In physical geography, a dune is a hill of loose sand built by aeolian processes or the flow of water. Dunes occur in different sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the stoss side, where the sand is pushed up the dune, have a shorter "slip face" in the lee side; the valley or trough between dunes is called a slack. A "dune field" or erg is an area covered by extensive dunes. Dunes occur along some coasts; some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds. Dunes can form under the action of water flow, on sand or gravel beds of rivers and the sea-bed; the modern word "dune" came into English from French c.
1790, which in turn came from Middle Dutch dūne. Dunes are made of sand-sized particles, may consist of quartz, calcium carbonate, gypsum, or other materials; the upwind/upstream/upcurrent side of the dune is called the stoss side. Sand is pushed or bounces up the stoss side, slides down the lee side. A side of a dune that the sand has slid down is called a slip face; the Bagnold formula gives the speed. Five basic dune types are recognized: crescentic, star and parabolic. Dune areas may occur in three forms: simple and complex. Barchan dunes are crescent-shaped mounds which are wider than they are long; the lee-side slipfaces are on the concave sides of the dunes. These dunes form under winds that blow from one direction, they form separate crescents. When the sand supply is greater, they may merge into barchanoid ridges, transverse dunes; some types of crescentic dunes move more over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China's Ningxia Province, similar speeds have been recorded in the Western Desert of Egypt.
The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert. See lunettes and parabolic dues, for dunes similar to crescent-shaped ones. Abundant barchan dunes may merge into barchanoid ridges, which grade into linear transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest. Seif dunes are linear dunes with two slip faces; the two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for "sword", they may be more than 160 kilometres long, thus visible in satellite images. Seif dunes are associated with bidirectional winds; the long axes and ridges of these dunes extend along the resultant direction of sand movement. Some linear dunes merge to form Y-shaped compound dunes. Formation is debated. Bagnold, in The Physics of Blown Sand and Desert Dunes, suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, one arm or wing of the crescent elongates.
Others suggest. In the sheltered troughs between developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. Seif dunes are common in the Sahara, they range up to 300 km in length. In the southern third of the Arabian Peninsula, a vast erg, called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for 200 km and reach heights of over 300 m. Linear loess hills known; these hills appear to have been formed during the last ice age under permafrost conditions dominated by sparse tundra vegetation. Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound, they tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally, they dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth.
Oval or circular mounds that lack a slipface. Dome dunes occur at the far upwind margins of sand seas. Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to the direction of prevailing winds, are known as lunettes, source-bordering dunes and clay dunes, they may be composed of clay, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, up to 50 metres high. They occur in southern and West Africa, in parts of the western United States Texas. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes; these dunes are formed from blowout dunes where the erosion
Wind tunnels are large tubes with air moving inside. The tunnels are used to copy the actions of an object in flight. Researchers use wind tunnels to learn more about. NASA uses wind tunnels to test scale models of spacecraft; some wind tunnels are big enough to hold full-size versions of vehicles. The wind tunnel moves air around an object, making it seem like the object is flying. Most of the time, powerful fans move air through the tube; the object to be tested is fastened in the tunnel. The object can be a small model of a vehicle, it can be just a piece of a vehicle. It can be spacecraft, it can be a common object like a tennis ball. The air moving around the still object shows what would happen if the object were moving through the air. How the air moves can be studied in different ways. Smoke or dye can be seen as it moves. Threads can be attached to the object to show. Special instruments are used to measure the force of the air on the object; the earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautic research, when many attempted to develop successful heavier-than-air flying machines.
The wind tunnel was envisioned as a means of reversing the usual paradigm: instead of the air standing still and an object moving at speed through it, the same effect would be obtained if the object stood still and the air moved at speed past it. In that way a stationary observer could study the flying object in action, could measure the aerodynamic forces being imposed on it; the development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during World War II. Wind tunnel testing was considered of strategic importance during the Cold War development of supersonic aircraft and missiles. On, wind tunnel study came into its own: the effects of wind on man made structures or objects needed to be studied when buildings became tall enough to present large surfaces to the wind, the resulting forces had to be resisted by the building's internal structure. Determining such forces was required before building codes could specify the required strength of such buildings and such tests continue to be used for large or unusual buildings.
