An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge. The evolution of an anticyclone depends on a few variables such as its size, intensity, moist-convection, Coriolis force etc .
Sir Francis Galton first discovered anticyclones in the 1860s. Preferred areas within a synoptic flow pattern in higher levels of the hydrosphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as cum sole. Subtropical high pressure zones form under the descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature.
Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold.
Heating of the earth near the equator forces upward motion and convection along the monsoon trough or intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres. This circulation known as the Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas. Because these anticyclones strengthen with height, they are known as warm core ridges.
The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by the formation of clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow.
In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure. The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by Earth's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward (more ageostrophically) from the center.
High-pressure systems are frequently associated with light winds at the surface and subsidence of air from higher portions of the troposphere. Subsidence will generally warm an air mass by adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during the day, there is more incoming solar radiation and temperatures rise rapidly near the surface. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the high pressure, leading to widespread haze. If the surface level relative humidity rises towards 100 percent overnight, fog can form.
The movement of continental arctic air masses to lower latitudes produces strong but vertically shallow high-pressure systems. The surface level, sharp temperature inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter because they pick up moisture as they move over the warmer oceans. High pressures that build to the north and move southwards often bring clear weather because they are cooled at the base (as opposed to warmed) which helps prevent clouds from forming.
Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans, polar lows can develop. However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses.
The circulation around mid-level (altitude) ridges, and the air subsidence at their center, act to steer tropical cyclones around their periphery. Due to the subsidence within this type of system, a cap can develop which inhibits free convection and hence mixing of the lower with the middle level troposphere. This limits thunderstorm activity near their centers and traps low-level pollutants such as ozone as haze under their base, which is a significant problem in large urban centers during summer months such as Los Angeles, California and Mexico City.
The existence of upper-level (altitude) high pressure allows upper level divergence which leads to surface convergence. If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because a positive feedback loop develops between the convective tropical cyclone and the upper level high, the two system are strengthened. This loop stops once ocean temperatures cool to below 26.5 °C (79.7 °F), reducing the thunderstorm activity, which then weakens the upper level high pressure system.
When the subtropical ridge in the Northwest Pacific is stronger than normal, it leads to a wet monsoon season for Asia. The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States. Typically, the subtropical ridge across North America migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September. When the subtropical ridge is farther north than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona. When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime.
On weather maps, high-pressure centers are associated with the letter H in English, within the isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within the highest height line contour.
On Jupiter, there are two examples of an extraterrestrial anticyclonic storm; the Great Red Spot and the recently formed Oval BA. They are powered by smaller storms merging unlike any typical anticyclonic storm that happens on Earth where water powers them. Another theory is that warmer gases rise in a column of cold air, creating a vortex as is the case of other storms that include Anne's Spot on Saturn, and the Great Dark Spot on Neptune. Anticyclones have been detected near the poles of Venus.
The winter of 1981–82 in the United Kingdom (also called The Big Snow of 1982 by the press) was a severe cold wave that was formed in early December 1981 and lasted until mid-late January in 1982, and was one of the coldest Decembers recorded in the United Kingdom.At the end of November 1981, a strong high-pressure anticyclone over southern England was keeping temperatures around the average for the time of year. Numerous strong low-pressure extratropical cyclones passing to the north of Scotland dragged cold upper-level air down from the Arctic, but the anticyclone to the south of the United Kingdom deflected the coldest air away from the British Isles. On 23 November 1981, a cold front crossing the United Kingdom, fuelled by humid subtropical air from the south colliding with this colder Arctic air from the north, spawned 104 tornadoes as part of a record-breaking nationwide tornado outbreak. Following the passing of the cold front, the anticyclone to the south began to break down, allowing the colder Arctic air to move in over the British Isles from the north and precipitating the start of the severe cold wave at the start of December.
The CET Central England Station recorded a daily mean temperature of 0.3 °C (32.5 °F) and a daily minimum temperature of −2.7 °C (27.1 °F), for December, and is the coldest December recorded in the 20th Century. The CET Central England Station also recorded its coldest minimum December temperature at −15.9 °C (3.4 °F) on the 13th. The coldest temperature recorded in December was −25.2 °C (−13.4 °F) recorded in Shawbury, Shropshire on the 13th, and is the coldest December temperature recorded in England. Wales also records its coldest recorded December temperature during the cold wave, with a temperature of −22.7 °C (−8.9 °F) recorded at Corwen, Denbighshire also on the 13th.The coldest temperature recorded in the United Kingdom during the cold wave was recorded in Scotland with a temperature of −27.2 °C (−17.0 °F) recorded in Braemar, Aberdeenshire on 10 January, and is the coldest temperature ever recorded in the United Kingdom. England also records its coldest temperature during the cold wave at −26.1 °C (−15.0 °F) which was recorded at Newport, Telford and Wrekin, also on 10 January.2018 Great Britain and Ireland cold wave
Beginning on 22 February 2018, Great Britain and Ireland were affected by a cold wave, dubbed the Beast from the East by the media and officially named Anticyclone Hartmut, which brought widespread unusually low temperatures and heavy snowfall to large areas. The cold wave combined with Storm Emma, part of the 2017–18 UK and Ireland windstorm season, which made landfall in southwest England and southern Ireland on 2 March.
