History.

Cooling towers originated in the 19th century through the development of condensers for use with the steam engine. Condensers use relatively cool water, via various means, to condense the steam coming out of the cylinders or turbines. This reduces the back pressure, which in turn reduces the steam consumption, and thus the fuel consumption, while at the same time increasing power and recycling boiler-water. However the condensers require an ample supply of cooling water, without which they are impractical. The consumption of cooling water by inland processing and power plants is estimated to reduce power availability for the majority of thermal power plants by 2040–2069. While water usage is not an issue with marine engines, it forms a significant limitation for many land-based systems.  By the turn of the 20th century, several evaporative methods of recycling cooling water were in use in areas lacking an established water supply, as well as in urban locations where municipal water mains may not be of sufficient supply; reliable in times of demand; or otherwise adequate to meet cooling needs. In areas with available land, the systems took the form of cooling ponds; in areas with limited land, such as in cities, they took the form of cooling towers.  These early towers were positioned either on the rooftops of buildings or as free-standing structures, supplied with air by fans or relying on natural airflow. An American engineering textbook from 1911 described one design as "a circular or rectangular shell of light plate—in effect, a chimney stack much shortened vertically (20 to 40 ft. high) and very much enlarged laterally. At the top is a set of distributing troughs, to which the water from the condenser must be pumped; from these it trickles down over "mats" made of wooden slats or woven wire screens, which fill the space within the tower."  A hyperboloid cooling tower was patented by the Dutch engineers Frederik van Iterson and Gerard Kuypers in 1918. The first hyperboloid cooling towers were built in 1918 near Heerlen. The first ones in the United Kingdom were built in 1924 at Lister Drive power station in Liverpool, England, to cool water used at a coal-fired electrical power station.

Classification by use.

Heating, ventilation and air conditioning (HVAC).

An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. In areas with a hot climate, large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers. HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A "ton" of air-conditioning is defined as the removal of. The "equivalent ton" on the cooling tower side actually rejects about due to the additional waste heat-equivalent of the energy needed to drive the chiller's compressor. This "equivalent ton" is defined as the heat rejection in cooling or of water 10 °F (6 °C), which amounts to, assuming a chiller coefficient of performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 14.  Cooling towers are also used in HVAC systems that have multiple water source heat pumps that share a common piping "water loop". In this type of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the atmosphere. By contrast, when the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated. When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized to prevent freeze damage), and heat is supplied by other means, usually from separate boilers.

Industrial cooling towers.

Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 US gallons per minute) and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour, equivalent to one cubic metre every second).  If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour A large cooling water intake typically kills millions of fish and larvae annually, as the organisms are impinged on the intake screens. A large amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. Elevated water temperatures can kill fish and other aquatic organisms (see "thermal pollution"), or can also cause an increase in undesirable organisms such as invasive species of zebra mussels or algae. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water. Evaporative cooling water cannot be used for subsequent purposes (other than rain somewhere), whereas surface-only cooling water can be re-used. Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.  Petroleum refineries also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day ( per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.  The world's tallest cooling tower is the tall cooling tower of Kalisindh Thermal Power Station in Jhalawar, Rajasthan, India.

Classification by build.

Package type.

These types of cooling towers are factory preassembled, and can be simply transported on trucks, as they are compact machines. The capacity of package type towers is limited and, for that reason, they are usually preferred by facilities with low heat rejection requirements such as food processing plants, textile plants, some chemical processing plants, or buildings like hospitals, hotels, malls, automotive factories etc.  Due to their frequent use in or near residential areas, sound level control is a relatively more important issue for package type cooling towers.

Field erected type.

Facilities such as power plants, steel processing plants, petroleum refineries, or petrochemical plants usually install field erected type cooling towers due to their greater capacity for heat rejection. Field erected towers are usually much larger in size compared to the package type cooling towers.  A typical field erected cooling tower has a pultruded fiber-reinforced plastic (FRP) structure, FRP cladding, a mechanical unit for air draft, and a drift eliminator.

Heat transfer methods.

With respect to the heat transfer mechanism employed, the main types are: In a wet cooling tower (or open circuit cooling tower), the warm water can be cooled to a temperature "lower" than the ambient air dry-bulb temperature, if the air is relatively dry (see dew point and psychrometrics). As ambient air is drawn past a flow of water, a small portion of the water evaporates, and the energy required to evaporate that portion of the water is taken from the remaining mass of water, thus reducing its temperature. Approximately of heat energy is absorbed for the evaporated water. Evaporation results in saturated air conditions, lowering the temperature of the water processed by the tower to a value close to wet-bulb temperature, which is lower than the ambient dry-bulb temperature, the difference determined by the initial humidity of the ambient air.  To achieve better performance (more cooling), a medium called "fill" is used to increase the surface area and the time of contact between the air and water flows. "Splash fill" consists of material placed to interrupt the water flow causing splashing. "Film fill" is composed of thin sheets of material (usually PVC) upon which the water flows. Both methods create increased surface area and time of contact between the fluid (water) and the gas (air), to improve heat transfer.

Air flow generation methods.

With respect to drawing air through the tower, there are three types of cooling towers:  Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency. These designs are popularly associated with nuclear power plants. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants as well. Conversely, not all nuclear power plants have cooling towers, and some instead cool their heat exchangers with lake, river or ocean water.  Thermal efficiencies up to 92% have been observed in hybrid cooling towers.

Categorization by air-to-water flow.

Crossflow.

Typically lower initial and long-term cost, mostly due to pump requirements. Crossflow is a design in which the air flow is directed perpendicular to the water flow (see diagram at left). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum volume. Lastly, a fan forces the air out into the atmosphere.  A "distribution" or "hot water basin" consisting of a deep pan with holes or "nozzles" in its bottom is located near the top of a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material.  Advantages of the crossflow design:  Disadvantages of the crossflow design:

Counterflow.

In a counterflow design, the air flow is directly opposite to the water flow (see diagram at left). Air flow first enters an open area beneath the fill media, and is then drawn up vertically. The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow. Advantages of the counterflow design:  Disadvantages of the counterflow design:

Common aspects.

Common aspects of both designs:  Both crossflow and counterflow designs can be used in natural draft and in mechanical draft cooling towers.

Wet cooling tower material balance.