Still wind-tunnel testing was applied to automobiles, not so much to determine aerodynamic forces per se but more to determine ways to reduce the power required to move the vehicle on roadways at a given speed. In these studies, the interaction between the road and the vehicle plays a significant role, this interaction must be taken into consideration when interpreting the test results. In an actual situation the roadway is moving relative to the vehicle but the air is stationary relative to the roadway, but in the wind tunnel the air is moving relative to the roadway, while the roadway is stationary relative to the test vehicle; some automotive-test wind tunnels have incorporated moving belts under the test vehicle in an effort to approximate the actual condition, similar devices are used in wind tunnel testing of aircraft take-off and landing configurations. Wind tunnel testing of sporting equipment has been prevalent over the years, including golf clubs, golf balls, Olympic bobsleds, Olympic cyclists, race car helmets.
Helmet aerodynamics is important in open cockpit race cars. Excessive lift forces on the helmet can cause considerable neck strain on the driver, flow separation on the back side of the helmet can cause turbulent buffeting and thus blurred vision for the driver at high speeds; the advances in computational fluid dynamics modelling on high speed digital computers has reduced the demand for wind tunnel testing. However, CFD results are still not reliable and wind tunnels are used to verify CFD predictions. Air velocity and pressures are measured in several ways in wind tunnels. Air velocity through the test section is determined by Bernoulli's principle. Measurement of the dynamic pressure, the static pressure, the temperature rise in the airflow; the direction of airflow around a model can be determined by tufts of yarn attached to the aerodynamic surfaces. The direction of airflow approaching a surface can be visualized by mounting threads in the airflow ahead of and aft of the test model. Smoke or bubbles of liquid can be introduced into the airflow upstream of the test model, their path around the model can be photographed.
Aerodynamic forces on the test model are measured with beam balances, connected to the test model with beams, strings, or cables. The pressure distributions across the test model have been measured by drilling many small holes along the airflow path, using multi-tube manometers to measure the pressure at each hole. Pressure distributions can more conveniently be measured by the use of pressure-sensitive paint, in which higher local pressure is indicated by lowered fluorescence of the paint at that point. Pressure distributions can be conveniently measured by the use of pressure-sensitive pressure belts, a recent development in which multiple ultra-miniaturized pressure sensor modules are integrated into a flexible strip; the strip is attached to the aerodynamic surface with tape, it sends signals depicting the pressure distribution along its surface. Pressure distributions on a test model can be determined by performing a wake survey, in which either a single pitot tube is used to obtain multiple readings downstream of the test model, or a multiple-tube manometer is mounted downstream and all its readings are taken.
The aerodynamic properties of an object can not all remain the same for a
Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, the use of quantitative methods for their analysis. The term geophysics sometimes refers only to the geological applications: Earth's shape. However, modern geophysics organizations use a broader definition that includes the water cycle including snow and ice. Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times; the first magnetic compasses were made from lodestones, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton applied his theory of mechanics to the precession of the equinox. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, geophysics played an essential role in the development of the theory of plate tectonics.
Geophysics is applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. In Exploration Geophysics, Geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, locate groundwater, find archaeological relics, determine the thickness of glaciers and soils, assess sites for environmental remediation. Geophysics is a interdisciplinary subject, geophysicists contribute to every area of the Earth sciences. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings; the gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide. Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.
Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can be used to look for mineral deposits. The surface gravitational field provides information on the dynamics of tectonic plates; the geopotential surface called. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents; the Earth is cooling, the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are the primordial heat and radioactivity, although there are contributions from phase transitions. Heat is carried to the surface by thermal convection, although there are two thermal boundary layers – the core-mantle boundary and the lithosphere – in which heat is transported by conduction; some heat is carried up from the bottom of the mantle by mantle plumes. The heat flow at the Earth's surface is about 4.2 × 1013 W, it is a potential source of geothermal energy.
Seismic waves are vibrations that travel along its surface. The entire Earth can oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source; the locations of earthquakes provide information on mantle convection. Recording of seismic waves from controlled sources provide information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth. Earthquakes pose a risk to humans.
Understanding their mechanisms, which depend on the type of earthquake, can lead to better estimates of earthquake risk and improvements in earthquake engineering. Although we notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 volts per meter. Relative to the solid Earth, the atmosphere has a net positive charge due to bombardment by cosmic rays. A current of about 1800 amperes flows in the global circuit, it flows downward from the ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the sprites above. A variety of electric methods are used in geophysical survey; some measure spontaneous potential, a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in the oceans, they have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies across the Earth's per
Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than coarser than silt. Sand can refer to a textural class of soil or soil type; the composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica in the form of quartz. The second most common type of sand is calcium carbonate, for example, created, over the past half billion years, by various forms of life, like coral and shellfish. For example, it is the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean. Sand is a non-renewable resource over human timescales, sand suitable for making concrete is in high demand. Desert sand, although plentiful, is not suitable for concrete, 50 billion tons of beach sand and fossil sand is needed each year for construction; the exact definition of sand varies.
