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Low-pressure area

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),[1] while high-pressure areas are associated with lighter winds and clear skies.[2] 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:

A clockwise spinning low-pressure area or cyclone of southern Australia. The center of the spiral-shaped cloud system is also the center of the low.
A low-pressure system over Iceland.

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 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 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 the air temperature drops below the dew point as it rises, although the low-pressure area typically brings cloudy skies, which 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 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".

Formation

Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere.[3] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areasanticyclogenesis.[4] 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 the direction of the Earth's rotation,[5][6] which normally coincides with areas of low pressure.[7][8] 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.[9][10] Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger-scale troughs, also called Rossby waves, are synoptic in scale.[11] Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature.[12] 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.[13] All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure.[14]

Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper-level jet streak[clarification needed] before occluding later in their life cycle as cold-core cyclones.[15][16][17][18] Polar lows are small-scale, short-lived atmospheric low-pressure systems that occur over 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).[19]

 
This depiction of the Hadley cell shows the process which sustains low-pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.

Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm-core with well-defined circulations.[20] 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 a depth of at least 50 m (160 ft);[21] waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.[22] Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.[21] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.[21] Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.[21] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.[21] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[23] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[24]

In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of 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.[25] 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 the land, bringing the moist near-surface air over the oceans with it.[26] Similar rainfall is caused by the moist ocean-air being lifted upwards by mountains,[27] surface heating,[28] convergence at the surface,[29] divergence aloft, or from storm-produced outflows at the surface.[30] However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation. In winter, 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.[26] 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.[31]

Climatology

Mid-latitudes and subtropics

 
QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds on the poleward side of the occluded front.

Large polar cyclones help determine 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.[32] Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans.[33] A study of extratropical cyclones in 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.[34] A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.[35] In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low-pressure weather systems are typically known as depressions.[36][37][38] These tend to bring wet weather throughout 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.[25] The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.[33]

Monsoon trough

 
February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific

Elongated areas of low pressure form at the monsoon trough or intertropical convergence zone as part of the Hadley cell circulation.[39] 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 is characterized by the development of lower air pressure over the warmest part of the various continents.[40][41] The large-scale thermal lows over continents help create pressure gradients which drive monsoon circulations.[42] In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February,[43] oriented along a west-northwest/east-southeast axis. Many of the world's rainforests are associated with these climatological low-pressure systems.[44]

Tropical cyclone

 
Infrared image of a powerful northern hemisphere cyclone, Megi, at its peak intensity

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.[21] Worldwide, tropical cyclone activity peaks in late summer, when 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. November is the only month that activity in all the tropical cyclone basins is possible.[45] 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.[46]

Associated weather

 
Schematic representation of flow (represented in black) around a low-pressure area in the Northern hemisphere. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows.

Wind is initially accelerated from areas of high pressure to areas of low pressure.[47] 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 the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind.[48] 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 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).[49] A tropical cyclone differs from a hurricane or typhoon based only on geographic location.[50] Note that a tropical cyclone is fundamentally different from a mid-latitude cyclone.[51] A hurricane 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.[50][52] 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.[48] Tornadoes are often too small, and of too short duration, to be influenced by the Coriolis force, but may be so-influenced when arising from a low-pressure system.[53][54]