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up volumetric flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles.  In the adjacent diagram, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage or drift (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off or blown down (D) for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.  Using these flow rates and concentration dimensional units: A water balance around the entire system is then:  Since the evaporated water (E) has no salts, a chloride balance around the system is:  and, therefore:  From a simplified heat balance around the cooling tower:  Windage (or drift) losses (W) is the amount of total tower water flow that is entrained in the flow of air to the atmosphere. From large-scale industrial cooling towers, in the absence of manufacturer's data, it may be assumed to be:

Cycles of concentration.

Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Discharge of draw-off (or blowdown) is used principally to control the buildup of these minerals.  The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Make-up waters from ground water supplies (such as wells) are usually higher in minerals, and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.  Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for New York City, has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.  Since higher cycles of concentration represent less make-up water, water conservation efforts may focus on increasing cycles of concentration. Highly treated recycled water may be an effective means of reducing cooling tower consumption of potable water, in regions where potable water is scarce.

Maintenance.

Clean visible dirt & debris from the cold water basin and surfaces with any visible biofilm (i.e., slime).  Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored.  Regular checks of water quality (specifically the aerobic bacteria levels) using dipslides should be taken as the presence of other organisms can support legionella by producing the organic nutrients that it needs to thrive.

Water treatment.

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered to remove particulates, and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. Under certain conditions, a biofilm of micro-organisms such as bacteria, fungi and algae can grow very rapidly in the cooling water, and can reduce the heat transfer efficiency of the cooling tower. Biofilm can be reduced or prevented by using chlorine or other chemicals. A normal industrial practice is to use two biocides, such as oxidizing and non-oxidizing types to complement each other's strengths and weaknesses, and to ensure a broader spectrum of attack. In most cases, a continual low level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides.  The water consumption of the cooling tower comes from Drift, Bleed-off, Evaporation loss, The water that is immediately replenished into the cooling tower due to loss is called Make-up Water. The function of make-up water is to make machinery and equipment run safely and stably.

Legionnaires' disease.

Another very important reason for using biocides in cooling towers is to prevent the growth of "Legionella", including species that cause legionellosis or Legionnaires' disease, most notably "L. pneumophila", or "Mycobacterium avium". The various "Legionella" species are the cause of Legionnaires' disease in humans and transmission is via exposure to aerosols—the inhalation of mist droplets containing the bacteria. Common sources of "Legionella" include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks.  French researchers found that "Legionella" bacteria travelled up to through the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.  Drift (or windage) is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0.001–0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow to prevent the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for "Legionella" or water treatment chemical exposure.  The CDC does not recommend that health-care facilities regularly test for the "Legionella pneumophila" bacteria. Scheduled microbiologic monitoring for "Legionella" remains controversial because its presence is not necessarily evidence of a potential for causing disease. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit "Legionella", but does not recommend regularly-scheduled microbiologic assays for the bacteria. However, scheduled monitoring of potable water within a hospital might be considered in certain settings where persons are highly susceptible to illness and mortality from "Legionella" infection (e.g. hematopoietic stem cell transplantation units, or solid organ transplant units). Also, after an outbreak of legionellosis, health officials agree that monitoring is necessary to identify the source and to evaluate the efficacy of biocides or other prevention measures.  Studies have found "Legionella" in 40% to 60% of cooling towers.

Fog production.

Under certain ambient conditions, plumes of water vapor can be seen rising out of the discharge from a cooling tower, and can be mistaken as smoke from a fire. If the outdoor air is at or near saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged, which is seen as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates. Fog and clouds associated with cooling towers can be described as homogenitus, as with other clouds of man-made origin, such as contrails and ship tracks.  This phenomenon can be prevented by decreasing the relative humidity of the saturated discharge air. For that purpose, in hybrid towers, saturated discharge air is mixed with heated low relative humidity air. Some air enters the tower above drift eliminator level, passing through heat exchangers. The relative humidity of the dry air is even more decreased instantly as being heated while entering the tower. The discharged mixture has a relatively lower relative humidity and the fog is invisible.

Salt emission pollution.

When wet cooling towers with seawater make-up are installed in various industries located in or near coastal areas, the drift of fine droplets emitted from the cooling towers contain nearly 6% sodium chloride which deposits on the nearby land areas. This deposition of sodium salts on the nearby agriculture/vegetative lands can convert them into sodic saline or sodic alkaline soils depending on the nature of the soil and enhance the sodicity of ground and surface water. The salt deposition problem from such cooling towers aggravates where national pollution control standards are not imposed or not implemented to minimize the drift emissions from wet cooling towers using seawater make-up.  Respirable suspended particulate matter, of less than 10 micrometers (μm) in size, can be present in the drift from cooling towers. Larger particles above 10 μm in size are generally filtered out in the nose and throat via cilia and mucus but particulate matter smaller than 10 μm, referred to as PM, can settle in the bronchi and lungs and cause health problems. Similarly, particles smaller than 2.5 μm, (PM), tend to penetrate into the gas exchange regions of the lung, and very small particles (less than 100 nanometers) may pass through the lungs to affect other organs. Though the total particulate emissions from wet cooling towers with fresh water make-up is much less, they contain more PM and PM than the total emissions from wet cooling towers with sea water make-up. This is due to lesser salt content in fresh water drift (below 2,000 ppm) compared to the salt content of sea water drift (60,000 ppm).

Use as a flue-gas stack.

At some modern power stations equipped with flue gas purification, such as the Großkrotzenburg Power Station and the Rostock Power Station, the cooling tower is also used as a flue-gas stack (industrial chimney), thus saving the cost of a separate chimney structure. At plants without flue gas purification, problems with corrosion may occur, due to reactions of raw flue gas with water to form acids.  Sometimes, natural draft cooling towers are constructed with structural steel in place of concrete (RCC) when the construction time of natural draft cooling tower is exceeding the construction time of the rest of the plant or the local soil is of poor strength to bear the heavy weight of RCC cooling towers or cement prices are higher at a site to opt for cheaper natural draft cooling towers made of structural steel.

Operation in freezing weather.

Some cooling towers (such as smaller building air conditioning systems) are shut down seasonally, drained, and winterized to prevent freeze damage.  During the winter, other sites continuously operate cooling towers with water leaving the tower. Basin heaters, tower draindown, and other freeze protection methods are often employed in cold climates. Operational cooling towers with malfunctions can freeze during very cold weather. Typically, freezing starts at the corners of a cooling tower with a reduced or absent heat load. Severe freezing conditions can create growing volumes of ice, resulting in increased structural loads which can cause structural damage or collapse.  To prevent freezing, the following procedures are used:

Fire hazard.

