflex large format reflex camera an automatic aperture control, not all early 35mm single lens reflex cameras had the feature. With a small aperture, this darkened the viewfinder, making viewing, focusing, and composition difficult. Korling's design enabled full-aperture viewing for accurate focus, closing to the pre-selected aperture opening when the shutter was fired and simultaneously synchronising the firing of a flash unit. From 1956 SLR camera manufacturers separately developed automatic aperture control (the Miranda T 'Pressure Automatic Diaphragm', and other solutions on the Exakta Varex IIa and Praktica FX2) allowing viewing at the lens's maximum aperture, stopping the lens down to the working aperture at the moment of exposure, and returning the lens to maximum aperture afterward. The first SLR cameras with internal ("through-the-lens" or "TTL") meters (e.g., the Pentax Spotmatic) required that the lens be stopped down to the working aperture when taking a meter reading. Subsequent models soon incorporated mechanical coupling between the lens and the camera body, indicating the working aperture to the camera for exposure while allowing the lens to be at its maximum aperture for composition and focusing; this feature became known as open-aperture metering.

For some lenses, including a few long telephotos, lenses mounted on bellows, and perspective-control and tilt/shift lenses, the mechanical linkage was impractical, and automatic aperture control was not provided. Many such lenses incorporated a feature known as a "preset" aperture, which allows the lens to be set to working aperture and then quickly switched between working aperture and full aperture without looking at the aperture control. A typical operation might be to establish rough composition, set the working aperture for metering, return to full aperture for a final check of focus and composition, and focusing, and finally, return to working aperture just before exposure. Although slightly easier than stopped-down metering, operation is less convenient than automatic operation. Preset aperture controls have taken several forms; the most common has been the use of essentially two lens aperture rings, with one ring setting the aperture and the other serving as a limit stop when switching to working aperture. Examples of lenses with this type of preset aperture control are the Nikon PC Nikkor 28 mm and the SMC Pentax Shift 6×7 75 mm. The Nikon PC Micro-Nikkor 85 mm lens incorporates a mechanical pushbutton that sets working aperture when pressed and restores full aperture when pressed a second time.

Canon EF lenses, introduced in 1987, have electromagnetic diaphragms, eliminating the need for a mechanical linkage between the camera and the lens, and allowing automatic aperture control with the Canon TS-E tilt/shift lenses. Nikon PC-E perspective-control lenses, introduced in 2008, also have electromagnetic diaphragms, a feature extended to their E-type range in 2013.

Optimal aperture
Optimal aperture depends both on optics (the depth of the scene versus diffraction), and on the performance of the lens.

Optically, as a lens is stopped down, the defocus blur at the Depth of Field (DOF) limits decreases but diffraction blur increases. The presence of these two opposing factors implies a point at which the combined blur spot is minimized (Gibson 1975, 64); at that point, the f -number is optimal for image sharpness, for this given depth of field – a wider aperture (lower f-number) causes more defocus, while a narrower aperture (higher f-number) causes more diffraction.

As a matter of performance, lenses often do not perform optimally when fully opened, and thus generally have better sharpness when stopped down some – this is sharpness in the plane of critical focus, setting aside issues of depth of field. Beyond a certain point, there is no further sharpness benefit to stopping down, and the diffraction occurred at the edges of the aperture begins to become significant for imaging quality. There is accordingly a sweet spot, generally in the –  range, depending on lens, where sharpness is optimal, though some lenses are designed to perform optimally when wide open. How significant this varies between lenses, and opinions differ on how much practical impact this has.

While optimal aperture can be determined mechanically, how much sharpness is required depends on how the image will be used – if the final image is viewed under normal conditions (e.g., an 8″×10″ image viewed at 10″), it may suffice to determine the f -number using criteria for minimum required sharpness, and there may be no practical benefit from further reducing the size of the blur spot. But this may not be true if the final image is viewed under more demanding conditions, e.g., a very large final image viewed at normal distance, or a portion of an image enlarged to normal size (Hansma 1996). Hansma also suggests that the final-image size may not be known when a photograph is taken, and obtaining the maximum practicable sharpness allows the decision to make a large final image to be made at a later time; see also critical sharpness.

In biology

In many living optical systems, the eye consists of an iris which adjusts the size of the pupil, through which light enters. The iris is analogous to the diaphragm, and the pupil (which is the adjustable opening in the iris) the aperture. Refraction in the cornea causes the effective aperture (the entrance pupil in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from as narrow as 2 mm in diameter in a brightly lit place to 8 mm  in the dark as part of adaptation. In rare cases in some individuals are able to dilate their pupils even beyond 8 mm (in scotopic lighting, close to the physical limit of the iris. In humans, the average iris diameter is about 11.5 mm, which naturally influences the maximal size of the pupil as well, where larger iris diameters would typically have pupils which are able to dilate to a wider extreme than those with smaller irises. Maximum dilated pupil size also decreases with age. The iris controls the size of the pupil via two complementary sets muscles, the sphincter and dilator muscles, which are innervated by the parasympathetic and sympathetic nervous systems respectively, and act to induce pupillary constriction and dilation respectively. The state of the pupil is closely influenced by various factors, primarily light (or absence of light), but also by emotional state, interest in the subject of attention, arousal, sexual stimulation, physical activity, accommodation state, and cognitive load. The field of view is not affected by the size of the pupil.

Some individuals are also able to directly exert manual and conscious control over their iris muscles and hence are able to voluntarily constrict and dilate their pupils on command. However, this ability is rare and potential use or advantages are unclear.

Equivalent aperture range
In digital photography, the 35mm-equivalent aperture range is sometimes considered to be more important than the actual f-number. Equivalent aperture is the f-number adjusted to correspond to the f-number of the same size absolute aperture diameter on a lens with a 35mm equivalent focal length. Smaller equivalent f-numbers are expected to lead to higher image quality based on more total light from the subject, as well as lead to reduced depth of field. For example, a Sony Cyber-shot DSC-RX10 uses a 1" sensor, 24 – 200 mm with maximum aperture constant along the zoom range;  has equivalent aperture range, which is a lower equivalent f-number than some other  cameras with smaller sensors.

