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44-672: The Australian High , also known as the Australian subtropical ridge , is a large, semi-permanent high pressure area or subtropical anticyclone that seasonally vacillates between the Great Australian Bight in the south to the Northern Territory in the north. It is generally located between 25 and 40 degrees of south latitude, depending on the season. In summer , it typically sits over southern Australia as an Australian Bight High , where it generally provides dry weather in

88-501: A La Niña phase. In March 2021 , and also in February and March 2022 , a stubborn blocking high in the Tasman caused heavy rain and flooding over large parts of Southeast Queensland and coastal New South Wales. High pressure system A high-pressure area , high , or anticyclone , is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in

132-400: A dry climate, bringing warm to hot weather in the southern Australian summer. The high influences the weather and climatic patterns of vast areas of Australia ; The aridity of the Australian deserts and the summer drought of southern Australia is due to the large-scale subsidence and sinking motion of air in the system. In winter, when the high remains stationary in southern Australia (in

176-456: A large high stationed below South Australia can bring persistent rainfall to the east coast of Australia, whilst proving clear conditions to Tasmania . In the western part of the high, hot dry northerly winds from the dry centre push through South Australia and Victoria, ensuing heatwave conditions in these regions. The Australian High is one the drivers of the Southerly buster , which occurs in

220-517: A month during the most frigid time of the year, making it unique in that regard. It is also a bit larger and more persistent than its counterpart in North America. Surface winds accelerating down valleys down the western Pacific Ocean coastline, causing the winter monsoon. Arctic high-pressure systems such as the Siberian High are cold core, meaning that they weaken with height. The influence of

264-510: A positive SAM phase), it can block or replace cold fronts from the Bight, thereby allowing warm weather to the southeast. However, when the high remains fixed south of Western Australia , it can drive in polar air towards the continent, whereby increasing snowfall and rain in the southeast, particularly areas that lie west of the Great Dividing Range (due to the foehn effect ). Conversely,

308-507: A tendency to produce anomalously mild conditions at very high latitudes, at least in those regions exposed to anomalous flow from the ocean as in Greenland and Beringia , or from chinook winds as in Interior Alaska . Such cold winters over the contiguous United States and southern Canada as 1911/12 , 1935/36 , 1949/50, 1977/78 and 1978/79, 1993/94, and 2017/18 resulted from blocks in

352-460: Is deflected right from the center of high pressure) and counterclockwise 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 than would be the case in the absence of friction. This results in the 'actual wind' or 'true wind', including ageostrophic corrections, which add to

396-456: Is near the 500 hPa pressure surface about midway up through the troposphere, and about half the atmospheric pressure at the surface. High pressure systems are also called anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English, within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within

440-462: Is roughly at the 30th parallel and is the source of warm high pressure systems. As the hot air closer to the equator rises, it cools, losing moisture; it is then transported poleward where it descends, creating the high-pressure area. This is part of the Hadley cell circulation and is known as the subtropical ridge or subtropical high. It follows the track of the sun over the year, expanding north (south in

484-632: The Azores High , also known as the Bermuda High, brings fair weather over much of the North Atlantic Ocean and mid to late summer heat waves in western Europe. Along its southerly periphery, the clockwise circulation often impels easterly waves , and tropical cyclones that develop from them, across the ocean towards landmasses in the western portion of ocean basins during the hurricane season . The highest barometric pressure ever recorded on Earth

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528-468: The Coriolis effect . High-pressure areas 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 convergence , near or above the level of non-divergence, which

572-667: The Gulf of Alaska or to the east of the Mackenzie Mountains directing very cold Arctic air with a long trajectory as far as the American South , as did the Western cold waves of 1889/90 and January 1950. In Northern and Western Europe , cold winters such as 1683/84, 1739/40, 1794/95, 1829/30, 1894/95 , 1916/17, 1941/42, February 1947 and 1962/63 are almost always associated with high latitude Atlantic blocking and an equatorward shift of

616-601: The equator due to the Coriolis effect . Between summer and autumn, the high over southern Australia may be linked, or would intertwine, with the Tasman High in the southern Tasman Sea . When the high stalls in the Tasman Sea and becomes a blocking high , New Zealand and Tasmania will generally experience warm and generally dry weather, whilst the east coast of Australia (particularly southern Queensland and New South Wales) will experience moist onshore flows, particularly during