See also

References

  1. ^ . American Meteorological Society. amsglossary.allenpress.com. Glossary of Meteorology. Allen Press. 2008. Archived from the original on 2008-10-04. Retrieved 2009-03-02.
  2. ^ Williams, Jack (2007). "What's happening inside highs and lows". Weather. USA Today. Retrieved 2009-02-16.
  3. ^ . nsidc.org. Arctic Climatology and Meteorology. National Snow and Ice Data Center. 2006. Archived from the original on 2006-08-30. Retrieved 2009-02-21.
  4. ^ "Cyclogenesis". American Meteorological Society. amsglossary.allenpress.com. Glossary of Meteorology. Allen Press. 2009. Retrieved 2009-02-21.
  5. ^ Glossary of Meteorology (June 2000). "Cyclonic circulation". American Meteorological Society. Retrieved 2008-09-17.
  6. ^ Glossary of Meteorology (June 2000). . American Meteorological Society. Archived from the original on 2008-10-04. Retrieved 2008-09-17.
  7. ^ BBC Weather Glossary (July 2006). . British Broadcasting Corporation. Archived from the original on 2006-08-29. Retrieved 2006-10-24.
  8. ^ "UCAR Glossary — Cyclone". meted.ucar.edu. Retrieved 2006-10-24.
  9. ^ Robert Hart (2003-02-18). "Cyclone Phase Analysis and Forecast: Help Page". Florida State University. Retrieved 2006-10-03.
  10. ^ I. Orlanski (1975). "A rational subdivision of scales for atmospheric processes". Bulletin of the American Meteorological Society. 56 (5): 527–530. Bibcode:1975BAMS...56..527.. doi:10.1175/1520-0477-56.5.527.
  11. ^ . American Meteorological Society. amsglossary.allenpress.com. Glossary of Meteorology. Allen Press. June 2000. Archived from the original on 2010-12-31. Retrieved 2009-11-06.
  12. ^ . American Meteorological Society. amsglossary.allenpress.com. Glossary of Meteorology. Allen Press. June 2000. Archived from the original on 2011-05-14. Retrieved 2009-11-06.
  13. ^ Glossary of Meteorology (June 2000). . American Meteorological Society. Archived from the original on 2011-01-09. Retrieved 2009-12-24.
  14. ^ Joel Norris (2005-03-19). (PDF). University of California, San Diego. Archived from the original (PDF) on 2010-06-26. Retrieved 2009-10-26.
  15. ^ Glossary of Meteorology (2009). Short Wave. 2009-06-09 at the Wayback Machine American Meteorological Society. Retrieved on 2009-03-02.
  16. ^ Glossary of Meteorology (2009). Upper-Level Trough. 2009-06-09 at the Wayback Machine American Meteorological Society. Retrieved on 2009-03-02.
  17. ^ Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss (1989). A Rapid Cyclogenesis Event during GALE IOP 9. Monthly Weather Review pp. 234–257. Retrieved on 2008-06-28.
  18. ^ Shay Johnson (2001-09-25). (PDF). weather.ou.edu. Archived from the original (PDF) on 2006-09-01. Retrieved 2006-10-11.
  19. ^ E. A. Rasmussen & J. Turner (2003). Polar Lows: Mesoscale Weather Systems in the Polar Regions. Cambridge University Press. p. 612. ISBN 978-0-521-62430-5.
  20. ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2004). "Frequently Asked Questions: What is an extra-tropical cyclone?". NOAA. Retrieved 2007-03-23.
  21. ^ a b c d e f Chris Landsea (2009-02-06). "Frequently Asked Questions: How do tropical cyclones form?". National Oceanic and Atmospheric Administration. Retrieved 2009-12-31.
  22. ^ Chris Landsea (2004-08-13). "Frequently Asked Questions: Why do tropical cyclones require 80 °F (27 °C) ocean temperatures to form?". National Oceanic and Atmospheric Administration. Retrieved 2006-07-25.
  23. ^ Glossary of Meteorology (2009). . American Meteorological Society. Archived from the original on 2006-07-09. Retrieved 2006-12-07.
  24. ^ Choy, Barry K.; Scott M. Spratt (2003-05-13). . NOAA. Archived from the original on 2008-06-17. Retrieved 2009-12-26.
  25. ^ a b Glossary of Meteorology (2009). Thermal Low. 2008-05-22 at the Wayback Machine American Meteorological Society. Retrieved on 2009-03-02.
  26. ^ a b Dr. Louisa Watts (2009). What causes the west African monsoon? National Centre for Environmental Science. Retrieved on 2009-04-04.
  27. ^ Dr. Michael Pidwirny (2008). CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes. Physical Geography. Retrieved on 2009-01-01.
