Editing Pressure system (section)

== 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>