Cooling towers constructed in whole or in part of combustible materials can support internal fire propagation. Such fires can become very intense, due to the high surface-volume ratio of the towers, and fires can be further intensified by natural convection or fan-assisted draft. The resulting damage can be sufficiently severe to require the replacement of the entire cell or tower structure. For this reason, some codes and standards recommend that combustible cooling towers be provided with an automatic fire sprinkler system. Fires can propagate internally within the tower structure when the cell is not in operation (such as for maintenance or construction), and even while the tower is in operation, especially those of the induced-draft type, because of the existence of relatively dry areas within the towers.

Structural stability.

Being very large structures, cooling towers are susceptible to wind damage, and several spectacular failures have occurred in the past. At Ferrybridge power station on 1 November 1965, the station was the site of a major structural failure, when three of the cooling towers collapsed owing to vibrations in winds. Although the structures had been built to withstand higher wind speeds, the shape of the cooling towers caused westerly winds to be funneled into the towers themselves, creating a vortex. Three out of the original eight cooling towers were destroyed, and the remaining five were severely damaged. The towers were later rebuilt and all eight cooling towers were strengthened to tolerate adverse weather conditions. Building codes were changed to include improved structural support, and wind tunnel tests were introduced to check tower structures and configuration.

A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of "closed circuit dry cooling towers", rely solely on air to cool the working fluid to near the dry-bulb air temperature.

Ein Kühlturm (auch Rückkühlwerk) ist eine Anlage, die mittels eines Wärmeübertragers überschüssige oder technisch nicht mehr nutzbare Wärme aus Kraftwerks- oder Industrieprozessen abführt.


Geography.

First established in the 6th-century as one of four administrative (military) divisions by the Sassanids, the scope of the region has varied considerably during its nearly 1,500-year history. Initially, the Khorasan division of the Sassanid empire covered the north-eastern military gains of the empire, at its height including cities such as Nishapur, Herat, Merv, Faryab, Talaqan (around modern Turkmenabat), Balkh, Bukhara, Badghis, Abiward, Gharjistan, Tus, Sarakhs and Gurgan.  With the rise of the Umayyad Caliphate, the designation was inherited and likewise stretched as far as their military gains in the east, starting off with the military installations at Nishapur and Merv, slowly expanding eastwards into Tokharistan and Sogdia. Under the Caliphs, Khorasan was the name of one of the three political zones under their dominion (the other two being "Eraq-e Arab" "Arabic Iraq" and "Eraq-e Ajam" "Non-Arabic Iraq or Persian Iraq"). Under the Umayyad and Abbasid caliphates, Khorasan was divided into four major sections or quarters ("rub′"), each section based on a single major city: Nishapur, Merv, Herat and Balkh. By the 10th-century, Ibn Khordadbeh and the Hudud al-'Alam mentions what roughly encompasses the previous regions of Abarshahr, Tokharistan and Sogdia as "Khwarasan" proper. They further report the southern part of the Hindu Kush, i.e. the regions of Sistan, Ghor, Rukhkhudh, Zabulistan and Kabul etc. to make up the "Khwarasan" "marches", a frontier region between Khwarasan and Hindustan which at the time would have been in a process of Islamization.  By the late Middle Ages, the term lost its administrative significance, in the west only being loosely applied among the Turko-Persian dynasties of modern Iran to all its territories that lay east and north-east of the Dasht-e Kavir desert. It was therefore subjected to constant change, as the size of their empires changed. In the east, "Khwarasan" likewise became a term associated with the great urban centers of Central Asia. It is mentioned in the Memoirs of Babur that: In modern times, the term has been source of great nostalgia and nationalism, especially amongst the Tajiks of Central Asia. Many Tajiks regard Khorasan as an integral part of their national myth, which has preserved an interest in the term, including its meaning and cultural significance, both in common discussion and academia, despite its falling out of political use in the region. According to Ghulam Mohammad Ghobar, Afghanistan's current Persian-speaking territories formed the major portion of Khorasān, as two of the four main capitals of Khorasān (Herat and Balkh) are now located in Afghanistan. Ghobar uses the terms ""Proper Khorasan"" and ""Improper Khorasan"" in his book to distinguish between the usage of Khorasān in its strict sense and its usage in a loose sense. According to him, Proper Khorasan contained regions lying between Balkh in the east, Merv in the north, Sistan in the south, Nishapur in the west and Herat, known as the "Pearl of Khorasan", in the center. Improper Khorasan's boundaries extended to as far as Hazarajat and Kabul in the east, Baluchistan in the south, Transoxiana and Khwarezm in the north, and Damghan and Gorgan in the west.

History.

Before the region fell to Alexander the Great in 330 BC, it was part of the Persian Achaemenid Empire and prior to that it was occupied by the Medes. The land that became known as Khorasan in geography of Eratosthenes was recognized as Ariana by Greeks at that time, which made up Greater Iran or the land where Zoroastrianism was the dominant religion. The southeastern region of Khorasan fell to the Kushan Empire in the 1st century AD. The Kushan rulers built a capital in modern-day Afghanistan at Bagram and are believed to have built the famous Buddhas of Bamiyan. Numerous Buddhist temples and buried cities have been found in Afghanistan. However, the region of Khorasan remained predominantly Zoroastrian but there were also Manichaeists, sun worshippers, Christians, Pagans, Shamanists, Buddhists, Jews, Hindus and others. One of the three great fire-temples of the Sassanids "Azar-burzin Mehr" is situated near Sabzevar in Iran. The boundary of the region began changing until the Kushans and Sassanids merged to form the Kushano-Sassanian civilization.

Sasanian era.

During the Sasanian era, likely in the reign of Khusrow I, Persia was divided into four regions (known as "kust" Middle Persian), Khwārvarān in the west, apāxtar in the north, nīmrūz in the south and Khurasan in the east. Since the Sasanian territories were more or less remained stable up to Islamic conquests, it can be concluded that Sasanian Khorasan was bordered to the south by Sistan and Kerman, to the west by the central deserts of modern Iran, and to the east by China and India.  In Sasanian era, Khurasan was further divided into four smaller regions, and each region was ruled by a marzban. These four regions were Nishapur, Marv, Herat and Balkh.  Khorasan in the east saw some conflict with the Hephthalites who became the new rulers in the area but the borders remained stable. Being the eastern parts of the Sassanids and further away from Arabia, Khorasan region was conquered after the remaining Persia. The last Sassanid king of Persia, Yazdgerd III, moved the throne to Khorasan following the Arab invasion in the western parts of the empire. After the assassination of the king, Khorasan was conquered by Arab Muslims in 647 AD. Like other provinces of Persia it became a province of the Umayyad Caliphate.

