Editing Pressure system

{{short description|Relative peak or lull in the sea level pressure distribution}}
{{good article}}
{{for|pressure systems and safety|Pressure vessel}}
[[File:Surface analysis.gif|thumb|Map of pressure systems across North America]] A '''pressure system''' is a  peak or lull in the [[Atmospheric pressure#Mean sea-level pressure|sea level pressure]] distribution, a feature of [[synoptic-scale weather]]. The surface pressure at sea level varies minimally, with the lowest value measured {{convert|87|kPa|inHg}} and the highest recorded  {{convert|108.57|kPa|inHg}}. High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause [[weather]] to be experienced locally. Low-pressure systems are associated with [[cloud]]s and [[precipitation (meteorology)|precipitation]] that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of [[meteorology]] within [[surface weather analysis|surface weather map]]s.

== Low-pressure system ==
[[File:Low pressure system over Iceland.jpg|thumb|250px|right|An [[extratropical cyclone]] swirls off the southwestern coast of Iceland.]]
{{Main article|Low-pressure area|Cyclone}}
A low-pressure area is a region where the [[atmospheric pressure]] at [[sea level]] is below that of surrounding locations. Low-pressure systems form under areas of [[wind]] divergence that occur in the upper levels of the [[troposphere]].<ref>{{cite web|url=http://meteora.ucsd.edu/~jnorris/sio217B/QG_Notes.pdf|title=QG Notes|author=Joel Norris|publisher=[[University of California]], San Diego|access-date=2009-10-26|date=2005-03-19|url-status=dead|archive-url=https://web.archive.org/web/20100626054418/http://meteora.ucsd.edu/~jnorris/sio217B/QG_Notes.pdf|archive-date=2010-06-26}}</ref> The formation process of a low-pressure area is known as [[cyclogenesis]].<ref>Arctic Climatologyal Snow and Ice Data Center. Retrieved on 2009-02-21.</ref> Within the field of [[Meteorology#Dynamic meteorology|atmospheric dynamics]], areas of wind divergence aloft occur in two areas:

* On the east side of upper troughs, which form half of a [[Rossby wave]] within the [[Westerlies]] (a trough with large [[wavelength]], which extends through the troposphere).
* Ahead of [[Shortwave (meteorology)|embedded shortwave troughs]], which have smaller wavelengths.

Diverging winds aloft ahead of these troughs cause [[atmospheric]] lift within the troposphere below, which [[pressure|lowers surface pressure]]s as upward motion partially counteracts the force of gravity.<ref>{{cite book|title=Atmosphere, Weather & Climate|author=Roger G. Barry and Richard J. Chorley|edition=5|date=187|pages=[https://archive.org/details/texasrealestate0000jaco/page/194 194–199]|publisher=Routledge|isbn=978-0-415-04585-8|url=https://archive.org/details/texasrealestate0000jaco/page/194}}</ref>