The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves, defines sand as particles with a diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between silt; the size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. The grains of sand in Archimedes Sand Reckoner written around 240 BCE, were 0.02 mm in diameter. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm. Sand feels gritty when rubbed between the fingers.
Silt, by comparison, feels like flour). ISO 14688 grades sands as fine and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In the United States, sand is divided into five sub-categories based on size: fine sand, fine sand, medium sand, coarse sand, coarse sand; these sizes are based on the Krumbein phi scale, where size in Φ = -log2D. On this scale, for sand the value of Φ varies from −1 to +4, with the divisions between sub-categories at whole numbers; the most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica in the form of quartz, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering. The composition of mineral sand is variable, depending on the local rock sources and conditions; the bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material, suggesting sand formation depends on living organisms, too.
The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from weathering and erosion of a granitic rock outcrop; some sands contain magnetite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are green in color, as are sands derived from basaltic lava with a high olivine content. Many sands those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow color. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones. Rocks erode/weather over a long period of time by water and wind, their sediments are transported downstream; these sediments continue to break apart into smaller pieces. The type of rock the sediment originated from and the intensity of the environment gives different compositions of sand.
The most common rock to form sand is Granite, where the Feldspar minerals dissolve faster than the Quartz, causing the rock to break apart into small pieces. In high energy environments rocks break apart much faster than in more calm settings. For example, Granite rocks this means more Feldspar minerals in the sand because it wouldn't have had time to dissolve; the term for sand formed by weathering is epiclastic. Sand from rivers are collected either from the river itself or its flood plain, accounts for the majority of the sand used in the construction industry; because if this, many small rivers have been depleted, causing environmental concern and economic losses to adjacent land. The rate of sand mining in such areas outweighs the rate the sand can replenish, making it a non-renewable resource. Sand dunes are a consequence of wind deposition; the Sahara Desert is dry because of its geographic location and is known for its vast sand dunes. They exist here because little vegetation is able to grow and there's not a lot of water.
Over time, wind blow
Wind speed, or wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure due to changes in temperature. Note that wind direction is almost parallel to isobars, due to Earth's rotation. Wind speed affects weather forecasting and maritime operations, construction projects and metabolism rate of many plant species, countless other implications. Wind speed is now measured with an anemometer, but can be classified using the older Beaufort scale, based on personal observation of defined wind effects. Wind speed is affected by a number of situations, operating on varying scales; these include the pressure gradient, Rossby waves and jet streams, local weather conditions. There are links to be found between wind speed and wind direction, notably with the pressure gradient and terrain conditions. Pressure gradient is a term to describe the difference in air pressure between two points in the atmosphere or on the surface of the Earth, it is vital to wind speed, because the greater the difference in pressure, the faster the wind flows to balance out the variation.
The pressure gradient, when combined with the Coriolis effect and friction influences wind direction. Rossby waves are strong winds in the upper troposphere; these operate on a global move from West to East. The Rossby waves are themselves a different wind speed from what we experience in the lower troposphere. Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes and cyclones as freak weather conditions can drastically affect the flow velocity of the wind; the fastest wind speed not related to tornadoes recorded was during the passage of Tropical Cyclone Olivia on 10 April 1996: an automatic weather station on Barrow Island, registered a maximum wind gust of 408 km/h. The wind gust was evaluated by the WMO Evaluation Panel who found that the anemometer was mechanically sound and the gust was within statistical probability and ratified the measurement in 2010; the anemometer was mounted 10 m above ground level. During the cyclone, several extreme gusts of greater than 300 km/h were recorded, with a maximum 5-minute mean speed of 176 km/h, the extreme gust factor was in the order of 2.27–2.75 times the mean wind speed.
The pattern and scales of the gusts suggest that a mesovortex was embedded in the strong eyewall of the cyclone. The second-highest surface wind speed officially recorded is 372 km/h at the Mount Washington Observatory: 6,288 ft above sea level in the US on 12 April 1934, using a heated anemometer; the anemometer designed for use on Mount Washington was tested by the US National Weather Bureau and confirmed to be accurate. Wind speeds within certain atmospheric phenomena may exceed these values but have never been measured. Directly measuring these tornadic winds is done as the violent wind would destroy the instruments. A method of estimating speed is to use Doppler on Wheels to sense the wind speeds remotely, using this method, the figure of 486 km/h during the 1999 Bridge Creek–Moore tornado in Oklahoma on 3 May 1999 is quoted as the highest-recorded surface wind speed, although another figure of 512 kilometres per hour has been quoted for the same tornado, yet another number used by the Center for Severe Weather Research for that measurement is 486 ± 32 km/h.