Arab conquest.

The first movement against the Arab conquest was led by Abu Muslim Khorasani between 747 and 750. He helped the Abbasids come to power but was later killed by Al-Mansur, an Abbasid Caliph. The first independent kingdom from Arab rule was established in Khorasan by Tahir Phoshanji in 821, but it seems that it was more a matter of political and territorial gain. Tahir had helped the Caliph subdue other nationalistic movements in other parts of Persia such as Maziar's movement in Tabaristan.  Other major independent dynasties who ruled over Khorasan were the Saffarids from Zaranj (861–1003), Samanids from Bukhara (875–999), Ghaznavids from Ghazni (963–1167), Seljuqs (1037–1194), Khwarezmids (1077–1231), Ghurids (1149–1212), and Timurids (1370–1506). Some of these dynasties were not Persian by ethnicity. The periods of Turkic Ghaznavids and Turco-Mongol Timurids are considered as some of the most brilliant eras of Khorasan's history. During these periods, there was a great cultural awakening. Many famous poets, scientists and scholars lived in this area. Numerous valuable works in Persian literature were written.  Between the early 16th and early 18th centuries, parts of Khorasan were contested between the Safavids and the Uzbeks. A part of the Khorasan region was conquered in 1722 by the Ghilji Pashtuns from Kandahar and became part of the Hotaki dynasty from 1722 to 1729. Nader Shah recaptured Khorasan in 1729 and chose Mashhad as the capital of Persia. Following his assassination in 1747, the eastern parts of Khorasan, including Herat was annexed with the Durrani Empire. Mashhad area was under control of Nader Shah's grandson Shahrukh Afshar until it was captured by the Qajar dynasty in 1796. In 1856, the Iranians, under the Qajar dynasty, briefly recaptured Herat; by the Treaty of Paris of 1857, signed between Iran and the British Empire to end the Anglo-Persian War, the Iranian troops withdrew from Herat. Later, in 1881, Iran relinquished its claims to a part of the northern areas of Khorasan to the Russian Empire, principally comprising Merv, by the Treaty of Akhal (also known as the "Treaty of Akhal-Khorasan").

Cultural importance.

Khorasan has had a great cultural importance among other regions in Greater Iran. The literary New Persian language developed in Khorasan and Transoxiana and gradually supplanted the Parthian language. The New Persian literature arose and flourished in Khorasan and Transoxiana where the early Iranian dynasties such as Tahirids, Samanids, Saffirids and Ghaznavids (a Turco-Persian dynasty) were based.The early Persian poets such as Rudaki, Shahid Balkhi, Abu al-Abbas Marwazi, Abu Hafas Sughdi, and others were from Khorasan. Moreover, Ferdowsi and Rumi were also from Khorasan.  Until the devastating Mongol invasion of the thirteenth century, Khorasan remained the cultural capital of Persia. It has produced scientists such as Avicenna, Al-Farabi, Al-Biruni, Omar Khayyam, Al-Khwarizmi, Abu Ma'shar al-Balkhi (known as Albumasar or Albuxar in the west), Alfraganus, Abu Wafa, Nasir al-Din al-Tusi, Sharaf al-Dīn al-Ṭūsī, and many others who are widely well known for their significant contributions in various domains such as mathematics, astronomy, medicine, physics, geography, and geology. Khorasan artisans contributed to the spread of technology and goods along the ancient trade routes have been traced to this ancient culture, including art objects, textiles and zoomorphic metalworks. Decorative antecedents of the famous "singing bowls" of Asia may have been invented in ancient Khorasan.  In Islamic theology, jurisprudence and philosophy, and in Hadith collection, many of the greatest Islamic scholars came from Khorasan, namely Imam Bukhari, Imam Muslim, Abu Dawood, Al-Tirmidhi, Al-Nasa'i, Al-Ghazali, Al-Juwayni, Abu Mansur Maturidi, Fakhruddin al-Razi, and others. Shaykh Tusi, a Shi'a scholar and Al-Zamakhshari, the famous Mutazilite scholar, also lived in Khorasan.

Khorāsān (;, ), sometimes called Greater Khorasan, is a historical region which formed the northeast province of Greater Iran. The name signifies "the Land of the Sun" or "the Eastern Province."

Chorasan oder Churasan (, manchmal auch "Chorassan" oder "Khurasan", im Englischen meist "Khorassan" oder "Khorasan" geschrieben), mit nördlicheren Regionen zusammengefasst als Chorasan und Mā warā’ an-nahr (arabisch-), ist eine historische Region in Zentralasien im Gebiet der heutigen Staaten Afghanistan, Iran, Tadschikistan, Usbekistan und Turkmenistan.


Geography.

Location.

The city of Poitiers is strategically situated on the Seuil du Poitou, a shallow gap between the Armorican and the Central Massif. The Seuil du Poitou connects the Aquitaine Basin to the South to the Paris Basin to the North. This area is an important geographic crossroads in France and Western Europe.

Situation.

Poitiers's primary site sits on a vast promontory between the valleys of the Boivre and the Clain. The old town occupies the slopes and the summit of a plateau which rises above the streams which surround benefits from a very strong tactical situation. This was an especially important factor before and throughout the Middle Ages.

Inhabitants and demography.

Inhabitants of Poitiers are referred to as Poitevins or Poitevines, although this denomination can be used for anyone from the Poitou province.  One out of three people in Poitiers is under the age of 30 and one out of four residents in Poitiers is a student.

Climate.

The climate in the Poitiers area is mild with mild temperature amplitudes, and adequate rainfall throughout the year although with a drying tendency during summer. The Köppen Climate Classification subtype for this type of climate is "" (Marine West Coast Climate/Oceanic climate).

History.

Antiquity.

