In meteorology , a low-pressure area , low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible rain or storms), while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces . Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis . In meteorology , atmospheric divergence aloft occurs in two kinds of places:
33-622: The Asiatic Low is a low-pressure trough which lies over southern Asia , during early summer. It is located roughly over India , heading over the Bay of Bengal . It is a major action centre for the Northern Hemisphere during that time of the year. It is created by more intense July sun, causing desert land areas of Northern Africa and Asia to warm rapidly. Winds round it circle counterclockwise, from May to September or October giving persistent southwest monsoon winds from over
66-495: A remnant low . However, these remnant cyclones may persist for a few more days. Excessive wind shear can also cause cyclolysis as it causes the convection that powers the tropical cyclone to move away from the center. Mesocyclones are localized storms, approximately 2 km (1.2 mi) to 10 km (6.2 mi) in diameter within strong thunderstorms. Thunderstorms containing persistent mesocyclones are supercell thunderstorms. Mesocyclones typically undergo cyclolysis when
99-413: A depth of at least 50 m (160 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed, especially in the lower-to-mid troposphere ; when there is a great deal of moisture in
132-522: Is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean , a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the south Pacific or Indian Ocean . Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically , toward their centers. Tornadoes are often too small, and of too short duration, to be influenced by
165-535: Is characterized by the development of lower air pressure over the warmest part of the various continents. The large-scale thermal lows over continents help create pressure gradients which drive monsoon circulations. In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, oriented along a west-northwest/east-southeast axis. Many of
198-416: Is initially accelerated from areas of high pressure to areas of low pressure. This is due to density (or temperature and moisture) differences between two air masses . Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts . The stronger
231-601: Is not connected to any weather fronts . Extratropical cyclones in the northern hemisphere tend to undergo cyclolysis over the northeast Atlantic Ocean , northwestern Europe , the Gulf of Alaska , the Pacific Northwest , northeast Canada , central Russia , or the eastern Mediterranean sea . In the southern hemisphere , most extratropical cyclones undergo cyclolysis north of 50°S. Tropical cyclones often undergo cyclolysis after making landfall , especially if
264-572: The Hadley cell circulation. Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest. It can reach as far as the 40th parallel in East Asia during August and 20th parallel in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which
297-683: The Southern Hemisphere shows that between the 30th and 70th parallels there are an average of 37 cyclones in existence during any 6-hour period. A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter. In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low-pressure weather systems are typically known as depressions. These tend to bring wet weather throughout
330-455: The Coriolis force, but may be so-influenced when arising from a low-pressure system. cyclolysis Cyclolysis is a process in which a cyclonic circulation weakens and deteriorates. Cyclolysis is the opposite of cyclogenesis . Extratropical cyclones usually undergo cyclolysis after an occluded front forms, which separates the center of the cyclone from the warm air. With
363-466: The air temperature drops below the dew point as it rises, the cloudy skies typical of low-pressure areas act to dampen diurnal temperature extremes . Since clouds reflect sunlight , incoming shortwave solar radiation decreases, which causes lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation , such as heat energy from the surface, allows for warmer night-time minimums in all seasons. The stronger
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#1732802597870396-651: The area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains . In Europe (particularly in the British Isles and Netherlands ), recurring low-pressure weather systems are typically known as "low levels". Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within
429-705: The atmosphere, conditions are more favorable for disturbances to develop. Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear . In deserts , lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of
462-407: The atmosphere. Cyclogenesis is the opposite of cyclolysis , and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas — anticyclogenesis . Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone . Meteorologists use the term "cyclone" where circular pressure systems flow in
495-433: The center of the cyclone surrounded by cool air, the temperature difference or baroclinity begins to decline, and the jet stream persisting over the strongest areas of baroclinity which is not over the center of the cyclone, which makes it so that the jet stream is unable to support the system. This usually causes decay and dissipation of the cyclone. When an extratropical cyclone has completed cyclolysis, it usually
528-405: The difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month while September is the most active month. Nearly one-third of the world's tropical cyclones form within the western Pacific Ocean, making it the most active tropical cyclone basin on Earth . Wind