  28. ^ Bart van den Hurk and Eleanor Blyth (2008). Global maps of Local Land-Atmosphere coupling. 2009-02-25 at the Wayback Machine KNMI. Retrieved on 2009-01-02.
  29. ^ Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press, p. 66. ISBN 978-0-12-548035-2. Retrieved on 2009-01-02.
  30. ^ Glossary of Meteorology (June 2000). . American Meteorological Society. Archived from the original on 2011-05-05. Retrieved 2008-07-09.
  31. ^ BBC Weather (2004-09-01). . Archived from the original on August 31, 2007. Retrieved 2008-05-22.
  32. ^ Todd Mitchell (2004). Arctic Oscillation (AO) time series, 1899 – June 2002. 2003-12-12 at the Wayback Machine University of Washington. Retrieved on 2009-03-02.
  33. ^ a b L. de la Torre, Nieto R., Noguerol M., Añel J.A., Gimeno L. (2008). A climatology based on reanalysis of baroclinic developmental regions in the extratropical northern hemisphere. Annals of the New York Academy of Sciences; vol. 1146: pp. 235–255. Retrieved on 2009-03-02.
  34. ^ Ian Simmonds & Kevin Keay (February 2000). "Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958–97". Journal of Climate. 13 (3): 550–561. Bibcode:2000JCli...13..550S. doi:10.1175/1520-0442(2000)013<0550:VOSHEC>2.0.CO;2. ISSN 1520-0442.
  35. ^ S.K. Gulev; O. Zolina & S. Grigoriev (2001). "Winter Storms in the Northern Hemisphere (1958–1999) via the Internet Wayback Machine". Climate Dynamics. 17 (10): 795–809. Bibcode:2001ClDy...17..795G. doi:10.1007/s003820000145. S2CID 129364159.
  36. ^ Met Office (2009). Frontal Depressions. 2009-02-24 at the Wayback Machine Retrieved on 2009-03-02.
  37. ^ "Understanding weather".
  38. ^ "KNMI - Depressie".
  39. ^ Becca Hatheway (2008). "Hadley Cell". University Corporation for Atmospheric Research. Archived from the original on 2012-05-26. Retrieved 2009-02-16.
  40. ^ National Centre for Medium Range Forecasting (2004-10-23). (PDF). Ministry of Earth Sciences (India). Archived from the original (PDF) on 2011-07-21. Retrieved 2008-05-03.
  41. ^ Australian Broadcasting Corporation (1999-08-11). "Monsoon". Australian Broadcasting Corporation. Retrieved 2008-05-03.
  42. ^ Mary E. Davis & Lonnie G. Thompson (2005). "Forcing of the Asian monsoon on the Tibetan Plateau: Evidence from high-resolution ice core and tropical coral records". Journal of Geophysical Research. 110 (D4): 1 of 13. Bibcode:2005JGRD..110.4101D. doi:10.1029/2004JD004933.
  43. ^ U. S. Navy (1998-01-22). "1.2 Pacific Ocean Surface Streamline Pattern". Retrieved 2006-11-26.
  44. ^ Hobgood (2008). . Ohio State University. Archived from the original on 2009-03-18. Retrieved 2009-03-08.
  45. ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2009-02-06). "Frequently Asked Questions: When is hurricane season?". National Oceanic and Atmospheric Administration. Retrieved 2009-12-24.
  46. ^ "Examining the ENSO" (PDF). James B Elsner, Kam-Biu Liu. 2003-10-08. Retrieved 2007-08-18.
  47. ^ BWEA (2007). Education and Careers: What is wind? 2011-03-04 at the Wayback Machine British Wind Energy Association. Retrieved on 2009-02-16.
  48. ^ a b JetStream (2008). Origin of Wind. National Weather Service Southern Region Headquarters. Retrieved on 2009-02-16.
  49. ^ Nelson, Stephen (Fall 2014). "Tropical Cyclones (Hurricanes)". Wind Systems: Low Pressure Centers. Tulane University. Retrieved 2016-12-24.
  50. ^ a b "What is the difference between a hurricane, a cyclone, and a typhoon?". OCEAN FACTS. National Ocean Service. Retrieved 2016-12-24.
  51. ^ "COMPARE AND CONTRAST: MID-LAT CYCLONE AND HURRICANE". www.theweatherprediction.com. Retrieved 2020-02-24.
  52. ^ "What is a Hurricane, Typhoon, or Tropical Cyclone? | Precipitation Education". pmm.nasa.gov. Retrieved 2020-02-24.
  53. ^ Horton, Jennifer (20 July 2009). "Does the rotation of the Earth affect toilets and baseball games?". SCIENCE, EVERYDAY MYTHS. HowStuffWorks. Retrieved 2016-12-25.
  54. ^ "Do Tornadoes Always Twist in the Same Direction?". SCIENCE — Earth and Space. WONDEROPOLIS. Retrieved 2016-12-25.