In contrast to usual winter storms, Emma was not formed as a normal low pressure area along with the jetstream; the initial event was an arctic outbreak due to a disordered polar vortex into Central Europe, transporting not only cold air from Siberia to Europe, but on the way to the British Islands according to the lake effect sent a lot of snow into areas of Great Britain and Ireland.
This weather situation repeated itself on the weekend of 17 and 18 March, but was less severe than on the previous occasion due to the onset of spring. This briefer cold snap was given the name "Mini Beast from the East".Anticyclonic storm
An anticyclonic storm is a weather storm where winds around the storm flow in the direction opposite to that of the flow about a region of low pressure.Azores High
The Azores High (Portuguese: Anticiclone dos Açores) also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda-Azores High, is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean, at the Horse latitudes. It forms one pole of the North Atlantic oscillation, the other being the Icelandic Low. The system influences the weather and climatic patterns of vast areas of North Africa and southern Europe, and to a lesser extent, eastern North America. The aridity of the Sahara Desert and the summer drought of the Mediterranean Basin is due to the large-scale subsidence and sinking motion of air in the system. In its summer position (the Bermuda High), the high is centered near Bermuda, and creates a southwest flow of warm tropical air toward the East Coast of the United States. In summer, the Azores-Bermuda High is strongest. The central pressure hovers around 1024 mbar (hPa).
This high-pressure block exhibits anticyclonic nature, circulating the air clockwise. Due to this direction of movement, African eastern waves are impelled along the southern periphery of the Azores High away from coastal West Africa towards the Caribbean, Central America, or the Bahamas, favouring tropical cyclogenesis, especially during the hurricane season.Block (meteorology)
Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively “blocking” or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans.Centers of action
Centers of action are extensive and almost stationary low-pressure areas or anticyclones which control the movement of atmospheric disturbances over a large area. This does not mean that the position of the center is constant over a specific area but that the monthly atmospheric pressure corresponds to a high or a low pressure.The French meteorologist Léon Teisserenc de Bort was the first in 1881 to apply this term to maxima and minima of pressure on daily charts. The main centers of action in the Northern Hemisphere are the Icelandic Low, the Aleutian Low, the Azores/Bermuda High, the Pacific High, the Siberian High (in winter), and the Asiatic Low (in summer). Sir Gilbert Walker used the same term to relate meteorological elements in a region to weather in the following season in other regions for the Southern Oscillation.Great Smog of London
The Great Smog of London, or Great Smog of 1952, was a severe air-pollution event that affected the British capital of London in early December 1952. A period of cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants—mostly arising from the use of coal—to form a thick layer of smog over the city. It lasted from Friday, 5 December, to Tuesday, 9 December 1952, and then dispersed quickly when the weather changed.
It caused major disruption by reducing visibility and even penetrating indoor areas, far more severe than previous smog events experienced in the past, called "pea-soupers". Government medical reports in the following weeks, however, estimated that up until 8 December, 4,000 people had died as a direct result of the smog and 100,000 more were made ill by the smog's effects on the human respiratory tract. More recent research suggests that the total number of fatalities may have been considerably greater, one paper suggested about 6,000 more died in the following months as a result of the event.London had suffered since the 13th century from poor air quality, which worsened in the 1600s, but the Great Smog is known to be the worst air-pollution event in the history of the United Kingdom, and the most significant in terms of its effect on environmental research, government regulation, and public awareness of the relationship between air quality and health. It led to several changes in practices and regulations, including the Clean Air Act 1956.High-pressure area
A high-pressure area, high or anticyclone is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment.
Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center.
However, because the planet is rotating underneath the atmosphere, and frictional forces arise as the planetary surface drags some atmosphere with it, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect, or the merely apparent force that arise when the observer is in a rotating frame of reference. Viewed from above this twist in wind direction is in the same direction as the rotation of the planet.
The strongest high-pressure areas are associated with cold air masses which push away out of polar regions during the winter when there is less sun to warm neighboring regions. These Highs change character and weaken once they move further over relatively warmer water bodies.
Somewhat weaker but more common are high-pressure areas caused by atmospheric subsidence, that is, areas where large masses of cooler drier air descend from an elevation of 8 to 15 km after the lower temperatures have precipitated out the water vapor.