However, modern optical research concludes that sensor size does not actually play a part in the depth of field in an image. An aperture's f-number is not modified by the camera's sensor size because it is a ratio that only pertains to the attributes of the lens. Instead, the higher crop factor that comes as a result of a smaller sensor size means that, in order to get an equal framing of the subject, the photo must be taken from further away, which results in a less blurry background, changing the perceived depth of field. Similarly, a smaller sensor size with an equivalent aperture will result in a darker image because of the pixel density of smaller sensors with equivalent megapixels. Every photosite on a camera's sensor requires a certain amount of surface area that is not sensitive to light, a factor that results in differences in pixel pitch and changes in the signal-noise ratio. However, neither the changed depth of field, nor the perceived change in light sensitivity are a result of the aperture. Instead, equivalent aperture can be seen as a rule of thumb to judge how changes in sensor size might affect an image, even if qualities like pixel density and distance from the subject are the actual causes of changes in the image.

In scanning or sampling
The terms scanning aperture and sampling aperture are often used to refer to the opening through which an image is sampled, or scanned, for example in a Drum scanner, an image sensor, or a television pickup apparatus. The sampling aperture can be a literal optical aperture, that is, a small opening in space, or it can be a time-domain aperture for sampling a signal waveform.

For example, film grain is quantified as graininess via a measurement of film density fluctuations as seen through a 0.048 mm sampling aperture.

In popular culture

Aperture Science, a fictional company in the Portal fictional universe, is named after the optical system. The company's logo heavily features an aperture in its logo, and has come to symbolize the series, fictional company, and the Aperture Science Laboratories Computer-Aided Enrichment Center that the game series takes place in.

title: Aperture


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Altimeter

An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water.

Sonic altimeter
In 1931, the US Army Air Corps and General Electric tested a sonic altimeter for aircraft, which was considered more reliable and accurate than one that relied on air pressure when heavy fog or rain was present. The new altimeter used a series of high-pitched sounds like those made by a bat to measure the distance from the aircraft to the surface, which on return to the aircraft was converted to feet shown on a gauge inside the aircraft cockpit.

Radar altimeter
A radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. Alternatively, Frequency Modulated Continuous-wave radar can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the pulsed radar for the same outlay and radar altimeters that use frequency modulation are industry standard. The radar altimeter is used to measure height above ground level during landing in commercial and military aircraft. Radar altimeters are also a component of terrain avoidance warning systems, warning the pilot if the aircraft is flying too low, or if there is rising terrain ahead. Radar altimeter technology is also used in terrain-following radar allowing combat aircraft to fly at very low height above the terrain.

After extensive research and experimentation, it has been shown that "phase radio-altimeters" are most suitable for ground effect vehicles, as compared to laser, isotropic or ultrasonic altimeters.

Laser altimeter
Lidar technology is used to help navigate the helicopter Ingenuity on its record-setting flights over the terrain of Mars by means of a downward-facing Lidar altimeter.

Global Positioning System
Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined using autonomous GPS is not reliable enough to supersede the pressure altimeter without using some method of augmentation. In hiking and climbing, it is common to find that the altitude measured by GPS is off by as much as 400 ft depending on satellite orientation.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Altimeter


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Ames Research Center

The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laboratory. That agency was dissolved and its assets and personnel transferred to the newly created National Aeronautics and Space Administration (NASA) on October 1, 1958. NASA Ames is named in honor of Joseph Sweetman Ames, a physicist and one of the founding members of NACA. At last estimate NASA Ames had over US$3 billion in capital equipment, 2,300 research personnel and a US$860 million annual budget.

Ames was founded to conduct wind-tunnel research on the aerodynamics of propeller-driven aircraft; however, its role has expanded to encompass spaceflight and information technology. Ames plays a role in many NASA missions. It provides leadership in astrobiology; small satellites; robotic lunar exploration; the search for habitable planets; supercomputing; intelligent/adaptive systems; advanced thermal protection; planetary science; and airborne astronomy. Ames also develops tools for a safer, more efficient national airspace. The center's current director is Eugene Tu.

The site was mission center for several key missions (Kepler, the Stratospheric Observatory for Infrared Astronomy (SOFIA), Interface Region Imaging Spectrograph) and a major contributor to the "new exploration focus" as a participant in the Orion crew exploration vehicle.

Missions
Although Ames is a NASA Research Center, and not a flight center, it has nevertheless been closely involved in a number of astronomy and space missions.

The Pioneer program's eight successful space missions from 1965 to 1978 were managed by Charles Hall at Ames, initially aimed at the inner Solar System. By 1972, it supported the bold flyby missions to Jupiter and Saturn with Pioneer 10 and Pioneer 11. Those two missions were trail blazers (radiation environment, new moons, gravity-assist flybys) for the planners of the more complex Voyager 1 and Voyager 2 missions, launched five years later. In 1978, the end of the program brought about a return to the inner solar system, with the Pioneer Venus Orbiter and Multiprobe, this time using orbital insertion rather than flyby missions.

Lunar Prospector was the third mission selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was put into a low polar orbit of the Moon, accomplishing mapping of surface composition and possible polar ice deposits, measurements of magnetic and gravity fields, and study of lunar outgassing events. Based on Lunar Prospector Neutron Spectrometer (NS) data, mission scientists have determined that there is indeed water ice in the polar craters of the Moon. The mission ended July 31, 1999, when the orbiter was guided to an impact into a crater near the lunar south pole in an (unsuccessful) attempt to analyze lunar polar water by vaporizing it to allow spectroscopic characterization from Earth telescopes.

The 11-pound (5 kg) GeneSat-1, carrying bacteria inside a miniature laboratory, was launched on December 16, 2006. The very small NASA satellite has proven that scientists can quickly design and launch a new class of inexpensive spacecraft—and conduct significant science.

The Lunar Crater Observation and Sensing Satellite (LCROSS) mission to look for water on the Moon was a 'secondary payload spacecraft.' LCROSS began its trip to the Moon on the same rocket as the Lunar Reconnaissance Orbiter (LRO), which continues to conduct a different lunar task. It launched in April 2009 on an Atlas V rocket from Kennedy Space Center, Florida.

The Kepler mission was NASA's first mission capable of finding Earth-size and smaller planets. The Kepler mission monitored the brightness of stars to find planets that pass in front of them during the planets' orbits. During such passes or 'transits,' the planets will slightly decrease the star's brightness.

Stratospheric Observatory for Infrared Astronomy (SOFIA) was a joint venture of the U.S. and German aerospace agencies, NASA and the German Aerospace Center (DLR) to make an infrared telescope platform that can fly at altitudes high enough to be in the infrared-transparent regime above the water vapor in the Earth's atmosphere. The aircraft was supplied by the U.S., and the infrared telescope by Germany. Modifications of the Boeing 747SP airframe to accommodate the telescope, mission-unique equipment and large external door were made by L-3 Communications Integrated Systems of Waco, Texas.