Poitiers was founded by the Celtic tribe of the Pictones and was known as the oppidum "Lemonum" before Roman influence. The name is said to have come from the Celtic word for elm, "Lemo". After Roman influence took over, the town became known as "Pictavium", or later "Pictavis", after the original Pictones inhabitants themselves.  There is a rich history of archeological finds from the Roman era in Poitiers. In fact until 1857 Poitiers hosted the ruins of a vast Roman amphitheatre, which was larger than that of Nîmes. Remains of Roman baths, built in the 1st century and demolished in the 3rd century, were uncovered in 1877.  In 1879 a burial-place and tombs of a number of Christian martyrs were discovered on the heights to the south-east of the town. The names of some of the Christians had been preserved in paintings and inscriptions. Not far from these tombs is a huge dolmen (the "Pierre Levée"), which is long, broad and high, and around which used to be held the great fair of Saint Luke.  The Romans also built at least three aqueducts. This extensive ensemble of Roman constructions suggests Poitiers was a town of first importance, possibly even the capital of the Roman province of "Gallia Aquitania" during the 2nd century.  As Christianity was made official and gradually introduced across the Roman Empire during the 3rd and 4th centuries, the first bishop of Poitiers from 350 to 367, Hilary of Poitiers or Saint Hilarius, proceeded to evangelize the town. Exiled by Constantius II, he risked death to return to Poitiers as Bishop. The first foundations of the Baptistère Saint-Jean can be traced to that era of open Christian evangelization. He was named "Doctor of The Church" by Pope Pius IX.  In the 4th century, a thick wall 6m wide and 10m high was built around the town. It was long and stood lower on the naturally defended east side and at the top of the promontory. Around this time, the town began to be known as Poitiers.  Fifty years later Poitiers fell into the hands of the Arian Visigoths, and became one of the principal residences of their kings. Visigoth King Alaric II was defeated by Clovis I at Vouillé, not far from Poitiers, in 507, and the town thus came under Frankish dominion.

Middle Ages.

During most of the Early Middle Ages, the town of Poitiers took advantage of its defensive tactical site and of its location, which was far from the centre of Frankish power. As the seat of an "évêché" (bishopric) since the 4th century, the town was a centre of some importance and the capital of the Poitou county. At the height of their power, the Counts of Poitiers governed a large domain, including both Nouvelle-Aquitaine and Poitou.  The town was often referred to as "Poictiers", a name commemorated in warships of the Royal Navy, after the battle of Poi(c)tiers.  The first decisive victory of a Western European Christian army over a Muslim power, the Battle of Tours, was fought by Charles Martel's men in the vicinity of Poitiers on 10 October 732. For many historians, it was one of the world's pivotal moments.  Eleanor of Aquitaine frequently resided in the town, which she embellished and fortified, and in 1199 entrusted with communal rights. In 1152 she married the future King Henry II of England in Poitiers Cathedral.  During the Hundred Years' War, the Battle of Poitiers, an English victory, was fought near the town of Poitiers on 19 September 1356. Later in the war In 1418, under duress, the royal parliament moved from Paris to Poitiers, where it remained in exile until the Plantagenets finally withdrew from the capital in 1436. During this interval, in 1429 Poitiers was the site of Joan of Arc's formal inquest.  The University of Poitiers was founded in 1431. During and after the Reformation, John Calvin had numerous converts in Poitiers and the town had its share of the violent proceedings which underlined the Wars of Religion throughout France.  In 1569 Poitiers was defended by Gui de Daillon, comte du Lude, against Gaspard de Coligny, who after an unsuccessful bombardment and seven weeks, retired from a siege he had laid to the town.

16th century.

The type of political organisation existing in Poitiers during the late medieval or early modern period can be glimpsed through a speech given on 14 July 1595 by Maurice Roatin, the town's mayor. He compared it to the Roman state, which combined three types of government: monarchy (rule by one person), aristocracy (rule by a few), and democracy (rule by the many). He said the Roman consulate corresponded to Poitiers' mayor, the Roman senate to the town's peers and "échevins", and the democratic element in Rome corresponded to the fact that most important matters "can not be decided except by the advice of the "Mois et Cent"" (broad council). The mayor appears to have been an advocate of a mixed constitution; not all Frenchmen in 1595 would have agreed with him, at least in public; many spoke in favour of absolute monarchy. The democratic element was not as strong as the mayor's words may seem to imply: in fact, Poitiers was similar to other French cities, Paris, Nantes, Marseille, Limoges, La Rochelle, Dijon, in that the town's governing body ("corps de ville") was "highly exclusive and oligarchical": a small number of professional and family groups controlled most of the city offices. In Poitiers many of these positions were granted for the lifetime of the office holder.  The city government in Poitiers based its claims to legitimacy on the theory of government where the mayor and "échevins" held jurisdiction of the city's affairs in fief from the king: that is, they swore allegiance and promised support for him, and in return he granted them local authority. This gave them the advantage of being able to claim that any townsperson who challenged their authority was being disloyal to the king. Every year the mayor and the 24 "échevins" would swear an oath of allegiance "between the hands" of the king or his representative, usually the lieutenant général or the sénéchaussée. For example, in 1567, when Maixent Poitevin was mayor, king Henry III came for a visit, and, although some townspeople grumbled about the licentious behaviour of his entourage, Henry smoothed things over with a warm speech acknowledging their allegiance and thanking them for it.  In this era, the mayor of Poitiers was preceded by sergeants wherever he went, consulted deliberative bodies, carried out their decisions, "heard civil and criminal suits in first instance", tried to ensure that the food supply would be adequate, visited markets.  In the 16th century, Poitiers impressed visitors because of its large size, and important features, including "royal courts, university, prolific printing shops, wealthy religious institutions, cathedral, numerous parishes, markets, impressive domestic architecture, extensive fortifications, and castle."  16th-century Poitiers is closely associated with the life of François Rabelais and with the community of Bitards.

17th century.

The town saw less activity during the Renaissance. Few changes were made in the urban landscape, except for laying way for the "rue de la Tranchée". Bridges were built where the inhabitants had used "gués". A few "hôtels particuliers" were built at that time, such as the hôtels Jean Baucé, Fumé and Berthelot. Poets Joachim du Bellay and Pierre Ronsard met at the University of Poitiers, before leaving for Paris.  During the 17th century, many people emigrated from Poitiers and the Poitou to the French settlements in the new world and thus many Acadians or Cajuns living in North America today can trace ancestry back to this region.

18th century.