pressure, area, weather, system, redirects, here, general, discussion, pressure, areas, pressure, system, opposite, effect, high, pressure, area, meteorology, pressure, area, areaor, region, where, atmospheric, pressure, lower, than, that, surrounding, locatio. Weather system redirects here For a general discussion on pressure areas see Pressure system For the opposite effect see High pressure area In meteorology a low pressure area low areaor 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 1 while high pressure areas are associated with lighter winds and clear skies 2 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 The first is in the area on the east side of upper troughs which form half of a Rossby wave within the Westerlies a trough with large wavelength that extends through the troposphere A second is an area where wind divergence aloft occurs ahead of embedded shortwave troughs which are of smaller wavelength A clockwise spinning low pressure area or cyclone of southern Australia The center of the spiral shaped cloud system is also the center of the low A low pressure system over Iceland 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 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 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 the air temperature drops below the dew point as it rises although the low pressure area typically brings cloudy skies which 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 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 Contents 1 Formation 2 Climatology 2 1 Mid latitudes and subtropics 2 2 Monsoon trough 2 3 Tropical cyclone 3 Associated weather 4 See also 5 ReferencesFormation EditMain article Cyclogenesis Cyclogenesis is the development and strengthening of cyclonic circulations or low pressure areas within the atmosphere 3 Cyclogenesis is the opposite of cyclolysis and has an anticyclonic high pressure system equivalent which deals with the formation of high pressure areas anticyclogenesis 4 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 the direction of the Earth s rotation 5 6 which normally coincides with areas of low pressure 7 8 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 9 10 Cyclogenesis can occur at various scales from the microscale to the synoptic scale Larger scale troughs also called Rossby waves are synoptic in scale 11 Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale or mesoscale in nature 12 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 13 All share one important aspect that of upward vertical motion within the troposphere Such upward motions decrease the mass of local atmospheric columns of air which lowers surface pressure 14 Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak clarification needed before occluding later in their life cycle as cold core cyclones 15 16 17 18 Polar lows are small scale short lived atmospheric low pressure systems that occur over 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 19 This depiction of the Hadley cell shows the process which sustains low pressure areas Diverging winds aloft allow for lower pressure and convergence at the Earth s surface which leads to upward motion Tropical cyclones form due to latent heat driven by significant thunderstorm activity and are warm core with well defined circulations 20 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 a depth of at least 50 m 160 ft 21 waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms 22 Another factor is rapid cooling with height which allows the release of the heat of condensation that powers a tropical cyclone 21 High humidity is needed especially in the lower to mid troposphere when there is a great deal of moisture in the atmosphere conditions are more favorable for disturbances to develop 21 Low amounts of wind shear are needed as high shear is disruptive to the storm s circulation 21 Lastly a formative tropical cyclone needs a pre existing system of disturbed weather although without a circulation no cyclonic development will take place 21 Mesocyclones form as warm core cyclones over land and