[[Thermal low]]s form due to localized heating caused by greater sunshine 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.<ref name="thermal">{{cite web|author=Glossary of Meteorology|date=June 2000|url=http://amsglossary.allenpress.com/glossary/search?id=thermal-low1|title=Thermal Low|publisher=[[American Meteorological Society]]|access-date=2009-03-02|url-status=dead|archive-url=https://web.archive.org/web/20080522195848/http://amsglossary.allenpress.com/glossary/search?id=thermal-low1|archive-date=2008-05-22}}</ref>  Large-scale thermal lows over [[continent]]s help create pressure gradients that drive [[monsoon]] circulations.<ref name="cause">{{cite journal|title=Forcing of the Asian monsoon on the Tibetan Plateau: Evidence from high-resolution ice core and tropical coral records|author=Mary E. Davis and Lonnie G. Thompson|journal=Journal of Geophysical Research|year=2005|volume=110|page=1 of 13|doi=10.1029/2004JD004933|bibcode=2005JGRD..110.4101D |doi-access=free}}</ref>  Low-pressure areas can also form due to organized thunderstorm activity over warm water.<ref name = "AOML FAQ A7">{{cite web | author = [[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division | title = Frequently Asked Questions: What is an extra-tropical cyclone? | publisher = [[NOAA]] | access-date = 2007-03-23 | year=2004 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A7.html}}</ref>  When this occurs over the tropics in concert with the [[Intertropical Convergence Zone]], it is known as a [[monsoon trough]].<ref name="ams">{{cite web|title=Monsoon trough|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon+trough&submit=Search|author=Glossary of Meteorology|publisher=[[American Meteorological Society]]|date=June 2000|access-date=2009-06-04|url-status=dead|archive-url=https://web.archive.org/web/20090617192300/http://amsglossary.allenpress.com/glossary/search?p=1&query=monsoon+trough&submit=Search|archive-date=2009-06-17}}</ref>  Monsoon troughs reach their northerly extent in August and their southerly extent in February.<ref name="NCFMRF">{{cite web|author=National Centre for Medium Range Forecasting |date=2004-10-24 |url=http://www.ncmrwf.gov.in/Chapter-II.pdf |title=Chapter-II Monsoon-2004: Onset, Advancement and Circulation Features |access-date=2010-11-23 |url-status=dead |archive-url=https://web.archive.org/web/20110721161408/http://www.ncmrwf.gov.in/Chapter-II.pdf |archive-date=2011-07-21 }}</ref><ref>[[Australian Broadcasting Corporation]] (2000). [http://www.abc.net.au/storm/monsoon/print.htm Monsoon.] Retrieved on 2008-05-03.</ref><ref name="NAVY2">U. S. Navy. [https://web.archive.org/web/20000530113146/http://www.nrlmry.navy.mil/~chu/chap2/se102.htm 1.2 Pacific Ocean Surface Streamline Pattern.] Retrieved on 2006-11-26.</ref>  When a convective low acquires a well-defined circulation in the tropics it is termed a [[tropical cyclone]].<ref name = "AOML FAQ A7"/>  Tropical cyclones can form during any month of the year globally, but can occur in either the northern hemisphere or the southern hemisphere during November.<ref name="AOML FAQ G1">{{cite web|author=[[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division|title=Frequently Asked Questions: When is hurricane season?|publisher=[[National Oceanic and Atmospheric Administration]]|access-date=2010-11-23|date=2010-01-21|url=http://www.aoml.noaa.gov/hrd/tcfaq/G1.html}}</ref>

Atmospheric lift caused by low-level wind convergence into the surface low brings clouds and potentially [[precipitation (meteorology)|precipitation]].<ref name="convection">{{cite book|author=Robert Penrose Pearce|year=2002|url=https://books.google.com/books?id=QECy_UBdyrcC&q=ways+to+moisten+the+atmosphere&pg=PA66|title=Meteorology at the Millennium|publisher=Academic Press|page=66|isbn=978-0-12-548035-2|access-date=2009-01-02}}</ref> The low-pressure area's cloudy skies act to minimize [[diurnal temperature variation]]. Since clouds reflect [[sunlight]], incoming [[shortwave radiation|shortwave]] [[solar radiation]] is less, which causes lower [[temperature]]s during the day. At night, the absorptive effect of clouds on [[outgoing longwave radiation]], such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons.  The stronger the area of low pressure, the stronger the [[wind]]s experienced in its vicinity.<ref name="jet">JetStream (2008). [http://www.srh.noaa.gov/jetstream//synoptic/wind.htm Origin of Wind.] [[National Weather Service]] Southern Region Headquarters. Retrieved on 2009-02-16.</ref> Around the world, low-pressure systems are most frequently located over the [[Tibetan Plateau]] and in the lee of the [[Rocky Mountains]].<ref name="extra">L. de la Torre, Nieto R., Noguerol M., Añel J.A., Gimeno L. (2008). [https://www.ncbi.nlm.nih.gov/pubmed/19076418 A climatology based on reanalysis of baroclinic developmental regions in the extratropical northern hemisphere.] Ann [[New York Academy of Science]];vol. 1146: pp. 235–255.  Retrieved on 2009-03-02.</ref> In Europe—in particular, the [[United Kingdom]] and the [[Netherlands]]—recurring low-pressure weather systems are typically known as depressions. The lowest recorded non-tornadic barometric pressure was {{convert|870|hPa|inHg}}, occurring in the Western Pacific during [[Typhoon Tip]] on 12 October 1979.<ref name=FAQ1>{{cite web|url=http://www.aoml.noaa.gov/hrd/tcfaq/E1.html |title=Subject: E1), Which is the most intense tropical cyclone on record? |author=Chris Landsea|publisher=[[Atlantic Oceanographic and Meteorological Laboratory]]|date=2010-04-21 |access-date=2010-11-23|author-link=Chris Landsea }}</ref>