561-484: The direction of the Earth's rotation, which normally coincides with areas of low pressure. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale . Warm-core cyclones such as tropical cyclones, mesocyclones , and polar lows lie within the smaller mesoscale . Subtropical cyclones are of intermediate size. Cyclogenesis can occur at various scales, from
594-591: The force of gravity packing the air close to the ground. Thermal lows form due to localized heating caused by greater solar incidence over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth 's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over
627-545: The land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons resemble sea and land breezes , terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal. Large polar cyclones help determine
660-452: The land is mountainous as the friction with the terrain is increased. Its supply of warm, moist maritime air is also cut off when it moves over land. However, the brown ocean effect may cause a tropical cyclone to maintain its strength, or even intensify over land. A tropical cyclone may also undergo cyclolysis if it moves over water cooler than 26.5 °C (79.7 °F) as this will cause it to lose its tropical characteristics and become
693-403: The land, bringing the moist near-surface air over the oceans with it. Similar rainfall is caused by the moist ocean-air being lifted upwards by mountains , surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation . In winter,
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#1732802597870726-407: The lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in a low-pressure area called a thermal low . Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean. This generates a steady wind blowing toward
759-531: The microscale to the synoptic scale. Larger-scale troughs, also called Rossby waves, are synoptic in scale. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres. All share one important aspect, that of upward vertical motion within
792-648: The north Indian Ocean and South China Sea , also south-south-west or south winds over the west Pacific Ocean . Its counterpart during the winter is the Siberian High . The Asian Low is part of the Intertropical Convergence Zone (ITCZ). Winds from May to October are persistent southwesterly from the Indian Ocean and South China Sea as well as south-southwesterly or southerly over the western Pacific Ocean. This gradually generates
825-423: The northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure). A tropical cyclone differs from a hurricane or typhoon based only on geographic location. A tropical cyclone is fundamentally different from a mid-latitude cyclone. A hurricane
858-722: The ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale weather-systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and offshore platforms . They are vigorous systems that have near-surface winds of at least 17 metres per second (38 mph). Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm-core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to
891-414: The pressure difference, or pressure gradient , between a high-pressure system and a low-pressure system, the stronger the wind. Thus, stronger areas of low pressure are associated with stronger winds. The Coriolis force caused by the Earth 's rotation is what gives winds around low-pressure areas (such as in hurricanes , cyclones , and typhoons ) their counter-clockwise (anticlockwise) circulation in
924-583: The steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic . The Arctic oscillation provides an index used to gauge the magnitude of this effect in the Northern Hemisphere. Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans. A study of extratropical cyclones in
957-479: The summer monsoon over the Indian subcontinent and Southeast Asia. This article related to a specific weather event is a stub . You can help Misplaced Pages by expanding it . Low-pressure trough Diverging winds aloft, ahead of these troughs, cause atmospheric lift within the troposphere below as air flows upwards away from the surface, which lowers surface pressures as this upward motion partially counteracts
990-588: The tropics in concert with the Intertropical Convergence Zone , it is known as a monsoon trough . Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone . Tropical cyclones can form during any month of the year globally but can occur in either the northern or southern hemisphere during December. Atmospheric lift will also generally produce cloud cover through adiabatic cooling once
1023-407: The troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure. Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper-level jet streak before occluding later in their life cycle as cold-core cyclones. Polar lows are small-scale, short-lived atmospheric low-pressure systems that occur over
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1056-470: The world's rainforests are associated with these climatological low-pressure systems. Tropical cyclones generally need to form more than 555 km (345 mi) or poleward of the 5th parallel north and 5th parallel south , allowing the Coriolis effect to deflect winds blowing towards the low-pressure center and creating a circulation. Worldwide, tropical cyclone activity peaks in late summer, when
1089-724: The year. Thermal lows also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert , the Mexican Plateau , the Sahara , South America , and Southeast Asia. The lows are most commonly located over the Tibetan Plateau and in the lee of the Rocky Mountains. Elongated areas of low pressure form at the monsoon trough or Intertropical Convergence Zone as part of
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