During the 18th century, the town's activity mainly depended on its administrative functions as a regional centre: Poitiers served as the seat for the regional administration of royal justice, the évêché, the monasteries and the intendance of the "Généralité du Poitou".  The Vicomte de Blossac, intendant of Poitou from 1750 to 1784, had a French garden landscaped in Poitiers. He also had Aliénor d'Aquitaine's ancient wall razed and modern boulevards were built in its place.

19th century.

During the 19th century, many army bases were built in Poitiers because of its central and strategic location. Poitiers became a garrison town, despite its distance from France's borders.  The Poitiers train station was built in the 1850s, and connected Poitiers to the rest of France.

20th century and contemporary Poitiers.

Poitiers was bombed during World War II, particularly the area around the railway station which was heavily hit on 13 June 1944.  From the late 1950s until the late 1960s when Charles de Gaulle ended the American military presence, the U.S. Army and U.S. Air Force had an array of military installations in France, including a major Army logistics and communications hub in Poitiers, part of what was called the Communication Zone (ComZ), and consisting of a logistics headquarters and communications agency located at Aboville Caserne, a military compound situated on a hill above the city. Hundreds of graduates ("Military Brats") of Poitiers American High School, a school operated by the Department of Defense School System (DODDS), have gone on to successful careers, including the recent commander-in-chief of U.S. Special Forces Command, Army General Bryan (Doug) Brown. The Caserne also housed a full support community, with a theater, commissary, recreation facilities and an affiliate radio station of the American Forces Network, Europe, headquartered in Frankfurt (now Mannheim, Germany).  The town benefited from industrial "décentralisation" in the 1970s, for instance with the installation during that decade of the Michelin and Compagnie des compteurs Schlumberger factories. The "Futuroscope" theme-park and research park project, built in 1986–1987 in nearby Chasseneuil-du-Poitou, after an idea by René Monory, consolidated Poitiers' place as a touristic destination and as a modern university centre, and opened the town to the era of information technology.

Sports.

The Stade Poitevin, founded in 1900, is a multi-sports club, which fields several top-level teams in a variety of sports. These include a volleyball team that play in the French Pro A volleyball league, a basketball team, an amateur football team and a professional rugby team (as of the 2008–2009 season.)  The PB86 or Poitiers Basket 86 (www.pb86.fr) play in the French Pro A basketball league. In the 2009–10 season, three Americans played for PB86: Rasheed Wright, Kenny Younger and Tommy Gunn. The team played the French championship playoffs in the 2009–10 season and was the Pro B French Champion for the 2008–2009 season. The team's communication strategy is considered by some to be one of the best in the French basketball league.  Brian Joubert, the figure skating champion, practices at an ice rink in Poitiers and lives with his family in the city.

Tourism.

Historic churches, in particular Romanesque church buildings, are the main attraction inside Poitiers itself. The town's centre is picturesque, with generally well-preserved architecture and a recently re-zoned pedestrian area. There are numerous shops, cafes and restaurants in the town centre.  Since 1987, Poitiers' tourist industry has indirectly benefited from the "Futuroscope" theme-park and research park in nearby Chasseneuil-du-Poitou. The centre of town receives visits in complement to the theme-park and benefits from a larger proportion of European tourists, notably from the United Kingdom. In conjunction, Poitiers' tourism has directly benefited from the TGV high-speed rail link to Paris.

Transport.

Poitiers' railway station lies on the TGV Atlantique line between Paris and Bordeaux. The station is in the valley to the west of the old town centre. Services run to Angoulême, Limoges and La Rochelle in addition to Paris and Bordeaux. The direct TGV puts Poitiers 1h40 from Paris' Gare Montparnasse.  Poitiers - Biard Airport is located west of Poitiers with flights to Lyon-Saint Exupéry, London-Stansted, Edinburgh and Shannon, Ireland on Ryanair.  Urban transportation in Poitiers is provided by a company called Vitalis. Regional ground transportation in the department of the Vienne is provided by private bus companies such as "Ligne en Vienne". Rail transportation in the region is provided by the public TER Nouvelle-Aquitaine (regional express train).  From January 2009 to December 2012, Poitiers' town centre went through deep changes to make it less accessible to motor vehicles. The project, named "Projet Coeur d'Agglo", focused on re-thinking the way people use individual cars to access the town centre and as an everyday way of transportation. On 29 September 2010, 12 streets were permanently closed off to motor vehicles and transformed into an entirely pedestrian zone.  Eventually, a new line of fast buses will be added around 2017.

Education.

The city of Poitiers has a very old tradition as a university centre, starting in the Middle Ages. The University of Poitiers was established in 1431 and has welcomed many famous philosophers and scientists throughout the ages (notably François Rabelais; René Descartes; Francis Bacon; Samir Amin).  Today Poitiers has more students per inhabitant than any other large town or city in France. All around, there are over 27,000 university students in Poitiers, nearly 4,000 of which are foreigners, hailing from 117 countries. The University covers all major fields from sciences to geography, history, languages economics and law.  The law degree at the University of Poitiers is considered to be one of the best in France. The program was ranked second by "l'Étudiant magazine" in 2005.  In addition to the University, Poitiers also hosts two engineering schools and two business schools:  Since 2001, the city of Poitiers has hosted the first cycle of "the South America, Spain and Portugal" program from the Paris Institute of Political Studies, also known as Sciences Po.

International relations.

Twin towns – sister cities.

Poitiers is twinned with:

Notable people.

This is a list of people of interest who were born or resided in Poitiers:

Poitiers (,,, ) is a city on the Clain river in west-central France. It is a commune and the capital of the Vienne department and the historical centre of Poitou. Poitiers is a significant university centre. The centre of town is picturesque and its streets include predominantly historical architecture, especially religious architecture and especially from the Romanesque period. Two major battles took place near the city. The first, in 732, also known as the Battle of Tours (to avoid confusion with the second), saw the Franks commanded by Charles Martel defeat an expeditionary army of the Umayyad Caliphate. The second, in 1356, was one of the key battles of the Hundred Years War. It saw the defeat of a large French royal army by the English and the capture of King John II by the victorious Prince of Wales.

Poitiers [] ist die Hauptstadt des Départements Vienne im Westen Frankreichs. Bis 2016 war sie auch Hauptstadt der dann aufgelösten Region Poitou-Charentes. Sie liegt am Fluss Clain und zählt Einwohner (Stand ).


History.