== High-pressure system ==
{{Main article|High-pressure area|Anticyclone}}
[[File:High Pressure.jpg|thumb|250px|Satellite image of a high-pressure area south of Australia, evidenced by the clearing in the clouds<ref>{{cite web|url=http://earthobservatory.nasa.gov/IOTD/view.php?id=78208|title=An Australian "Anti-storm"|date=2012-06-08|access-date=2013-02-12|publisher=NASA}}</ref>]]
High-pressure systems are frequently associated with light winds at the surface and [[Subsidence (atmosphere)|subsidence]] through the lower portion of the [[troposphere]]. In general, subsidence will dry out an air mass by [[adiabatic process|adiabatic]] or compressional heating.<ref>Office of the Federal Coordinator for Meteorology (2006). [http://www.ofcm.gov/fmh3/pdf/15-app-g.pdf Appendix G: Glossary.] {{webarchive|url=https://web.archive.org/web/20090225025158/http://www.ofcm.gov/fmh3/pdf/15-app-g.pdf |date=2009-02-25 }} [[NOAA]]. Retrieved on 2009-02-16.</ref> Thus, high pressure typically brings clear skies.<ref>Jack Williams (2007). [https://www.usatoday.com/weather/tg/whighlow/whighlow.htm What's happening inside highs and lows.] [[USA Today]]. Retrieved on 2009-02-16.</ref>  During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave [[solar radiation]] and temperatures rise.  At night, the absence of clouds means that [[outgoing longwave radiation]] (i.e. heat energy from the surface) is not absorbed, giving cooler [[Diurnal temperature variation|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]].<ref>{{cite web|author=Myanmar government|year=2007|url=http://www.kjc.gov.my/english/education/weather/haze01.html |title=Haze|publisher=Internet Wayback Machine|access-date=2007-02-11 |archive-url = https://web.archive.org/web/20080224035453/http://www.kjc.gov.my/english/education/weather/haze01.html |archive-date = 2008-02-24}}</ref>  If the low-level [[relative humidity]] rises towards 100 percent overnight, [[fog]] can form.<ref>Robert Tardif (2002). [http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm Fog characteristics.] {{webarchive|url=https://web.archive.org/web/20110520021633/http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm |date=2011-05-20 }} [[NCAR]] National Research Laboratory. Retrieved on 2007-02-11.</ref>

Strong but vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses.<ref>[[CBC News]] (2009). [https://www.cbc.ca/news/canada/north/blame-yukon-arctic-air-mass-chills-rest-of-north-america-1.860626?ref=rss Blame Yukon: Arctic air mass chills rest of North America.] Canadian Broadcasting Centre. Retrieved on 2009-02-16.</ref>  The low, sharp [[inversion (meteorology)|temperature inversion]] can lead to areas of persistent [[stratocumulus]] or [[stratus cloud]], known in colloquial terms as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high bubble pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressure systems that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed), which helps prevent clouds from forming.  The highest barometric pressure ever recorded on Earth was {{convert|1085.7|hPa|inHg}} measured in [[Tosontsengel, Zavkhan|Tonsontsengel, Mongolia]] on 19 December 2001.<ref>{{cite book|page=[https://archive.org/details/extremeweathergu00burt_0/page/234 234]|title=Extreme Weather|author=Christopher C. Burt|year=2004|edition=1|isbn=978-0-393-32658-1|publisher=Twin Age Ltd.|url-access=registration|url=https://archive.org/details/extremeweathergu00burt_0/page/234}}</ref>