can lead to tornado formation 23 Waterspouts can also form from mesocyclones but more often develop from environments of high instability and low vertical wind shear 24 In deserts lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense rapid solar heating of 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 25 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 the land bringing the moist near surface air over the oceans with it 26 Similar rainfall is caused by the moist ocean air being lifted upwards by mountains 27 surface heating 28 convergence at the surface 29 divergence aloft or from storm produced outflows at the surface 30 However the lifting occurs the air cools due to expansion in lower pressure which in turn produces condensation In winter 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 26 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 31 Climatology EditMid latitudes and subtropics Edit QuikSCAT image of typical extratropical cyclones over the ocean Note the maximum winds on the poleward side of the occluded front See also Arctic oscillation Extratropical cyclone and Thermal low Large polar cyclones help determine 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 32 Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents or west side of oceans 33 A study of extratropical cyclones in 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 34 A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter 35 In Europe particularly in the United Kingdom and in the Netherlands recurring extratropical low pressure weather systems are typically known as depressions 36 37 38 These tend to bring wet weather throughout 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 25 The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains 33 Monsoon trough Edit February position of the ITCZ and monsoon trough in the Pacific Ocean depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific See also Monsoon trough Elongated areas of low pressure form at the monsoon trough or intertropical convergence zone as part of the Hadley cell circulation 39 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 is characterized by the development of lower air pressure over the warmest part of the various continents 40 41 The large scale thermal lows over continents help create pressure gradients which drive monsoon circulations 42 In the southern hemisphere the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February 43 oriented along a west northwest east southeast axis Many of the world s rainforests are associated with these climatological low pressure systems 44 Tropical cyclone Edit Infrared image of a powerful northern hemisphere cyclone Megi at its peak intensity See also Tropical cyclone 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 21 Worldwide tropical cyclone activity peaks in late summer when 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 November is the only month that activity in all the tropical cyclone basins is possible 45 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 46 Associated weather Edit Schematic representation of flow represented in black around a low pressure area in the Northern hemisphere The pressure gradient force is represented by blue arrows the Coriolis acceleration always perpendicular to the velocity by red arrows See also Geostrophic wind and Precipitation meteorology Wind is initially accelerated from areas of high pressure to areas of low pressure 47 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 the pressure difference or pressure gradient between a high pressure system and a low pressure system the stronger the wind 48 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 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 49 A tropical cyclone differs from a hurricane or typhoon based only on geographic location 50 Note that a tropical cyclone is fundamentally different from a mid latitude cyclone 51 A hurricane 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 50 52 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 48 Tornadoes are often too small