The origins of the madrigal are obscure, and debated, with one school of thought seeing it as a secular mutation of the conductus of the "ars antiqua", and another seeing it as deriving from 13th-century secular monophonic song with an improvised accompaniment. Little Italian music from the 13th century has survived, so links between medieval forms such as the conductus and troubadour song and the music of the trecento are largely inferential. The origin of the name (which appears in early sources as "madriale", "matricale", "madregal", and "marigalis") is also unclear; two possibilities are derivation from "materialis" (in contrast to "formalis"), designating a poem without a definite form, or from "matrix", meaning mother, either as in a song in the mother tongue or music used for Mother Church.  The earliest stage in the development of the madrigal is seen in the Rossi Codex, a collection of music from ca. 1350 or earlier, compiled around 1370. It has been suggested that the ornamentation of the upper voices may be improvised above a skeletal structure.  In the madrigal's later stages of development its uppermost voice was often highly elaborate, with the lower voice, the tenor, much less so. The form at this time was probably a development of connoisseurs, and sung by small groups of cognoscenti; there is no evidence of its widespread popularity, unlike the madrigal of the 16th century. By the end of the 14th century it had fallen out of favor, with other forms (in particular, the ballata and imported French music) taking precedence, some of which were even more highly refined and ornamented.  By the beginning of 15th century the term was no longer used musically. The later, 16th-century madrigal is unrelated, although it often used texts written in the 14th century (for instance by Petrarch).

Notable composers.

Important composers of the madrigal in the Trecento include:

The Trecento Madrigal is an Italian musical form of the 14th century. It is quite distinct from the madrigal of the Renaissance and early Baroque, with which it shares only the name. The madrigal of the Trecento flourished ca. 1300–1370 with a short revival near 1400. It was a composition for two (or rarely three) voices, sometimes on a pastoral subject. In its earliest development it was simple construction: Francesco da Barberino in 1300 called it a "raw and chaotic singalong".

Das Trecento-Madrigal ist eine italienische Musikform des 14. Jahrhunderts (von ca. 1300 bis 1370), meist zwei-, selten dreistimmige, unbegleitete Vokalkompositionen in einfacher Manier, bei der die Oberstimme die Hauptstimme darstellt. Durch seine schlichte Konstruktion und die primitive Rhythmik sowie die weltlichen Texte galt das Trecento-Madrigal nicht unbedingt als eine edle Form, Francesco da Barberino bezeichnete um 1300 das Madrigal als „raues und ungeordnetes Zusammensingen“.


Historical views.

In the Pacific, traditional landfinding techniques used by Micronesians and Polynesians suggest that bird migration was observed and interpreted for more than 3000 years. In Samoan tradition, for example, Tagaloa sent his daughter Sina to Earth in the form of a bird, Tuli, to find dry land, the word tuli referring specifically to landfinding waders, often to the Pacific golden plover. Records of bird migration were also known in Europe from at least 3,000 years ago as indicated by the Ancient Greek writers Hesiod, Homer, Herodotus and Aristotle. The Bible also notes migrations, as in the Book of Job, where the inquiry is made: "Is it by your insight that the hawk hovers, spreads its wings southward?" The author of Jeremiah wrote: "Even the stork in the heavens know its seasons, and the turtle dove, the swift and the crane keep the time of their arrival."  Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. Pliny the Elder, in his "Historia Naturalis", repeats Aristotle's observations.

Swallow migration versus hibernation.

Aristotle, however, suggested that swallows and other birds hibernated. This belief persisted as late as 1878, when Elliott Coues listed the titles of no less than 182 papers dealing with the hibernation of swallows. Even the "highly observant" Gilbert White, in his posthumously published 1789 "The Natural History of Selborne", quoted a man's story about swallows being found in a chalk cliff collapse "while he was a schoolboy at Brighthelmstone", though the man denied being an eyewitness. However, he also writes that "as to swallows being found in a torpid state during the winter in the Isle of Wight or any part

"Pfeilstörche".

In 1822, a white stork was found in the German state of Mecklenburg with an arrow made from central African hardwood,

General patterns of migration.

Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat, or weather. Sometimes, journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Migration is marked by its annual seasonality. Non-migratory birds are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants.  Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south. Of course, in the southern hemisphere the directions are reversed, but there is less land area in the far south to support long-distance migration.  The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. Also, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.  These advantages offset the high stress, physical exertion costs, and other risks of the migration. Predation can be heightened during migration: Eleonora's falcon "Falco eleonorae", which breeds on Mediterranean islands, has a very late breeding season, coordinated with the autumn passage of southbound passerine migrants, which it feeds to its young. A similar strategy is adopted by the greater noctule bat, which preys on nocturnal passerine migrants. The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response.  Within a species not all populations may be migratory; this

Nocturnal migratory behavior.

While participating in nocturnal migration, many birds give 'Nocturnal Flight Calls', which are short, contact-type calls. These calls likely serve to maintain the composition of a migrating flock, and can sometimes encode the gender of a migrating individual. They also likely serve to avoid collision in the air. Nocturnal migration can also be monitored using weather radar data, which can be used to estimate the number of birds migrating on a given night, as well

Long-distance migration.

The typical image of migration is of northern landbirds, such as swallows (Hirundinidae) and birds of prey, making long flights to the tropics. However, many Holarctic wildfowl and finch (Fringillidae) species winter in the North Temperate Zone, in regions with milder winters than their summer breeding grounds. For example, the pink-footed goose migrates from Iceland to Britain and neighbouring countries, whilst the dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon. Some ducks, such as the garganey "Anas querquedula", move completely or partially into the tropics. The European pied flycatcher "Ficedula hypoleuca" also follows this migratory trend, breeding in Asia and Europe and wintering in Africa.  Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. In long-lived, social species such as white storks "(Ciconia ciconia)," flocks are often led by the oldest members and young storks learn the route on their first journey. In short-lived species that migrate alone, such as the Eurasian blackcap "Sylvia atricapilla" or the yellow-billed cuckoo "Coccyzus americanus", first-year migrants follow a genetically determined route that is alterable with selective breeding.  Often, the migration route of a long-distance migratory bird doesn't follow a straight line between breeding and wintering grounds. Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land-birds, such barriers could consist of seas, large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (important for broad-winged birds). Additionally, many migration routes are circuitous due to evolutionary history: the breeding range of Northern wheatears "Oenanthe oenanthe" has expanded to cover the entire Northern Hemisphere, but the species still migrates up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas.  The same considerations about barriers and detours that apply to long-distance land-bird migration apply to water birds, but in reverse: a large area of land without bodies of water that offer feeding sites may also be a barrier to a bird that feeds in coastal waters. Detours avoiding such barriers are observed: for example, brent geese "Branta bernicla" migrating from the Taymyr Peninsula to the Wadden Sea travel via the White Sea coast and the Baltic Sea rather than directly across the Arctic Ocean and northern Scandinavia.