However, speeds measured by Doppler radar are not considered official records. An anemometer is one of the tools used to measure wind speed. A device consisting of a vertical pillar and three or four concave cups, the anemometer captures the horizontal movement of air particles. Another tool used to measure wind velocity includes a GPS combined with pitot tube. A fluid flow velocity tool, the Pitot tube is used to determine the air velocity of an aircraft. Wind speed is a common factor in the design of buildings around the world, it is the governing factor in the required lateral strength of a structure's design. In the United States, the wind speed used in design is referred to as a "3-second gust", the highest sustained gust over a 3-second period having a probability of being exceeded per year of 1 in 50; this design wind speed is accepted by most building codes in the United States and governs the lateral design of buildings and structures. In Canada, reference wind pressures are used in design and are based on the "mean hourly" wind speed having a probability of being exceeded per year of 1 in 50.
The reference wind pressure is calculated in Pascals using the following equation: q=pV² where p is the air density in kg/m³ and V is wind speed in m/s. Wind speeds have been reported with a variety of averaging times which designers may have to take into account. To convert wind speeds from one averaging time to another, the Durst Curve was developed which defines the relation between probable maximum wind speed averaged over t seconds, Vt, mean wind speed over one hour V3600. Beaufort scale Fujita scale and Enhanced Fujita Scale Prevailing wind Saffir–Simpson Hurricane Scale TORRO scale Wind direction Knot International Building Code American Society of Civil Engineers Media related to Wind spe
Ralph Alger Bagnold
Ralph Alger Bagnold, OBE, FRS, was a 20th Century English desert explorer and soldier. In 1932 he staged the first recorded East-to-West crossing of the Libyan Desert, his work in the field of Aeolian processes produced the book'The Physics of Blown Sand and Desert Dunes', used by United States' space agency "NASA" in its study of the terrain of the planet Mars, the "Bagnold Dunes" on Mars' surface being named after him by the organization. During World War 2 he was a soldier in the British Army, in which he founded the behind-the-lines reconnaissance and raiding unit the "Long Range Desert Group", serving as its first commanding officer in the North Africa Campaign. Bagnold was born in England, his father, Colonel Arthur Henry Bagnold, participated in the rescue expedition of 1884–85 to rescue General Gordon in Khartoum. His sister was playwright Enid Bagnold, who wrote the 1935 novel National Velvet. After Malvern College, he attended Woolwich. In 1915, Ralph Bagnold followed in his father's footsteps and was commissioned into the Royal Engineers.
He spent three years in the trenches in France, being Mentioned in Despatches in 1917 and receiving the Belgian Order of Leopold in 1919. After the war Bagnold studied engineering at Gonville and Caius College, Cambridge University, obtaining an MA before returning to active duty with the British Army in 1920 with the Royal Corps of Signals, he served in Cairo and the North West Frontier, where he was again mentioned in despatches. In both of these locations he spent much of his leave exploring the local deserts. After having read Ahmed Hassanein's "Lost Oasis" he spent one such expedition in 1929 using a Ford Model A automobile and two Ford lorries exploring the vast swathe of desert from Cairo to Ain Dalla, an area reputed to contain the mythical city of Zerzura. After a brief period of half-pay, he left the Army in 1935 but rejoined upon the outbreak of World War II. Bagnold and his travelling companions were early pioneers in the use of motor vehicles to explore the desert. In 1932, Bagnold explored the Mourdi Depression, now in Chad, found implements dated to the Palaeolithic period in the valley.
Bagnold wrote of his travels in the book Libyan Sands: Travel in a Dead World. He is credited with developing a sun compass, not affected by the large iron ore deposits found in the desert areas or by metal vehicles as a magnetic compass might be. During the 1930s his group began the practice of reducing tyre pressure when driving over loose sand. In addition, Bagnold is credited with devising a method of driving over the large sand dunes found in the "sand seas" of the Libyan Desert, he wrote, "I increased speed.... A huge glaring wall of yellow shot up high into the sky; the lorry tipped violently backwards—and we rose as in a lift, smoothly without vibration. We floated up on a yellow cloud. All the accustomed car movements had ceased, it was incredible..." However, noted Fitzroy Maclean, "too much dash had its penalties. Many of the dunes fell away at the far side and if you arrived at the top at full speed, you were to plunge headlong over the precipice.... and end up with your truck upside down on top of you."