The Interface Region Imaging Spectrograph mission is a partnership with the Lockheed Martin Solar and Astrophysics Laboratory to understand the processes at the boundary between the Sun's chromosphere and corona. This mission is sponsored by the NASA Small Explorer program.

The Lunar Atmosphere Dust Environment Explorer (LADEE) mission has been developed by NASA Ames. This successfully launched to the Moon on September 6, 2013.

In addition, Ames has played a support role in a number of missions, most notably the Mars Pathfinder and Mars Exploration Rover missions, where the Ames Intelligent Robotics Laboratory played a key role. NASA Ames was a partner on the Mars Phoenix, a Mars Scout Program mission to send a high-latitude lander to Mars, deployed a robotic arm to dig trenches up to 1.6 feet (one half meter) into the layers of water ice and analyzing the soil composition. Ames is also a partner on the Mars Science Laboratory and its Curiosity rover, a next generation Mars rover to explore for signs of organics and complex molecules.

Air traffic control automation research
The Aviation Systems Division conducts research and development in two primary areas: air traffic management, and high-fidelity flight simulation. For air traffic management, researchers are creating and testing concepts to allow for up to three times today's level of aircraft in the national airspace. Automation and its attendant safety consequences are key foundations of the concept development. Historically, the division has developed products that have been implemented for the flying public, such as the Traffic Management Adviser, which is being deployed nationwide. For high-fidelity flight simulation, the division operates the world's largest flight simulator (the Vertical Motion Simulator), a Level-D 747-400 simulator, and a panoramic air traffic control tower simulator. These simulators have been used for a variety of purposes including continued training for Space Shuttle pilots, development of future spacecraft handling qualities, helicopter control system testing, Joint Strike Fighter evaluations, and accident investigations. Personnel in the division have a variety of technical backgrounds, including guidance and control, flight mechanics, flight simulation, and computer science. Customers outside NASA have included the FAA, DOD, DHS, DOT, NTSB, Lockheed Martin, and Boeing.

The center's flight simulation and guidance laboratory was listed on the National Register of Historic Places in 2017.

Information technology

Ames is the home of NASA's large research and development divisions in Advanced Supercomputing, Human Factors, and Artificial Intelligence (Intelligent Systems ). These Research & Development organizations support NASA's Exploration efforts, as well as the continued operations of the International Space Station, and the space science and Aeronautics work across NASA. The center also runs and maintains the E Root nameserver of the DNS.

The Intelligent Systems Division (Code TI) is NASA's leading R&D Division developing advanced intelligent software and systems for all of NASA Mission Directorates. It provides software expertise for earth science applications, aeronautics, space science missions, International Space Station, and the Crewed Exploration Vehicle (CEV).

The first AI in space (Deep Space 1) was developed from Code TI, as is the MAPGEN software that daily plans the activities for the Mars Exploration Rovers, the same core reasoner is used for Ensemble to operate Phoenix Lander, and the planning system for the International Space Station's solar arrays. Integrated System Health Management for the International Space Station's control moment gyroscopes, collaborative systems with semantic search tools, and robust software engineering round out the scope of Code TI's work.

The Human Systems Integration Division "advances human-centered design and operations of complex aerospace systems through analysis, experimentation, and modeling of human performance and human-automation interaction to make dramatic improvements in safety, efficiency and mission success". For decades, the Human Systems Integration Division has been on the leading edge of human-centered aerospace research. The Division is home to over 100 researchers, contractors and administrative staff.

The NASA Advanced Supercomputing Division at Ames operates several of the agency's most powerful supercomputers, including the petaflop-scale Pleiades, Aitken, and Electra systems. Originally called the Numerical Aerodynamic Simulation Division, the facility has housed more than 40 production and test supercomputers since its construction in 1987, and has served as a leader in high-performance computing, developing technology used across the industry, including the NAS Parallel Benchmarks and the Portable Batch System (PBS) job scheduling software.

In September 2009, Ames launched NEBULA as a fast and powerful Cloud Computing Platform to handle NASA's massive data sets that complied with security requirements. This innovative pilot uses open-source components, complies with FISMA and can scale to Government-sized demands while being extremely energy efficient. In July 2010, NASA CTO Chris C. Kemp open sourced Nova, the technology behind the NEBULA Project in collaboration with Rackspace, launching OpenStack. OpenStack has subsequently become one of the largest and fastest growing open source projects in the history of computing, and has been included in most major distributions of Linux including Red Hat, Oracle, HP, SUSE, and Canonical.

Image processing
NASA Ames was one of the first locations to conduct research on image processing of satellite-platform aerial photography. Some of the pioneering techniques of contrast enhancement using Fourier analysis were developed at Ames in conjunction with researchers at ESL Inc.

Wind tunnels
The NASA Ames Research Center wind tunnels are known not only for their immense size, but also for their diverse characteristics that enable various kinds of scientific and engineering research.

ARC Unitary Plan Wind Tunnel
The Unitary Plan Wind Tunnel (UPWT) was completed in 1956 at a cost of $27 million under the Unitary Plan Act of 1949. Since its completion, the UPWT facility has been the most heavily used NASA wind tunnel within the NASA Wind Tunnel Fleet. Every major commercial transport and almost every military jet built in the United States over the last 40 years has been tested in this facility. Mercury, Gemini, and Apollo spacecraft, as well as Space Shuttle, were also tested in this tunnel complex.

National Full-Scale Aerodynamics Complex (NFAC)
Ames Research Center also houses the world's largest wind tunnel, part of the National Full-Scale Aerodynamic Complex (NFAC): it is large enough to test full-sized planes, rather than scale models. The complex of wind tunnels was listed on the National Register in 2017.

The 40 by 80 foot wind tunnel circuit was originally constructed in the 1940s and is now capable of providing test velocities up to 300 kn. It is used to support an active research program in aerodynamics, dynamics, model noise, and full-scale aircraft and their components. The aerodynamic characteristics of new configurations are investigated with an emphasis on estimating the accuracy of computational methods. The tunnel is also used to investigate the aeromechanical stability boundaries of advanced rotorcraft and rotor-fuselage interactions. Stability and control derivatives are also determined, including the static and dynamic characteristics of new aircraft configurations. The acoustic characteristics of most of the full-scale vehicles are also determined, as well as acoustic research aimed at discovering and reducing aerodynamic sources of noise. In addition to the normal data gathering methods (e.g., balance system, pressure measuring transducers, and temperature sensing thermocouples), state-of-the-art, non-intrusive instrumentation (e.g., laser velocimeters and shadowgraphs) are available to help determine flow direction and velocity in and around the lifting surfaces of aircraft. The 40 by 80 Foot Wind Tunnel is primarily used for determining the low- and medium-speed aerodynamic characteristics of high-performance aircraft, rotorcraft, and fixed wing, powered-lift V/STOL aircraft.