Many of the features of Highs may be understood in context of middle- or meso-scale and relatively enduring dynamics of a planet's atmospheric circulation. For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells. Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most of the world's deserts.
On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols.Horse latitudes
Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds.
It is the product of the global air circulation cell known as the Hadley Cell. The subtropical ridge is characterized by mostly calm winds, which act to reduce air quality under its axis by causing fog overnight, and haze during daylight hours as a result of the stable atmosphere found near its location. The air descending from the upper troposphere flows out from its center at surface level toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies. The subtropical ridge moves poleward during the summer, reaching its most northern latitude in early fall, before moving equatorward during the cold season. The El Niño southern climate oscillation (ENSO) can displace the northern hemisphere subtropical ridge, with La Niñas allowing for a more northerly axis for the ridge, while El Niños show flatter, more southerly ridges. The change of the ridge position during ENSO cycles changes the tracks of tropical cyclones that form around their equatorward and western peripheries. As the subtropical ridge varies in position and strength, it can enhance or depress monsoon regimes around their low-latitude periphery.
The horse latitudes are associated with the subtropical anticyclone. The belt in the Northern Hemisphere is sometimes called the "calms of Cancer" and that in the Southern Hemisphere the "calms of Capricorn".
The consistently warm, dry, and sunny conditions of the horse latitudes are the main cause for the existence of the world's major non-polar deserts, such as the Sahara Desert in Africa, the Arabian and Syrian deserts in the Middle East, the Mojave and Sonoran deserts in the southwestern United States and northern Mexico, all in the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.Hurricane Kate (2003)
Hurricane Kate was the eleventh tropical storm, fifth hurricane, and third major hurricane of the 2003 Atlantic hurricane season, Kate developed from a tropical wave in the central tropical Atlantic on September 25. Its unusual track included four major changes in direction. The storm moved northwestward until a weakness in the subtropical ridge forced it eastward. Kate strengthened to a hurricane, turned sharply westward while moving around a mid-level low, and intensified to a 125 mph (205 km/h) major hurricane on October 4. Kate turned sharply northward around the periphery of an anticyclone, weakened, and became extratropical after passing to the east of Newfoundland. The extratropical storm persisted for three days until losing its identity near Scandinavia.
The storm had minimal effects on land, limited to moderately strong winds and heavy rainfall over Newfoundland. Kate threatened Atlantic Canada just one week after Hurricane Juan caused severe damage in Nova Scotia.Kalahari High
The Kalahari High is a semi-permanent anticyclone situated over the interior of southern Africa. It is part of the subtropical ridge system and the reason the Kalahari is a desert. It is the descending limb of a Hadley cell.Kona District, Hawaii
Kona is a moku or district on the Big Island of Hawaiʻi in the State of Hawaii. In the current system of administration of Hawaiʻi County, the moku of Kona is divided into North Kona District (Kona ‘Akau) and South Kona District (Kona Hema). The term "Kona" is sometimes used inaccurately to refer to its largest town, Kailua-Kona. Other towns in Kona include Kealakekua, Keauhou, Holualoa, Hōnaunau and Honalo.
In the Hawaiian language, kona means leeward or dry side of the island, as opposed to ko‘olau which means windward or the wet side of the island. In the times of Ancient Hawaiʻi, Kona was the name of the leeward district on each major island. In Hawai‘i, the Pacific anticyclone provides moist prevailing northeasterly winds to the Hawaiian islands, resulting in rain when the winds contact the windward landmass of the islands – the winds subsequently lose their moisture and travel on to the leeward (or kona) side of the island. When this pattern reverses, it can produce a Kona storm from the west. Kona has cognates with the same meaning in other Polynesian languages. In Tongan, the equivalent cognate would be tonga; for windward, the associated cognate would be tokelau.
Kona is the home of the world-famous Ironman World Championship Triathlon which is held each year in October in Kailua-Kona.
The Kealakekua Bay State Historical Park marks the place where Captain James Cook was killed in 1779. Puʻuhonua o Hōnaunau National Historical Park and Honokohau Settlement and Kaloko-Honokohau National Historical Park are in Kona.
The volcanic slopes of Hualālai and Mauna Loa in the Kona district provide an ideal microclimate for growing coffee. Kona coffee is considered one of the premium specialty coffees of the world.In pop culture, the region served as the basis of the Beach Boys' song "Kona Coast" from their 1978 album M.I.U. Album.
Kona is the home of one of the main bases of the international Christian mission organization YWAM, and the University of the Nations, first founded here.North American High
The North American High (also Canadian High/Anticyclone, sometimes in Europe Greenland High/Anticyclone) is an impermanent high-pressure area or anticyclone created by a formative process that occurs when cool or cold dry air settles over North America. In summer it is replaced with an Arctic Low, or if it moves to continental land, a North American Low.
