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[[Archivo:Microburstnasa.JPG|250px|thumb|Ilustración de un microfrente. El régimen ventoso en el microfrente es opuesto al de un tornado.]]<!--
{{See also|Downburst|Squall line}}
A [[microburst]] is a very localized column of sinking air known as a downburst, producing damaging divergent and [[straight-line winds]] at the surface that are similar to but distinguishable from [[tornado]]es which generally have convergent damage.<ref>{{cite web|author=Dr. Nolan Atkins|date=2009|publisher=[[Lyndon State College]] Meteorology|url=http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/tornado_mb_damage.html|title=How to distinguish between tornado and microburst (straight-line) wind damage|accessdate=9 de julio de 2008-07-09}}</ref> The term was defined as affecting an area {{convert|4|km|mi}} in diameter or less,<ref>{{cite web|author=[[National Weather Association]]|date=23 de noviembre de 2003|url=http://www.nwas.org/committees/avnwxcourse/lesson5.htm|title=Welcome to Lesson 5|accessdate=9 de julio de 2008-07-09}}</ref> distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. They are normally associated with individual thunderstorms. Microburst soundings show the presence of mid-level dry air, which enhances evaporative cooling.<ref name="micro">{{cite web|author=Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III|date=26 de junio de 2002|publisher=Cooperative Institute for Mesoscale Meteorological Studies|url=http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html|title=Microbursts: A Handbook for Visual Identification|accessdate=9 de julio de 2008}}</ref>
 
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. This outrunning occurs within the [[westerlies]] in a pattern where the upper level jet splits into two streams. The resultant [[Mesoscale Convective System|mesoscale convective system]] (MCS) forms at the point of the upper level split in the wind pattern in the area of best low level inflow. The convection then moves east and toward the [[equator]] into the warm sector, parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a [[squall line]], with the feature placed at the leading edge of the significant wind shift and pressure rise which is normally just ahead of its radar signature.<ref>{{cite web|author=Office of the Federal Coordinator for Meteorology|date=2008|url=http://www.ofcm.gov/slso/pdf/slsochp2.pdf|title=Chapter 2: Definitions|page=2-1|publisher=[[NOAA]]|accessdate=3 de mayo de 2009-05-03}}</ref> This feature is commonly depicted in the warm season across the [[United States]] on surface analyses, as they lie within sharp surface troughs.
 
A macroburst, normally associated with squall lines, is a strong downburst larger than {{convert|4|km|mi}}.<ref>{{cite web|author=Dr. Ali Tokay|url=http://userpages.umbc.edu/~tokay/chapter13.html|title=Chapter #13: Thunderstorms|accessdate=9 de julio de 2008-07-09|date=2000-04-21 de abril de 2000|publisher=[[University of Maryland]] Baltimore College}}</ref> A wet microburst consists of precipitation and an atmosphere saturated in the low-levels. A dry microburst emminates from high-based thunderstorms with [[virga]] falling from their base.<ref name=autogenerated1>Fernando Caracena, Ronald L. Holle, and Charles A. Doswell III. [http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html Microbursts: A Handbook for Visual Identification.] Retrieved on 2008-07-09.</ref> All types are formed by precipitation-cooled air rushing to the surface. Downbursts can occur over large areas. In the extreme case, a [[derecho]] can cover a huge area more than {{convert|200|mi|km|abbr=off}} wide and over {{convert|1000|mi|km|abbr=off}} long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds,<ref>{{cite web|author=Peter S. Parke and Norvan J. Larson|date=23 de noviembre de 2005|url=http://www.crh.noaa.gov/dlh/science/event_archive/summer_archive/1999blowdown/1999blowdown.php|title=Boundary Waters Windstorm|accessdate=2008-07-30 de julio de 2008|publisher=[[National Weather Service]] Forecast Office, [[Duluth, Minnesota]]}}</ref> but the generative process is somewhat different from that of most downbursts.
 