and of too short duration to be influenced by the Coriolis force but may be so influenced when arising from a low pressure system 53 54 See also EditEast Asian Monsoon High pressure area Intertropical Convergence Zone North American Monsoon Surface weather analysis Tropical wave Trough meteorology Weather mapReferences Edit Cyclone American Meteorological Society amsglossary allenpress com Glossary of Meteorology Allen Press 2008 Archived from the original on 2008 10 04 Retrieved 2009 03 02 Williams Jack 2007 What s happening inside highs and lows Weather USA Today Retrieved 2009 02 16 Cyclogenesis nsidc org Arctic Climatology and Meteorology National Snow and Ice Data Center 2006 Archived from the original on 2006 08 30 Retrieved 2009 02 21 Cyclogenesis American Meteorological Society amsglossary allenpress com Glossary of Meteorology Allen Press 2009 Retrieved 2009 02 21 Glossary of Meteorology June 2000 Cyclonic circulation American Meteorological Society Retrieved 2008 09 17 Glossary of Meteorology June 2000 Cyclone American Meteorological Society Archived from the original on 2008 10 04 Retrieved 2008 09 17 BBC Weather Glossary July 2006 Cyclone British Broadcasting Corporation Archived from the original on 2006 08 29 Retrieved 2006 10 24 UCAR Glossary Cyclone meted ucar edu Retrieved 2006 10 24 Robert Hart 2003 02 18 Cyclone Phase Analysis and Forecast Help Page Florida State University Retrieved 2006 10 03 I Orlanski 1975 A rational subdivision of scales for atmospheric processes Bulletin of the American Meteorological Society 56 5 527 530 Bibcode 1975BAMS 56 527 doi 10 1175 1520 0477 56 5 527 Rossby wave American Meteorological Society amsglossary allenpress com Glossary of Meteorology Allen Press June 2000 Archived from the original on 2010 12 31 Retrieved 2009 11 06 Short wave American Meteorological Society amsglossary allenpress com Glossary of Meteorology Allen Press June 2000 Archived from the original on 2011 05 14 Retrieved 2009 11 06 Glossary of Meteorology June 2000 Polar vortex American Meteorological Society Archived from the original on 2011 01 09 Retrieved 2009 12 24 Joel Norris 2005 03 19 QG Notes PDF University of California San Diego Archived from the original PDF on 2010 06 26 Retrieved 2009 10 26 Glossary of Meteorology 2009 Short Wave Archived 2009 06 09 at the Wayback Machine American Meteorological Society Retrieved on 2009 03 02 Glossary of Meteorology 2009 Upper Level Trough Archived 2009 06 09 at the Wayback Machine American Meteorological Society Retrieved on 2009 03 02 Carlyle H Wash Stacey H Heikkinen Chi Sann Liou and Wendell A Nuss 1989 A Rapid Cyclogenesis Event during GALE IOP 9 Monthly Weather Review pp 234 257 Retrieved on 2008 06 28 Shay Johnson 2001 09 25 The Norwegian Cyclone Model PDF weather ou edu Archived from the original PDF on 2006 09 01 Retrieved 2006 10 11 E A Rasmussen amp J Turner 2003 Polar Lows Mesoscale Weather Systems in the Polar Regions Cambridge University Press p 612 ISBN 978 0 521 62430 5 Atlantic Oceanographic and Meteorological Laboratory Hurricane Research Division 2004 Frequently Asked Questions What is an extra tropical cyclone NOAA Retrieved 2007 03 23 a b c d e f Chris Landsea 2009 02 06 Frequently Asked Questions How do tropical cyclones form National Oceanic and Atmospheric Administration Retrieved 2009 12 31 Chris Landsea 2004 08 13 Frequently Asked Questions Why do tropical cyclones require 80 F 27 C ocean temperatures to form National Oceanic and Atmospheric Administration Retrieved 2006 07 25 Glossary of Meteorology 2009 Mesocyclone American Meteorological Society Archived from the original on 2006 07 09 Retrieved 2006 12 07 Choy Barry K Scott M Spratt 2003 05 13 Using the WSR 88D to Predict East Central Florida Waterspouts NOAA Archived from the original on 2008 06 17 Retrieved 2009 12 26 a b Glossary of Meteorology 2009 Thermal Low Archived 2008 05 22 at the Wayback Machine American Meteorological Society Retrieved on 2009 03 02 a b Dr Louisa Watts 2009 What causes the west African monsoon National Centre for Environmental Science Retrieved on 2009 04 04 Dr