Short-distance and altitudinal migration.

Many long-distance migrants appear to be genetically programmed to respond to changing day length. Species that move short distances, however, may not need such a timing mechanism, instead moving in response to local weather conditions. Thus mountain and moorland breeders, such as wallcreeper "Tichodroma muraria" and white-throated dipper "Cinclus cinclus", may move only altitudinally to escape the cold higher ground. Other species such as merlin "Falco columbarius" and Eurasian skylark "Alauda arvensis" move further, to the coast or towards the south. Species like the chaffinch are much less migratory in Britain than those of continental Europe, mostly not moving more than 5 km in their lives.  Short-distance passerine migrants have two evolutionary origins. Those that have long-distance

Irruptions and dispersal.

Sometimes circumstances such as a good breeding season followed by a food source failure the following year lead to irruptions in which large numbers of a species move far beyond the normal range. Bohemian waxwings "Bombycilla garrulus" well show this unpredictable variation in annual numbers, with five major arrivals in Britain during the nineteenth century, but 18 between the years 1937 and 2000. Red crossbills "Loxia curvirostra" too

Physiology and control.

The control of migration, its timing and response are genetically controlled and appear to be a primitive trait that is present even in non-migratory species of birds. The ability to navigate and orient themselves during migration is a much more complex phenomenon that may include both endogenous programs as well as learning.

Timing.

The primary physiological cue for migration is the changes in the day length. These changes are also related to hormonal changes in the birds. In the period before migration, many birds display higher activity or Zugunruhe (), first described by Johann Friedrich Naumann in 1795, as well as physiological changes such as increased fat deposition. The occurrence

Orientation and navigation.

Navigation is based on a variety of senses. Many birds have been shown to use a sun compass. Using the sun for direction involves the need for making compensation based on the time. Navigation has also been shown to be based on a combination of other abilities including the ability to detect magnetic fields (magnetoreception), use visual landmarks as well as olfactory cues.  Long-distance migrants are believed to disperse as young birds and form attachments to potential breeding sites and to favourite wintering sites. Once the site attachment is made they show high site-fidelity, visiting the same wintering sites year after year.  The ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps. Satellite tracking of day migrating raptors such as ospreys

Vagrancy.

Migrating birds can lose their way and appear outside their normal ranges. This can be due to flying past their destinations as in the "spring overshoot" in which birds returning to their breeding areas overshoot and end up further north than intended. Certain areas, because of their location, have become famous as watchpoints for such birds. Examples are the Point Pelee National Park in Canada, and Spurn

Migration conditioning.

It has been possible to teach a migration route to a flock of birds, for example in re-introduction

Adaptations.

Birds need to alter their metabolism to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and

Evolutionary and ecological factors.

Migration in birds is highly labile and is believed to have developed independently in many avian lineages. While it is agreed that the behavioral and physiological adaptations necessary for migration are under genetic control, some authors have argued that no genetic change is necessary for migratory behavior to develop in a sedentary species because the genetic framework for migratory behavior exists in nearly all avian lineages. This explains the rapid appearance of migratory behavior after the most recent glacial maximum.  Theoretical analyses show that detours that increase flight distance by up to 20% will often be adaptive on aerodynamic grounds – a bird that loads itself with food to cross a long barrier flies less efficiently. However some species show circuitous migratory routes that reflect historical range expansions and are far from optimal in ecological terms. An example is the migration of continental populations of Swainson's thrush "Catharus ustulatus", which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago. Detours may also be caused by differential wind conditions, predation risk, or other factors.

Climate change.

Large scale climatic changes are expected to have an effect on the timing of migration. Studies have shown a variety of effects including timing changes in migration, breeding as well as population

Ecological effects.

The migration of birds also aids the movement of other species, including those of ectoparasites such as ticks and lice, which in turn may carry micro-organisms including those of concern to human health. Due to the global spread of avian influenza, bird migration has been studied as a possible mechanism of disease transmission, but it has

Study techniques.

Early studies on the timing of migration began in 1749 in Finland, with Johannes Leche of Turku collecting the dates of arrivals of spring migrants.  Bird migration routes have been studied by a variety of techniques including the oldest, marking. Swans have been marked with a nick on the beak since about 1560 in England. Scientific ringing was pioneered by Hans Christian Cornelius Mortensen in 1899. Other techniques include radar and satellite tracking. The rate of bird migration over the Alps (up to a height of 150 m) was found to be highly comparable between fixed-beam radar measurements and visual bird counts, highlighting the potential use of this technique as an objective way of quantifying bird migration.  Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds.

Threats and conservation.

Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. and the African-Eurasian Migratory Waterbird Agreement  The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct; during the passenger pigeon's ("Ectopistes migratorius") migration the enormous flocks were a mile (1.6 km) wide, darkening the sky and long, taking several days to pass.  Other significant areas include stop-over sites between the wintering and breeding territories. A capture-recapture study of passerine migrants with high fidelity for breeding and wintering sites did not show similar strict association with stop-over sites.  Hunting along migration routes threatens some bird species. The populations of Siberian cranes ("Leucogeranus leucogeranus") that wintered in India declined due to hunting along the route, particularly in Afghanistan and Central Asia. Birds were last seen in their favourite wintering grounds in Keoladeo National Park in 2002. Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. Other migration hazards include pollution, storms, wildfires, and habitat destruction along migration routes, denying migrants food at stopover points. For example, in the East Asian–Australasian Flyway, up to 65% of key intertidal habitat at the Yellow Sea migration bottleneck has been destroyed since the 1950s.

Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. It occurs mainly in the northern hemisphere, where birds are funneled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea.

Als Vogelzug bezeichnet man den alljährlichen Flug der Zugvögel von ihren Brutgebieten zu ihren Winterquartieren und wieder zurück. Jährlich sind weltweit schätzungsweise 50 Milliarden Zugvögel unterwegs, davon etwa fünf Milliarden zwischen Europa und Afrika.