660-603: The geostrophic wind that is characterized by flow parallel to the isobars. Blocking high Polar cyclones are climatological features which hover near the poles year-round. They are weaker during summer and strongest during winter. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Extratropical cyclones which occlude and migrate into higher latitudes create cold-core lows within

704-554: The Northern Hemisphere resemble an Ω, the uppercase Greek letter omega . They typically have a low-high-low pattern, arranged in the west–east direction. Rex blocks (or dipole blocks) consist of a high situated poleward (north in the Northern Hemisphere; south in the Southern Hemisphere) of a low. Very often both the high and the low are closed, meaning that the isobars (or constant geopotential height lines) defining

748-471: The Northern Hemisphere, however, cold winters in Europe (e.g. 1916/17, 1962/63) are often very mild over Central Asia, which can gain warm air advection from subtropical cyclones pushed to the south under negative NAO conditions. Heat waves in summer are the result of similar blocking patterns, typically involving the placement of the semi-permanent subtropical ridge . Some unusually intense summers such as 1936 in

792-433: The Southern Hemisphere) in spring and retreating south (north in the Southern Hemisphere) in fall. The subtropical ridge is a warm core high-pressure system, meaning it strengthens with height. Many of the world's deserts are caused by these climatological high-pressure systems. Some climatological high-pressure areas acquire regionally based names. The land-based Siberian High often remains quasi-stationary for more than

836-538: The United States, 1999, 2002, and 2011, and in Europe summers such as 1976, 2003 European heat wave , and 2019, were the result of entrenched highs that became detached from the jet stream for a prolonged period of time and allowed warm, dry air to build in place. In many cases such as the 1999 US drought, the heat wave was preceded by prior months of below normal precipitation that prevented temperatures from cooling. The 2003 heat wave in Europe occurred, conversely, during

880-564: The absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, 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 ridge , leading to widespread haze . If the low level relative humidity rises towards 100 percent overnight, fog can form. Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in

924-524: The air anticlockwise. It remains almost stationary for a prolonged period over the Bight, hence obstructing the typical easterly procession of weather systems across southern Australia. A cloud hole with an expansion as far as 1,000 kilometers (620 miles) has been observed, with tops of 1,040 millibars . The high may be extensive enough to interconnect with the Tasman High over in the Tasman Sea , just near New Zealand . The anticyclonic circulation produces

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968-508: The amount of heat received from the Sun during daytime exceeds what is lost at night, and cold weather in winter when the amount of heat lost at night exceeds what is gained during daytime. In the Southern Hemisphere the result is similar. Australia and the southern cone of South America get hot, dry summer weather from the subtropical ridge and cooler wetter winter weather as cold fronts from

1012-515: The block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed, as happened in southeastern Australia in 2006 and 1967 with resultant extreme droughts . However, when a blocking high is situated in the Tasman Sea it can cause torrential rains in eastern Australia, as in the cases of the 2021 and 2022 flood events . A blocking high in the southern Tasman Sea directs low pressure systems and troughs towards eastern Australia, whereby providing rainfall on

1056-423: The center outward, the coriolis force given by the earth's rotation to the air circulation is in the opposite direction of earth's apparent rotation if viewed from above the hemisphere's pole. So, both the earth and winds around a low-pressure area rotate counter-clockwise in the northern hemisphere, and clockwise in the southern. The opposite to these two cases occurs in the case of a high. These results derive from

1100-594: The coast of Georgia that caused a drought in the Southeast that same year. Rainy, cooler weather results if the block is a low in the US. Hurricane Ian in the last week of September 2022 drifted northward and its remnants became detached from the jet stream, resulting in a stationary low pressure system spinning off the Northeastern US and bringing several days of precipitation until a front finally moved through on October 6. If

1144-430: The east coast of Australia. In Australia, blocking highs generally occur in the Great Australian Bight and the Tasman Sea , which are powerful high-pressure systems that usually develop further south than normal. They stay virtually unmoving for a lengthy period (i.e. several days to weeks) and thus block the regular easterly motion of weather systems across southern Australia. Blocking of atmospheric systems near

1188-459: The highest height line contour. Highs are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere . In general, subsidence will dry out an air mass by adiabatic , or compressional, heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night,

1232-412: The high–low close to form a circle. Rex blocks are named after meteorologist Daniel F. Rex, who first identified them in 1950. When an upper-level high - or low -pressure system becomes stuck in place due to a lack of steering currents , it is known as being "cut off". The usual pattern which leads to this is the jet stream retreating poleward, leaving the then cut-off system behind. Whether or not