The 80 by 120 Foot Wind Tunnel is the world's largest wind tunnel test section. This open circuit leg was added and a new fan drive system was installed in the 1980s. It is currently capable of air speeds up to 100 kn. This section is used in similar ways to the 40 by 80 foot section, but it is capable of testing larger aircraft, albeit at slower speeds. Some of the test programs that have come through the 80 by 120 Foot include: F-18 High Angle of Attack Vehicle, DARPA/Lockheed Common Affordable Lightweight Fighter, XV-15 Tilt Rotor, and Advance Recovery System Parafoil. The 80 by 120 foot test section is capable of testing a full size Boeing 737.

Although decommissioned by NASA in 2003, the NFAC is now being operated by the United States Air Force as a satellite facility of the Arnold Engineering Development Complex (AEDC).

Arc Jet Complex
The Ames Arc Jet Complex is an advanced thermophysics facility where sustained hypersonic- and hyperthermal testing of vehicular thermoprotective systems takes place under a variety of simulated flight- and re-entry conditions. Of its seven available test bays, four currently contain Arc Jet units of differing configurations, serviced by common facility support equipment. These are the Aerodynamic Heating Facility (AHF), the Turbulent Flow Duct (TFD), the Panel Test Facility (PTF), and the Interaction Heating Facility (IHF). The support equipment includes two D.C. power supplies, a steam ejector-driven vacuum system, a water-cooling system, high-pressure gas systems, data acquisition system, and other auxiliary systems.

The magnitude and capacity of these systems makes the Ames Arc Jet Complex unique. The largest power supply can deliver 75 megawatts (MW) for a 30-minute duration or 150 MW for a 15-second duration. This power capacity, in combination with a high-volume 5-stage steam ejector vacuum-pumping system, enables facility operations to match high-altitude atmospheric flight conditions with samples of relatively large size. The Thermo-Physics Facilities Branch operates four arc jet facilities. The Interaction Heating Facility (IHF), with an available power of over 60-MW, is one of the highest-power arc jets available. It is a very flexible facility, capable of long run times of up to one hour, and able to test large samples in both a stagnation and flat plate configuration. The Panel Test Facility (PTF) uses a unique semielliptic nozzle for testing panel sections. Powered by a 20-MW arc heater, the PTF can perform tests on samples for up to 20 minutes. The Turbulent Flow Duct provides supersonic, turbulent high temperature air flows over flat surfaces. The TFD is powered by a 20-MW Hüls arc heater and can test samples 203 by in size. The Aerodynamic Heating Facility (AHF) has similar characteristics to the IHF arc heater, offering a wide range of operating conditions, sample sizes and extended test times. A cold-air-mixing plenum allows for simulations of ascent or high-speed flight conditions. Catalycity studies using air or nitrogen can be performed in this flexible rig. A 5-arm model support system allows the user to maximize testing efficiency. The AHF can be configured with either a Hüls or segmented arc heater, up to 20-MW. 1 MW is enough power to supply 750 homes.

The Arc Jet Complex was listed on the National Register in 2017.

Ames Vertical Gun Range
The Ames Vertical Gun Range (AVGR) was designed to conduct scientific studies of lunar impact processes in support of the Apollo missions. In 1979, it was established as a National Facility, funded through the Planetary Geology and Geophysics Program. In 1995, increased scientific needs across various disciplines resulted in joint core funding by three different science programs at NASA Headquarters (Planetary Geology and Geophysics, Exobiology, and Solar System Origins). In addition, the AVGR provides programmatic support for various proposed and ongoing planetary missions (e.g. Stardust, Deep Impact).

Using its 0.30 cal light-gas gun and powder gun, the AVGR can launch projectiles to velocities ranging from 500 to 7000 m/s. By varying the gun's angle of elevation with respect to the target vacuum chamber, impact angles from 0° to 90° relative to the gravitational vector are possible. This unique feature is extremely important in the study of crater formation processes.

The target chamber is approximately 2.5 m in diameter and height and can accommodate a wide variety of targets and mounting fixtures. It can maintain vacuum levels below 0.03 Torr, or can be back filled with various gases to simulate different planetary atmospheres. Impact events are typically recorded with high-speed video/film, or Particle Image Velocimetry (PIV).

Hypervelocity Free-Flight Range
The Hypervelocity Free-Flight (HFF) Range currently comprises two active facilities: the Aerodynamic Facility (HFFAF) and the Gun Development Facility (HFFGDF). The HFFAF is a combined Ballistic Range and Shock-tube Driven Wind Tunnel. Its primary purpose is to examine the aerodynamic characteristics and flow-field structural details of free-flying aeroballistic models.

The HFFAF has a test section equipped with 16 shadowgraph-imaging stations. Each station can be used to capture an orthogonal pair of images of a hypervelocity model in flight. These images, combined with the recorded flight time history, can be used to obtain critical aerodynamic parameters such as lift, drag, static and dynamic stability, flow characteristics, and pitching moment coefficients. For very high Mach number (M > 25) simulations, models can be launched into a counter-flowing gas stream generated by the shock tube. The facility can also be configured for hypervelocity impact testing and has an aerothermodynamic capability as well. The HFFAF is currently configured to operate the 1.5 in light-gas gun in support of continuing thermal imaging and transition research for NASA's hypersonics program.

The HFFGDF is used for gun performance enhancement studies, and occasional impact testing. The Facility uses the same arsenal of light-gas and powder guns as the HFFAF to accelerate particles that range in size from 3.2 to 25.4 mm diameter to velocities ranging from 0.5 to 8.5 km/s. Most of the research effort to date has centered on Earth atmosphere entry configurations (Mercury, Gemini, Apollo, and Shuttle), planetary entry designs (Viking, Pioneer Venus, Galileo and MSL), and aerobraking (AFE) configurations. The facility has also been used for scramjet propulsion studies (National Aerospace Plane (NASP)) and meteoroid/orbital debris impact studies (Space Station and RLV). In 2004, the facility was utilized for foam-debris dynamics testing in support of the Return To Flight effort. As of March 2007, the GDF has been reconfigured to operate a cold gas gun for subsonic CEV capsule aerodynamics.