North American Highs move eastwards across the continent, often in the company of one or more low-pressure cells or cyclones. Its cold, dense air does not extend usually above 3 km (1.9 mi), lower than the Canadian Rockies. Sometimes, in winter it breaks free and passes over the Rockies and brings a cold front into Southwestern United States and Mexico, freezing crops and bringing snow into Mexico's mountains as far south as Jalisco. The high’s usual location east of the Rockies shelters it from the relatively warm Pacific Ocean and helps it maintain its strength. The average January sea level pressure at its centre is about 1,020 millibars (30.12 inches of mercury). The Canadian high often moves southeastward until it eventually reaches the Atlantic Ocean, where it merges with the Azores high. In the summer the Canadian high circulates cool, dry air to the United States east of the Rockies and parts of southern Canada.
The North American High is akin to the Siberian High of Eurasia, but it is much smaller, and it has much less influence, merely affecting the weather of the Northern Hemisphere. The sea-level pressure (atmospheric pressure) rarely, if ever, exceeds 1055.0 millibars (1055.0 hectopascals)(hPa)(SI).
Often, in the winter months, cool or cold dry air settles over the land in the vicinity of the Great Basin where it builds into a high-pressure cell or anticyclone that moves across the United States with a cold front on its leading edge. After reaching the Atlantic Ocean, the moist environment brings on changes of the qualities of the air and the dissipation of the high-pressure cell or anticyclone as the cold air warms and becomes humid.
In Europe, a portion of the North American/Canadian high usually over Greenland called the Greenland high which settles over Greenland affects northern European weather and may merge with the Scandinavian High.North Pacific High
The North Pacific High is a semi-permanent, subtropical anticyclone located in the northeastern portion of the Pacific Ocean, located northeast of Hawaii and west of California. It is strongest during the northern hemisphere summer and shifts towards the equator during the winter, when the Aleutian Low becomes more active. It is responsible for California's typically dry summer and fall and typically wet winter and spring, as well as Hawaii's year-round trade winds.Rapid intensification
Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center (NHC) defines rapid intensification as an increase in the maximum 1-min sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.Ridiculously Resilient Ridge
The "Ridiculously Resilient Ridge," sometimes shortened to "Triple R" or "RRR," is the nickname given to a persistent anticyclone that occurred over the far northeastern Pacific Ocean, causing the 2011–2017 California drought. The "Ridiculously Resilient Ridge" nickname was originally coined in December 2013 by Daniel Swain on the Weather West Blog, but has since been used widely in popular media as well as in peer-reviewed scientific literature.Siberian High
The Siberian High (also Siberian Anticyclone) is a massive collection of cold dry air that accumulates in the northeastern part of Eurasia from September until April. It is usually centered on Lake Baikal. It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than −40 °C (−40 °F). The atmospheric pressure is often above 1,040 millibars (31 inHg). The Siberian High is the strongest semi-permanent high in the northern hemisphere and is responsible for both the lowest temperature in the Northern Hemisphere, of −67.8 °C (−90.0 °F) on 15 January 1885 at Verkhoyansk, and the highest pressure, 1083.8 mbar (108.38 kPa, 32.01 inHg) at Agata, Krasnoyarsk Krai on 31 December 1968, ever recorded. The Siberian High is responsible both for severe winter cold and attendant dry conditions with little snow and few or no glaciers across Siberia, Mongolia, and China. During the summer, the Siberian High is largely replaced by the Asiatic low.South Pacific High
The South Pacific High is a subtropical anticyclone located in the southeast Pacific Ocean. The area of high atmospheric pressure and the presence of the Humboldt Current in the underlying ocean make the west coast of Peru and northern Chile extremely arid. The Sechura and Atacama deserts, as the whole climate of Chile, are heavily influenced by this semi-permanent high-pressure area.Tropical upper tropospheric trough
A tropical upper tropospheric trough (TUTT), also known as the mid-oceanic trough,or commonly called as Western Hemisphere or "upper cold low" is a trough situated in upper-level (at about 200 hPa) tropics. Its formation is usually caused by the intrusion of energy and wind from the mid-latitudes into the tropics. It can also develop from the inverted trough adjacent to an upper level anticyclone. TUTTs are different from mid-latitude troughs in the sense that they are maintained by subsidence warming near the tropopause which balances radiational cooling. When strong, they can present a significant vertical wind shear to the tropics and subdue tropical cyclogenesis. When upper cold lows break off from their base, they tend to retrograde and force the development, or enhance, surface troughs and tropical waves to their east. Under special circumstances, they can induce thunderstorm activity and lead to the formation of tropical cyclones.