1276-460: The letter H . Weather maps in other languages may use different letters or symbols. The direction of wind flow around an atmospheric high-pressure area and a low-pressure area , as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere. High pressure systems in the temperate latitudes generally bring warm weather in summer, when

1320-555: The northern hemisphere are associated with continental arctic air masses. 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 that move poleward from tropical sources are slower to modify than arctic air masses. The horse latitudes , or torrid zone,

1364-612: The polar jet stream to Portugal and even Morocco . Over Central Asia , unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and 1968/69 are associated with blocking near the Ural Mountains extending the Siberian High westwards to push the very cold air from the Siberian " cold pole " outward towards the Aral and Caspian Seas . Unlike other midlatitude regions of

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1408-523: The polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gauge its magnitude is the Arctic oscillation . Omega blocks are so-named because the isobars or geopotential height contours with which they are associated in

1452-523: The pressure is highest, at the center of the area, towards the periphery where the pressure is lower. However, the direction is not straight from the center outwards, but curved due to the Coriolis effect from Earth's rotation. Viewed from above, the wind direction is bent in the direction opposite to the planet's rotation; this causes the characteristic spiral shape of the tropical cyclones otherwise known as hurricanes and typhoons. On English-language weather maps , high-pressure centers are identified by

1496-491: The proximate region. In winter it normally moves north, therefore permitting cold fronts and low pressure systems to relocate up from the Great Australian Bight and bring rainfall to most parts of southern Australia. The high is part of the subtropical ridge system and it is the reason why a large part of Australia is arid to semi-arid. The Australian High tends to follow the seasonal variation in position of

1540-518: The southeast coast in the summer. In winter, when the high pressure system is broad over the Australian landmass, widespread frost may occur. During the southern winter, the Australian High is one of the driving forces behind the South Asian Monsoon . Easterly winds blow from the center of the high towards Asia , and are deflected in a southwest direction towards India after crossing

1584-554: The southern oceans take over. The term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India . A cyclone forms around a low-pressure area. Anticyclone , the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton . A simple rule is that for high-pressure areas, where generally air flows from

1628-478: The stable air mass of a cold air dam. In the middle latitudes of the Northern Hemisphere, areas on the eastern side of blocking anticyclones or under the influence of anomalous flows from colder continental interiors related to blocks experience severe winters, a phenomenon which has been known since the discovery of the North Atlantic Oscillation (NAO) in the 1840s. These blocking patterns also have

1672-412: The sun ; it is strongest and most persistent during the southern hemisphere summer and weakest during winter when it shifts towards the interior of Australia, as the westerly frontal systems becomes more active in the region around the Bight, thereby allowing cold fronts and low-pressure systems to perforate the southern states. This high-pressure block exhibits anticyclonic behaviour, circulating

1716-592: The surface of the Earth occurs when a well-established poleward high pressure system lies near or within the path of the advancing storm system. The thicker the cold air mass is, the more effectively it can block an invading milder air mass. The depth of the cold air mass is normally shallower than the mountain barrier which created the cold air damming, or CAD. Some events across the Intermountain West can last for ten days. Pollutants and smoke can remain suspended within

1760-671: The surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation . The strongest high-pressure areas result from masses of cold air which spread out from polar regions into cool neighboring regions. These highs weaken once they extend out over warmer bodies of water. Weaker—but more frequently occurring—are high-pressure areas caused by atmospheric subsidence : Air becomes cool enough to precipitate out its water vapor, and large masses of cooler, drier air descend from above. Within high-pressure areas, winds flow from where

1804-410: The system is of high- or low-pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southern United States during late spring and early summer of 2007, when a cut-off-low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to Texas and Oklahoma (see June 2007 Texas flooding ), and a cut-off-high near

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1848-458: The vicinity of low pressure areas in advance of their associated cold fronts . The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by the Earth 's rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and

1892-459: Was 1,085.7 hectopascals (32.06 inHg) measured in Tosontsengel, Zavkhan , Mongolia on 19 December 2001. A particularly hot summer such as 2003 which saw the subtropical ridge expand more than usual can bring heat waves as far north as Scandinavia —conversely, while Europe had record-breaking summer heat in 2003 due to a particularly strong subtropical ridge, its counterpart in North America

1936-408: Was unusually weak, and temperatures across the continent that spring and summer were wet and well below normal. Wind flows from areas of high pressure to areas of low pressure . This is due to density differences between the two air masses . Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in

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