Electric Arc Shock Tube
The Electric Arc Shock Tube (EAST) Facility is used to investigate the effects of radiation and ionization that occur during very high velocity atmospheric entries. In addition, the EAST can also provide air-blast simulations requiring the strongest possible shock generation in air at an initial pressure loading of 1 atm or greater. The facility has three separate driver configurations, to meet a range of test requirements: the driver can be connected to a diaphragm station of either a 102 mm or a 610 mm shock tube, and the high-pressure 102 mm shock tube can also drive a 762 mm shock tunnel. Energy for the drivers is supplied by a 1.25-MJ-capacitor storage system.

United States Geological Survey (USGS)
In September 2016, the United States Geological Survey (USGS) announced plans to relocate its West Coast science center from nearby Menlo Park to the Ames Research Center at Moffett Field. The relocation is expected to take five years and will begin in 2017 with 175 of the USGS employees moving to Moffett. The relocation is designed to save money on the $7.5 million annual rent the USGS pays for its Menlo Park campus. The land in Menlo Park is owned by the General Services Administration, which is required by federal law to charge market-rate rent.

NASA Ames Visitor Center
The NASA Experience exhibit at the Chabot Space and Science Center serves as the visitor center for NASA's Ames Research Center. The NASA Experience provides a dynamic and interactive space for the public to learn about local contributions to space exploration across the years. From models of spacecraft and genuine spacesuits from as early as the Mercury and Gemini missions to artifacts related to NASA's upcoming Artemis missions, the NASA Ames Visitor Center gives visitors access to over 80 years of Ames history and a look into current and future projects. Ames' expertise in wind tunnel testing, rover design and testing, space robotics, supercomputing, and more is on display. The exhibit was opened on November 12, 2021.

NASA Ames Exploration Center

The NASA Ames Exploration Center is a science museum and education center for NASA. There are displays and interactive exhibits about NASA technology, missions and space exploration. A Moon rock, meteorite, and other geologic samples are on display. The theater shows movies with footage from NASA's explorations of Mars and the planets, and about the contributions of the scientists at Ames. This facility is currently closed.

Robotics Alliance Project
In 1999, Mark León developed NASA's Robotics Education Project — now called the Robotics Alliance Project — under his mentor Dave Lavery, which has reached over 100,000 students nationwide using FIRST robotics and BOTBALL robotics competitions. The Project's FIRST branch originally comprised FRC Team 254: "The Cheesy Poofs", an all-male team from Bellarmine High School in San Jose, California. In 2006, Team 1868: "The Space Cookies", an all-female team, was founded in collaboration with the Girl Scouts. In 2012, Team 971: "Spartan Robotics" of Mountain View High School joined the Project, though the team continues to operate at their school. All three teams are highly decorated. All three have won Regional competitions, two have won the FIRST Championship, two have won the Regional Chairman's Award, and one is a Hall of Fame team. The three teams are collectively referred to as "House teams".

The mission of the project is "To create a human, technical, and programmatic resource of robotics capabilities to enable the implementation of future robotic space exploration missions."

Public-private partnerships
The federal government has re-tasked portions of the facility and human resources to support private sector industry, research, and education.

HP became the first corporate affiliate of a new Bio-Info-Nano Research and Development Institute (BIN-RDI); a collaborative venture established by the University of California Santa Cruz and NASA, based at Ames. The Bio|Info|Nano R&D Institute is dedicated to creating scientific breakthroughs by the convergence of biotechnology, information technology, and nanotechnology.

Singularity University hosts its leadership and educational program at the facility. The Organ Preservation Alliance is also headquartered there; the Alliance is a nonprofit organization that works in partnership with the Methuselah Foundation's New Organ Prize "to catalyze breakthroughs on the remaining obstacles towards the long-term storage of organs" to overcome the drastic unmet medical need for viable organs for transplantation. Kleenspeed Technologies is headquartered there.

Google
On September 28, 2005, Google and Ames Research Center disclosed details to a long-term research partnership. In addition to pooling engineering talent, Google planned to build a 1000000 sqft facility on the ARC campus. One of the projects between Ames, Google, and Carnegie Mellon University is the Gigapan Project – a robotic platform for creating, sharing, and annotating terrestrial gigapixel images. The Planetary Content Project seeks to integrate and improve the data that Google uses for its Google Moon and Google Mars projects. On 4 June 2008, Google announced it had leased 42 acre from NASA, at Moffett Field, for use as office space and employee housing.

Construction of the new Google project which is near Google's Googleplex headquarters began in 2013 and has a target opening date in 2015. It is called "Bay View" as it overlooks San Francisco Bay.

In May 2013, Google announced that it was launching the Quantum Artificial Intelligence Lab, to be hosted by ARC. The lab will house a 512 qubit quantum computer from D-Wave Systems, and the Universities Space Research Association (USRA) will invite researchers from around the world to share time on it. The goal being to study how quantum computing might advance machine learning.

Announced on November 10, 2014, Planetary Ventures LLC (a Google subsidiary) will lease the Moffett Federal Airfield from NASA Ames, a site of about 1,000 acres formerly costing the agency $6.3 million annually in maintenance and operation costs. The lease includes the restoration of the site's historic landmark Hangar One, as well as hangars Two and Three. The lease went into effect in March 2015, and spans 60 years.

Living and working at Ames
An official NASA ID is required to enter Ames.

There are myriad activities both inside the research center and around the base for full-time workers and interns alike. Portions of a fitness trail remain inside the base (also called a Parcourse trail), Sections of it are now inaccessible due to changes in base layout since it was installed.

title: Ames Research Center


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Talk:Stock market index

What is a Stock Market index?
Can someone explain how the index figures are calculated? adamsan 15:56, 31 December 2004 (UTC)

 * Something I would have thought this article might address... What actually is an index; what does it measure, how is it calculated, how is it used. What does it mean if an index is high or low, or rising or falling? They seem a bit more important than what types of index exist. 94.174.217.148 (talk) 15:27, 9 August 2011 (UTC)

inflation-correction
I'm a newcomer, so I'm just writing this as a suggestion.

I use the US as an example here. Stock prices are in US$, and so are the various indexes/averages of them. Values of an index from (very) different times are commonly compared. The purchasing power of the US$ has declined a lot over time. So, sound representation of an index is showing it in terms of purchasing power, i.e., after inflation-correction.