==Surface weather maps==
[[File:Streamlineanalysispacific.gif|thumb|right|250px|Streamline analysis of the tropical Pacific Ocean]]

{{See also|Surface weather analysis}}
A surface weather analysis is a type of [[weather map]] that depicts positions for [[high-pressure area|high]]- and [[low-pressure area]]s, as well as various types of [[synoptic scale]] systems such as [[weather fronts|frontal zones]].  Isotherms can be drawn on these maps, which are lines of equal temperature.  Isotherms are drawn normally as solid lines at a preferred temperature interval.<ref name="DataAir">{{cite web|author=DataStreme Atmosphere|publisher=American Meteorological Society|url=http://www.ametsoc.org/amsedu/dstreme/learn/sample.act.html |title=Air Temperature Patterns|date=2008-04-28|access-date=2010-02-07 |archive-url = https://web.archive.org/web/20080511124504/http://www.ametsoc.org/amsedu/dstreme/learn/sample.act.html |archive-date = 2008-05-11}}</ref>  They show temperature gradients, which can be useful in finding fronts, which are on the warm side of large temperature gradients.  By plotting the freezing line, isotherms can be useful in determination of precipitation type.  [[Mesoscale convective system]]s such as [[tropical cyclone]]s, outflow boundaries and [[squall line]]s also are analyzed on surface weather analyses.

Isobaric analysis is performed on these maps, which involves the construction of lines of equal mean [[sea level pressure]].  The innermost closed lines indicate the positions of relative maxima and minima in the pressure field.  The minima are called low-pressure areas, and the maxima are called [[high-pressure area]]s.  A ''High'' is often shown as '''H''', and a ''[[low-pressure area|low]]'' is shown as '''L'''.  Elongated areas of low pressure, or troughs, are sometimes plotted as thick, brown dashed lines down the trough axis.<ref>{{cite web|publisher=University of Wisconsin|author=Edward J. Hopkins, Ph.D.|date=1996-06-10|url=http://www.meteor.wisc.edu/~hopkins/aos100/sfc-anl.htm|title=Surface Weather Analysis Chart|access-date=2007-05-10}}</ref>  Isobars are commonly used to place surface boundaries from the [[horse latitudes]] poleward, while streamline analyses are used in the tropics.<ref>{{cite web|author=Bureau of Meteorology|url=http://www.bom.gov.au/nmoc/MSL/WeatherMap.shtml#types|title=The Weather Map|publisher=Commonwealth of Australia|year=2010|access-date=2010-02-06}}</ref>  A streamline analysis is a series of arrows oriented parallel to wind, showing wind motion within a certain geographic area.  Cs depict cyclonic flow or likely areas of low pressure, while As depict anticyclonic flow or likely positions of high-pressure areas.<ref>{{cite web|author=[[National Weather Service]] Forecast Office [[Honolulu, Hawaii]]|url=http://www.prh.noaa.gov/hnl/graphics/tropsl.tif|title=Pacific Streamline Analysis|publisher=Pacific Region Headquarters|date=2010-02-07|access-date=2010-02-07}}</ref>  An area of confluent streamlines shows the location of [[Weather front#Stationary front and shearline|shearlines]] within the tropics and subtropics.<ref name="DR">{{cite web|author=David M. Roth|publisher=[[Hydrometeorological Prediction Center]]|date=2006-12-14|url=http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf|title=Unified Surface Analysis Manual|access-date=2006-10-22}}</ref>
== References ==
{{reflist|2}}

{{DEFAULTSORT:Pressure System}}
[[Category:Meteorological phenomena]]