Michael Pidwirny 2008 CHAPTER 8 Introduction to the Hydrosphere e Cloud Formation Processes Physical Geography Retrieved on 2009 01 01 Bart van den Hurk and Eleanor Blyth 2008 Global maps of Local Land Atmosphere coupling Archived 2009 02 25 at the Wayback Machine KNMI Retrieved on 2009 01 02 Robert Penrose Pearce 2002 Meteorology at the Millennium Academic Press p 66 ISBN 978 0 12 548035 2 Retrieved on 2009 01 02 Glossary of Meteorology June 2000 Gust Front American Meteorological Society Archived from the original on 2011 05 05 Retrieved 2008 07 09 BBC Weather 2004 09 01 The Asian Monsoon Archived from the original on August 31 2007 Retrieved 2008 05 22 Todd Mitchell 2004 Arctic Oscillation AO time series 1899 June 2002 Archived 2003 12 12 at the Wayback Machine University of Washington Retrieved on 2009 03 02 a b L de la Torre Nieto R Noguerol M Anel J A Gimeno L 2008 A climatology based on reanalysis of baroclinic developmental regions in the extratropical northern hemisphere Annals of the New York Academy of Sciences vol 1146 pp 235 255 Retrieved on 2009 03 02 Ian Simmonds amp Kevin Keay February 2000 Variability of Southern Hemisphere Extratropical Cyclone Behavior 1958 97 Journal of Climate 13 3 550 561 Bibcode 2000JCli 13 550S doi 10 1175 1520 0442 2000 013 lt 0550 VOSHEC gt 2 0 CO 2 ISSN 1520 0442 S K Gulev O Zolina amp S Grigoriev 2001 Winter Storms in the Northern Hemisphere 1958 1999 via the Internet Wayback Machine Climate Dynamics 17 10 795 809 Bibcode 2001ClDy 17 795G doi 10 1007 s003820000145 S2CID 129364159 Met Office 2009 Frontal Depressions Archived 2009 02 24 at the Wayback Machine Retrieved on 2009 03 02 Understanding weather KNMI Depressie Becca Hatheway 2008 Hadley Cell University Corporation for Atmospheric Research Archived from the original on 2012 05 26 Retrieved 2009 02 16 National Centre for Medium Range Forecasting 2004 10 23 Chapter II Monsoon 2004 Onset Advancement and Circulation Features PDF Ministry of Earth Sciences India Archived from the original PDF on 2011 07 21 Retrieved 2008 05 03 Australian Broadcasting Corporation 1999 08 11 Monsoon Australian Broadcasting Corporation Retrieved 2008 05 03 Mary E Davis amp Lonnie G Thompson 2005 Forcing of the Asian monsoon on the Tibetan Plateau Evidence from high resolution ice core and tropical coral records Journal of Geophysical Research 110 D4 1 of 13 Bibcode 2005JGRD 110 4101D doi 10 1029 2004JD004933 U S Navy 1998 01 22 1 2 Pacific Ocean Surface Streamline Pattern Retrieved 2006 11 26 Hobgood 2008 Global Pattern of Surface Pressure and Wind Ohio State University Archived from the original on 2009 03 18 Retrieved 2009 03 08 Atlantic Oceanographic and Meteorological Laboratory Hurricane Research Division 2009 02 06 Frequently Asked Questions When is hurricane season National Oceanic and Atmospheric Administration Retrieved 2009 12 24 Examining the ENSO PDF James B Elsner Kam Biu Liu 2003 10 08 Retrieved 2007 08 18 BWEA 2007 Education and Careers What is wind Archived 2011 03 04 at the Wayback Machine British Wind Energy Association Retrieved on 2009 02 16 a b JetStream 2008 Origin of Wind National Weather Service Southern Region Headquarters Retrieved on 2009 02 16 Nelson Stephen Fall 2014 Tropical Cyclones Hurricanes Wind Systems Low Pressure Centers Tulane University Retrieved 2016 12 24 a b What is the difference between a hurricane a cyclone and a typhoon OCEAN FACTS National Ocean Service Retrieved 2016 12 24 COMPARE AND CONTRAST MID LAT CYCLONE AND HURRICANE www theweatherprediction com Retrieved 2020 02 24 What is a Hurricane Typhoon or Tropical Cyclone Precipitation Education pmm nasa gov Retrieved 2020 02 24 Horton Jennifer 20 July 2009 Does the rotation of the Earth affect toilets and baseball games SCIENCE EVERYDAY MYTHS HowStuffWorks Retrieved 2016 12 25 Do Tornadoes Always Twist in the Same Direction SCIENCE Earth and Space WONDEROPOLIS Retrieved 2016 12 25 Retrieved from https en wikipedia org w index php title Low pressure area amp oldid 1129899538, wikipedia, wiki, book, books, library,

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