I suggest that this point be strongly incorporated into the article "Stock market index". — Preceding unsigned comment added by Wttsmyf2 (talk • contribs) 19:25, 17 January 2006 (UTC)

 * "Stock prices are in US$, and so are the various indexes/averages of them."
 * Most of the world's stock prices are not in US$.
 * Indices/indexes are dimensionless, so it is meaningless to say they are in any currency.
 * But averages give you the average price of the shares in member companies. Strictly speaking, an average should be expressed in terms of currency. — Preceding unsigned comment added by 14.136.144.152 (talk) 23:14, 7 March 2013 (UTC)

Bad Info on the Definition Addemdum
In the Definition Addendum it is stated that a stock market index can be traded. That is not correct - you cannot own or trade an index. What you can do is own a share of an Exchange Traded Fund (ETF) which is constructed using the same structure and wieghting as an index, therefore it will follow the index exactly. Or you can buy (or sell) stock options based on the preformance of the index. In other words, you can own something that looks like the index or derives it's value from the index, but to say that you can own the index is incorrect. 173.197.29.146 (talk) 21:49, 1 July 2011 (UTC)

Averages are not indexes/indices
See above. — Preceding unsigned comment added by 14.136.144.152 (talk) 23:10, 7 March 2013 (UTC)

Closing milestones of stock market indices
In each of the three stock market indices on Wall Street (Dow Jones Industrial Average, NASDAQ Composite, and S&P 500), formatting of the article is inconsistent and I feel we should discuss possible consistent formats we can use for all three articles in question. Each of the issues will have its own subsection within this section. 9March2019 (talk) 01:25, 26 May 2019 (UTC)

title: Talk:Stock market index


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Aquifer

An aquifer is an underground layer of water-bearing material, consisting of permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude (or aquifuge), which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. The classification of aquifers is as follows: Saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; transboundary aquifer.

Groundwater from aquifers can be sustainably harvested by humans through the use of qanats leading to a well. This groundwater is a major source of fresh water for many regions, however can present a number of challenges such as overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, and the salinization or pollution of the groundwater.

Depth
Aquifers occur from near-surface to deeper than 9000 m. Those closer to the surface are not only more likely to be used for water supply and irrigation, but are also more likely to be replenished by local rainfall. Although aquifers are sometimes characterized as "underground rivers or lakes," they are actually porous rock saturated with water.

Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources. Part of the Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon ranges between Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra Nevada and neighboring ranges in the United States' Southwest, have shallow aquifers that are exploited for their water. Overexploitation can lead to the exceeding of the practical sustained yield; i.e., more water is taken out than can be replenished.

Along the coastlines of certain countries, such as Libya and Israel, increased water usage associated with population growth has caused a lowering of the water table and the subsequent contamination of the groundwater with saltwater from the sea.

In 2013 large freshwater aquifers were discovered under continental shelves off Australia, China, North America and South Africa. They contain an estimated half a million cubic kilometers of "low salinity" water that could be economically processed into potable water. The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100 times the amount of water extracted from other aquifers since 1900.

Classification
An aquitard is a zone within the Earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge. Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity.

Saturated versus unsaturated
Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain fresh water. The Earth's crust can be divided into two regions: the saturated zone or phreatic zone (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the unsaturated zone (also called the vadose zone), where there are still pockets of air that contain some water, but can be filled with more water.

Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).

Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero-gauge-pressure isobar) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than 1.8 m but can range between 0.3 and 10 m.

The capillary rise of water in a small-diameter tube involves the same physical process. The water table is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere.

Aquifers versus aquitards
Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials).

An aquitard is a zone within the Earth that restricts the flow of groundwater from one aquifer to another. A completely impermeable aquitard is called an aquiclude or aquifuge. Aquitards contain layers of either clay or non-porous rock with low hydraulic conductivity.

In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium, composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), whereas the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas—sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.

Confined versus unconfined
An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in the same geologic unit may be confined in one area and unconfined in another. Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface (see Biscayne Aquifer). Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.

If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as ), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content).

Isotropic versus anisotropic
In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense.

Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different (see hydraulic transmissivity and hydraulic resistance).

When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.

Porous, karst, or fractured
To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and contamination. Considerations include where and how much water enters the groundwater from rainfall and snowmelt, how fast and in what direction the groundwater travels, and how much water leaves the ground as springs. Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the hydrology has been characterized.

Porous

Porous aquifers typically occur in sand and sandstone. Porous aquifer properties depend on the depositional sedimentary environment and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers.

Sandy deposits formed in shallow marine environments and in windblown sand dune environments have moderate to high permeability while sandy deposits formed in river environments have low to moderate permeability. Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water.

Analyzing this type of information over an area gives an indication how much water can be pumped without overdrafting and how contamination will travel. In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d) is considered to be a high rate for porous aquifers, as illustrated by the water slowly seeping from sandstone in the accompanying image to the left.

Porosity is important, but, alone, it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps (a basaltic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous (Upper Cretaceous) Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.

Karst
Karst aquifers typically develop in limestone. Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs.

Characterization of karst aquifers requires field exploration to locate sinkholes, swallets, sinking streams, and springs in addition to studying geologic maps. Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize the complexity of karst aquifers. These conventional investigation methods need to be supplemented with dye traces, measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers.

Linear alignment of surface features such as straight stream segments and sinkholes develop along fracture traces. Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can initiate a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to the left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers.

In the extreme case, groundwater may exist in underground rivers (e.g., caves underlying karst topography.

Fractured
If a rock unit of low porosity is highly fractured, it can also make a good aquifer (via fissure flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water.

Human use of groundwater
Challenges for using groundwater include: overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, groundwater becoming saline, groundwater pollution.

Africa
Aquifer depletion is a problem in some areas, especially in northern Africa, where one example is the Great Manmade River project of Libya. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States.

Australia
The Great Artesian Basin situated in Australia is arguably the largest groundwater aquifer in the world (over 1.7 e6km2). It plays a large part in water supplies for Queensland, and some remote parts of South Australia.