Bagnold wrote, "Never in our peacetime travels had we imagined that war could reach the enormous empty solitudes of the inner desert, walled off by sheer distance, lack of water, impassable seas of sand dunes. Little did we dream that any of the special equipment and techniques we had evolved for long-distance travel, for navigation, would be put to serious use." On 10 June 1940 the Fascist dictator of Italy Benito Mussolini declared war on the United Kingdom in alliance with the III Reich whilst Bagnold was in Cairo due to an accident involving a troopship collision that he was on interrupting his journey elsewhere, on hearing the news and realizing that North Africa was about to become a theatre of war, he requested an interview with General General Wavell, Commander-in-Chief Middle East. Having secured it, Bagnold suggested to Wavell using his knowledge of the terrain in North Africa for the establishment of a mobile scouting force for desert operations against the Italian Armed Forces in Libya, which Wavell was charged with defeating in the field.
During the conversation Wavell asked Bagnold what he would do if he found that the Italians were not doing anything beyond the Libyan coast in the desert interior, Bagnold in reply stating that the new unit that he had in mind might be able to commit "acts of piracy". Wavell granted Bagnold authority to form a unit along these lines, with it being constituted in July 1940 with the name "Long Range Desert Group". After assembling its first formation Bagnold was the L. R. D. G.'s Commanding Officer until August 1941, when he handed over command to Guy Prendergast on being promoted to the post of "Inspector of Desert Troops". In the war he was promoted to the post of "Deputy Signal Officer-in-Chief Middle East", with the temporary rank of Brigadier. On 7 June 1944 Bagnold retired from the British Army with the end of military operations in North Africa after the Axis powers' defeat in that theatre, and returned to his scientific interests, being elected to a Fellowship of the Royal Society in the same year.
After the war Bagnold continued to work in the field of the geological science, he published academic papers into his nineties. He made significant contributions to the understanding of desert terrain such as sand dunes and sheets, he developed the dimensionless "Bagnold number" and "Bagnold formula" for characterising sand flo
A barchan or barkhan dune is a crescent-shaped dune. The term was introduced in 1881 by Russian naturalist Alexander von Middendorf, for crescent-shaped sand dunes in Turkestan and other inland desert regions. Barchans face the wind, appearing convex and are produced by wind action predominantly from one direction, they are a common landform in sandy deserts all over the world and are arc-shaped, markedly asymmetrical in cross section, with a gentle slope facing toward the wind sand ridge, comprising well-sorted sand. This type of dune possesses two "horns" that face downwind, with the steeper slope known as the slip face, facing away from the wind, downwind, at the angle of repose of the sand in question 30–35 degrees for medium-fine dry sand; the upwind side is packed by the wind, stands at about 15 degrees. Barchans may be 9–30 m high and 370 m wide at the base measured perpendicular to the wind. Simple barchan dunes may appear as larger, compound barchan or megabarchan dunes, which can migrate with the wind as a result of erosion on the windward side and deposition on the leeward side, at a rate of migration ranging from about a metre to a hundred metres per year.
Barchans occur as groups of isolated dunes and may form chains that extend across a plain in the direction of the prevailing wind. Barchans and megabarchans may coalesce into ridges. Dune collisions and changes in wind direction that spawn new barchans from the horns of the old govern the size distribution in a given field; as barchan dunes migrate, smaller dunes outpace larger dunes, catching-up the rear of the larger dune and appear to punch through the large dune to appear on the other side. The process appears superficially similar to waves of light, sound, or water that pass directly through each other, but the detailed mechanism is different; the dunes emulate soliton behavior, but unlike solitons, which flow through a medium leaving it undisturbed, the sand particles themselves are moved. When the smaller dune catches up the larger dune, the winds begin to deposit sand on the rear dune while blowing sand off the front dune without replenishing it; the rear dune has assumed dimensions similar to the former front dune which has now become a smaller, faster moving dune that pulls away with the wind.
Barchan dunes have been observed on Mars, where the thin atmosphere produces winds strong enough to move sand and dust. Great Sand Dunes National Park and Preserve – American national park, large sand dunes on eastern edge of the San Luis Valley, Sangre de Cristo Range, United States Sastrugi known as Zastruga, about snowforms of similar mechanism Dune types—Great Sand Dunes National Park Bibliography of Aeolian Research