{{clear}}
[[Archivo:DangerousShelfCloud.jpg|thumb|250px|Nube de frente, [[Minnesota]].]]<!--
At ground level, [[shelf cloud]]s and [[roll cloud]]s can be seen at the leading edge of outflow boundaries.<ref>{{cite web|url=http://www.met.utah.edu/class/jhorel/5140/radar_handbook.pdf|title=Federal Meteorological Handbook No. 11 Doppler RADAR Meteorological Observations Part B Doppler RADAR Theory and Meteorology|author=Office of the Federal Coordinator For
Meteorological Services and Supporting Research|publisher=[[Department of Commerce]]|date=December 2005|accessdate=1 de septiembre de 2009-09-01}}</ref> Through [[weather satellite|satellite]] imagery, an arc cloud is visible as an arc of low clouds spreading out from a thunderstorm. If the skies are cloudy behind the arc, or if the arc is moving quickly indicate that high wind gusts are likely behind the gust front.<ref>{{cite book|url=http://books.google.com/books?id=o2wfF7xNqGYC&pg=PA322&lpg=PA322&dq=satellite+arc+cloud+outflow+boundary+book&source=bl&ots=nIBpVqGJw5&sig=n7cafZP3T7-tKw3PcR43TDH3keU&hl=en&ei=d7OdSoH-G86w8QanycG4Aw&sa=X&oi=book_result&ct=result&resnum=2#v=onepage&q=&f=false|author=Pravas Mahapatra, R. J. Doviak, Vladislav Mazur, Dušan S. Zrnić|title=Aviation weather surveillance systems: advanced radar and surface sensors for flight safety and air traffic management, Volume 183|page=322|year=1999|publisher=Institution of Electrical Engineers|ISBN=9780852969373|accessdate=1 de septiembre de 2009-09-01}}</ref> Sometimes a gust front can be seen on [[weather radar]], showing as a thin arc or line of weak radar echos pushing out from a collapsing storm. The thin line of weak radar echoes is known as a fine line.<ref>{{cite web|author=Glossary of Meteorology|date=2009|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=fine+line&submit=Search|title=Fine Line|publisher=[[American Meteorological Society]]|accessdate=3 de julio de 2009-07-03}}</ref> Occasionally, winds caused by the gust front are so high in velocity, that they may also show up on radar. This cool outdraft can then energize other storms which it hits by assisting in [[updraft]]s. Gust fronts colliding from two storms can even create new storms. However, there is usually no rain accompanying the shifting winds. An expansion of the rain shaft near ground level, in the general shape of a foot, is a telltale sign of a downburst. Gustnadoes, short-lived vertical circulations near ground level, can be spawned by outflow boundaries.<ref name="micro" />
 
==Effects==
[[Image:Undular bore waves over Arabian Sea.jpg|250px|thumb|Satellite image of an undular bore.]]
{{See also|Wind shear|Undular bore}}
Gust fronts create low-level [[wind shear]] which can be hazardous to planes when they takeoff or land.<ref>{{cite journal|author=Diana L. Klingle, David R. Smith, and Marilyn M. Wolfson|url=http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0493(1987)115%3C0905%3AGFCADB%3E2.0.CO%3B2|title=Gust Front Characteristics as Detected by Doppler Radar|accessdate=9 de julio de 2008-07-09|journal=Monthly Weather Review|pages=905-918|volume=115|number=5|date=May 1987}}</ref> Flying [[insect]]s, a subset of [[arthropods]], are swept along by the [[prevailing winds]].<ref>{{cite web|author=Diana Yates|date=2008|url=http://news.illinois.edu/news/08/0707birds.html|title=Birds migrate together at night in dispersed flocks, new study indicates|publisher=[[University of Illinois]] at Urbana - Champaign|accessdate=2009-04-26 de abril de 2009}}</ref> As such, fine line patterns within [[weather radar]] imagery, associated with converging winds, are dominated by insect returns.<ref>{{cite web|author=Bart Geerts and Dave Leon|date=2003|url=http://www-das.uwyo.edu/wcr/projects/ihop02/coldfront_preprint.pdf|title=P5A.6 Fine-Scale Vertical Structure of a Cold Front As Revealed By Airborne 95 GHZ Radar|publisher=[[University of Wyoming]]|accessdate=2009-04-26 de abril de 2009}}</ref> At the surface, clouds of dust can be raised by outflow boundaries. If squall lines form over arid regions, a duststorm known as a [[haboob]] may result from the high winds in their wake picking up dust from the desert floor.<ref>{{cite web|author=Western Region Climate Center|date=2002|url=http://www.wrcc.dri.edu/ams/glossary.html#H|title=H|publisher=Desert Research Institute|accessdate=22 de octubre de 2006}}</ref> If outflow boundaries move into areas of the atmosphere which are stable in the low levels, such as over colder pockets of ocean or through the cold sector of [[extratropical cyclone]]s, they can create a phenomenon known as an undular bore, which shows up on satellite and radar imagery as a series of [[transverse wave]]s in the cloud field oriented perpendicular to the low-level winds.<ref>{{cite web|url=http://www.arpal.org/Pubbl/paper/eumetsat-mauritania.pdf|title=Outflow from convective storm, Mauritania and adjacent Atlantic Ocean (13 August 2006)|author=Martin Setvak, Jochen Kerkmann, Alexander Jacob, HansPeter Roesli, Stefano Gallino, and Daniel Lindsey|accessdate=3 de julio de 2009-07-03|date=2007-03-19 de marzo de 2007|publisher=Agenzia Regionale per la Protezione dell'Ambiente Ligure}}</ref>
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== Véase también ==
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