Canada
Discontinuous sand bodies at the base of the McMurray Formation in the Athabasca Oil Sands region of northeastern Alberta, Canada, are commonly referred to as the Basal Water Sand (BWS) aquifers. Saturated with water, they are confined beneath impermeable bitumen-saturated sands that are exploited to recover bitumen for synthetic crude oil production. Where they are deep-lying and recharge occurs from underlying Devonian formations they are saline, and where they are shallow and recharged by surface water they are non-saline. The BWS typically pose problems for the recovery of bitumen, whether by open-pit mining or by in situ methods such as steam-assisted gravity drainage (SAGD), and in some areas they are targets for waste-water injection.

South America
The Guarani Aquifer, located beneath the surface of Argentina, Brazil, Paraguay, and Uruguay, is one of the world's largest aquifer systems and is an important source of fresh water. Named after the Guarani people, it covers 1200000 km2, with a volume of about 40000 km3, a thickness of between 50 and 800 m and a maximum depth of about 1800 m.

United States
The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals. According to a 2013 report by the United States Geological Survey (USGS), the depletion between 2001 and 2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century.

In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. "Cumulative total groundwater depletion in the United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs."

An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer in central Texas. This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today, is full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas.

title: Aquifer


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Atmospheric pressure

Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101325 Pa, which is equivalent to 1,013.25 millibars, 760mm Hg, 29.9212inchesHg, or 14.696psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's gravitational acceleration as a function of altitude can be approximated as constant and contributes little to this fall-off. Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square metre, 1N/m2). On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm2 or 101kN/m2 (101 kilopascals, kPa). A column of air with a cross-sectional area of 1in2 would have a weight of about 14.7lbf, resulting in a pressure of 14.7lbf/in2.

Mechanism
Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere. It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.

Mean sea-level pressure

The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level. This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet.

The altimeter setting in aviation is an atmospheric pressure adjustment.

Average sea-level pressure is 1013.25 hPa. In aviation weather reports (METAR), QNH is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar), except in the United States, Canada, and Japan where it is reported in inches of mercury (to two decimal places). The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.

In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: 1013.2 hPa is transmitted as 132; 1000 hPa is transmitted as 000; 998.7hPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050 hPa, with record highs close to 1085 hPa. The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes, with a record low of 870 hPa.

Surface pressure
Surface pressure is the atmospheric pressure at a location on Earth's surface (terrain and oceans). It is directly proportional to the mass of air over that location.

For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure.

The average value of surface pressure on Earth is 985 hPa. This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea level for locations above or below sea level. The average pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA) is 1,013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury.

Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.

Altitude variation

Pressure on Earth varies with the altitude of the surface, so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude. Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure. The graph was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about 1.2 kPa for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h: $$\begin{align} p &=      p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R_0 \cdot L} \\ &= p_0 \cdot \left(1 - \frac{g \cdot h}{c_\text{p} \cdot T_0} \right)^{\frac{c_\text{p} \cdot M}{R_0}} \approx p_0 \cdot \exp \left(-\frac{g \cdot h \cdot M}{T_0 \cdot R_0} \right) \end{align}$$

The values in these equations are:

Local variation
Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with an amplitude of a few hectopascals, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle, and a semi-circadian (12 h) cycle.

Records
The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1084.8 hPa measured in Tosontsengel, Mongolia on 19 December 2001. The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug, Russia (66°53'N, 93°28'E, elevation: 261 m) on 31 December 1968 of 1083.8 hPa. The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.

The Dead Sea, the lowest place on Earth at 430 m below sea level, has a correspondingly high typical atmospheric pressure of 1,065hPa. A below-sea-level surface pressure record of 1081.8 hPa was set on 21 February 1961.

The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.

Measurement based on the depth of water
One atmosphere (101.325 kPa) is also the pressure caused by the weight of a column of freshwater of approximately 10.3 m. Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions.

Low pressures, such as natural gas lines, are sometimes specified in inches of water, typically written as w.c. (water column) gauge or w.g. (inches water) gauge. A typical gas-using residential appliance in the US is rated for a maximum of 1/2 psi, which is approximately 14 w.g. Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used.

Boiling point of liquids
Pure water boils at 100 C at earth's standard atmospheric pressure. The boiling point is the temperature at which the vapour pressure is equal to the atmospheric pressure around the liquid. Because of this, the boiling point of liquids is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes or pressure cooking. A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers. Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in distillation, the atmospheric pressure may be lowered by using a vacuum pump, as in a rotary evaporator.

Measurement and maps
An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains, thanks to reliable pressure measurement devices. In 1774, Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion mountain in Scotland, and he needed to measure elevations on the mountain's sides accurately. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations, the agreement being to be within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.

Experiments

 * Movies on atmospheric pressure experiments from Georgia State University's HyperPhysics website – requires QuickTime
 * Test showing a can being crushed after boiling water inside it, then moving it into a tub of ice-cold water.

title: Atmospheric pressure


text:
King (chess)

The king (♔, ♚) is the most important piece in the game of chess. It may move to any adjoining square; it may also perform, in tandem with the rook, a special move called castling. If a player's king is threatened with capture, it is said to be in check, and the player must remove the threat of immediately. If this cannot be done, the king is said to be in checkmate, resulting in a loss for that player. A player cannot make any move that places their own king in check. Despite this, the king can become a strong offensive piece in the endgame or, rarely, the middlegame.

In algebraic notation, the king is abbreviated by the letter K among English speakers. The white king starts the game on e1; the black king starts on e8. Unlike all other pieces, each player can have only one king, and the kings are never removed from the board during the game.

Placement and movement
The white king starts on e1, on the first to the right of the queen from White's perspective. The black king starts on e8, directly across from the white king. Each king starts on a square opposite its own color.

A king can move one square horizontally, vertically, and diagonally unless the square is already occupied by a friendly piece or the move would place the king in check. If the square is occupied by an undefended enemy piece, the king may capture it, removing it from play. Opposing kings may never occupy adjacent squares (see opposition) to give check, as that would put the moving king in check as well. The king can give discovered check, however, by unblocking a bishop, rook, or queen.

Castling
The king can make a special move, in conjunction with a rook of the same color, called castling. When castling, the king moves two squares horizontally toward one of its rooks, and that rook is placed on the square over which the king crossed.

Castling is permissible under the following conditions:
 * Neither the king nor the castling rook have previously moved.
 * No squares between the two pieces are occupied.
 * The king is not in check.
 * None of the squares the king would move across or to are under enemy attack.

Castling with the h-file rook is known as castling kingside or short castling (denoted 0-0 in algebraic notation), while castling with the a-file rook is known as castling queenside or long castling (denoted 0-0-0).

Check and checkmate
A king that is under attack is said to be in check, and the player in check must immediately remedy the situation. There are three possible ways to remove the king from check:
 * The king is moved to an adjacent non-threatened square. A player may not castle to get their king out of check. A king can capture an adjacent enemy piece if that piece is not protected by another enemy piece.
 * A piece is interposed between the king and the attacking piece to break the line of threat (not possible when the attacking piece is a knight or pawn, or when in double check).
 * The attacking piece is captured (not possible when in double check, unless the king captures).

If none of the three options are available, the player's king has been checkmated, and the player loses the game.

In casual games, when placing the opponent's king in check, it is common to announce "check", but this is not required by the rules of chess.

Stalemate
A stalemate occurs when a player, on their turn, has no legal moves, and the player's king is not in check.

If this happens, the king is said to have been stalemated, and the game ends in a draw. A player who has very little or no chance of winning will often, in order to avoid a loss, try to entice the opponent to inadvertently place the player's king in stalemate (see swindle).

Role in gameplay
In the opening and middlegame, the king will rarely play an active role in the development of an offensive or defensive position, with the notable exception of a king walk. Instead, it will normally castle and seek safety on the edge of the board behind friendly pawns. In the endgame, however, the king emerges to play an active role as an offensive piece, and can assist in the promotion of the player's remaining pawns.

It is not meaningful to assign a value to the king relative to the other pieces, as it cannot be captured or exchanged and must be protected at all costs. In this sense, its value could be considered infinite. As an assessment of the king's capability as an offensive piece in the endgame, it is often considered to be slightly stronger than a bishop or knight. Emanuel Lasker gave it the value of a knight plus a pawn (i.e. four points on the scale of chess piece relative value), though some other theorists evaluate it closer to three points. It is better at defending friendly pawns than the knight is, and it is better at attacking enemy pawns than the bishop is.

History
The king's predecessor is the piece of the same name in shatranj. Like the modern king, it is the most important piece in the game and can move to any neighboring square. However, in shatranj, baring the king is a win unless the opponent can do the same immediately afterward; stalemating the king is a win; and castling does not exist.

Unicode
Unicode defines three codepoints for a king:

♔ U+2654 White Chess King

♚ U+265A Black Chess King

🨀 U+1FA00 Neutral Chess King

title: King (chess)


text:
Atoll

An atoll is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most of the approximately 440 atolls in the world are in the Pacific Ocean.

Two different, well-cited models, the subsidence model and the antecedent karst model, have been used to explain the development of atolls. According to Charles Darwin's subsidence model, the formation of an atoll is explained by the sinking of a volcanic island around which a coral fringing reef has formed. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Eventually, reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. The lagoon is not the former volcanic crater. For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (sinking of the island or rising oceans).

An alternative model for the origin of atolls is called the antecedent karst model. In the antecedent karst model, the first step in the formation of an atoll is the development of a flat top, mound-like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level. Then, when relative sea level drops below the level of the flat surface of coral reef, it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst. Because of hydrologic properties of this karst, the rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island. As a result, a saucer shaped island with a raised rim forms. When relative sea level submerges the island again, the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms the lagoon within them.

Usage
The word atoll comes from the Dhivehi word atholhu (Dhivehi: އަތޮޅު, ). Dhivehi is an Indo-Aryan language spoken in the Maldives. The word's first recorded English use was in 1625 as atollon. Charles Darwin coined the term in his monograph, The Structure and Distribution of Coral Reefs. He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island".

More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".

Distribution and size
There are approximately 440 atolls in the world.

Most of the world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands, the Coral Sea Islands, the Marshall Islands, the Tuamotu Islands, Kiribati, Tokelau, and Tuvalu) and the Indian Ocean (the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles). The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.

Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls in the world are Elizabeth Reef at 29°57′ S, and nearby Middleton Reef at 29°27′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°41′ S. The atoll closest to the Equator is Aranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator.

Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different origin of formation.

In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 m. Measured by total area, Lifou (1146 km2) is the largest raised coral atoll of the world, followed by Rennell Island (660 km2). More sources, however, list Kiritimati as the largest atoll in the world in terms of land area. It is also a raised coral atoll (321 km2 land area; according to other sources even 575 km2), 160 km2 main lagoon, 168 km2 other lagoons (according to other sources 319 km2 total lagoon size).

The geological formation known as a reef knoll refers to the elevated remains of an ancient atoll within a limestone region, appearing as a hill. The second largest atoll by dry land area is Aldabra, with 155 km2. Huvadhu Atoll, situated in the southern region of the Maldives, holds the distinction of being the largest atoll based on the sheer number of islands it comprises, with a total of 255 individual islands.

Formation

In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836. Darwin's explanation suggests that several tropical island types: from high volcanic island, through barrier reef island, to atoll, represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, and Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll.

As formulated by J. E. Hoffmeister, F. S. McNeil, E. G. Prudy, and others, the antecedent karst model argues that atolls are Pleistocene features that are the direct result of the interaction between subsidence and preferential karst dissolution that occurred in the interior of flat topped coral reefs during exposure during glacial lowstands of sea level. The elevated rims along an island created by this preferential karst dissolution become the sites of coral growth and islands of atolls when flooded during interglacial highstands.

The research of A. W. Droxler and others supports the antecedent karst model as they found that the morphology of modern atolls are independent of any influence of an underlying submerged and buried island and are not rooted to an initial fringing reef/barrier reef attached to a slowly subsiding volcanic edifice. In fact, the Neogene reefs underlying the studied modern atolls overlie and completely bury the subsided island are all non-atoll, flat-topped reefs. In fact, they found that atolls did not form doing the subsidence of an island until MIS-11, Mid-Brunhes, long after the many the former islands had been completely submerged and buried by flat topped reefs during the Neogene.

Atolls are the product of the growth of tropical marine organisms, and so these islands are found only in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside, and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward seamounts or guyots; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll. However, ancient atolls during the Mesozoic appear to exhibit different growth and evolution patterns.

Coral atolls are important as sites where dolomitization of calcite occurs. Several models have been proposed for the dolomitization of calcite and aragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacement dolomites remains problematic and controversial, it is generally accepted that seawater was the source of magnesium for dolomitization and the fluid in which calcite was dolomitized to form the dolomites found within atolls. Various processes have been invoked to drive large amounts of seawater through an atoll in order for dolomitization to occur.

Investigation by the Royal Society of London

In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs. They wanted to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific.

The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897. The third expedition in 1898 was led by Alfred Edmund Finckh.

title: Atoll