Editing Squall

{{Short description|Short, sharp increase in wind speed}}
{{Other uses}}
{{Distinguish|squaw}}
A '''squall''' is a sudden, sharp increase in [[wind speed]] lasting minutes, as opposed to a [[wind gust]], which lasts for only seconds.<ref name=AMS>{{cite web|url=http://glossary.ametsoc.org/wiki/Squall|work=Glossary|publisher=American Meteorological Society|access-date=November 15, 2018|title=Squall}}</ref> They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow.<ref>{{cite web|publisher=The Weather Channel |url=http://www.weather.com/glossary/s.html |title=Weather Glossary: S.|archive-url= https://web.archive.org/web/20080801101119/http://www.weather.com/glossary/s.html |archive-date=August 1, 2008|access-date= November 15, 2018}}</ref> Squalls refer to the increase of the sustained winds over that time interval, as there may be higher gusts during a squall event.<ref name=BOM>{{cite web|publisher=Australian Bureau of Meteorology|url=http://www.bom.gov.au/info/wwords/ |title=Weather Words |access-date=November 15, 2018}}</ref> They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

[[File:Thunder and wind during a thunderstorm.webm|thumb|A video recorded nearing the end of a squall, caused by a dry [[microburst]] (strong downdraft); after this video was shot, it started raining.]]

==Etymology==
There are different versions of the word's origins:
* By one version, the word appears to be Nordic in origin, but its [[etymology]] is considered obscure. It probably has its roots in the word ''skvala''<ref>[http://www.answers.com/topic/squall?cat=technology squall: Definition and Much More from Answers.com<!-- Bot generated title -->]</ref> an [[Old Norse]] word meaning literally ''to squeal''.<ref>Georoots News. {{usurped|1=[https://archive.today/20030418173217/http://www.georoots.org/Georoots_News/Volume_1/georoots_v1_5.htm Georoots News V.1#5: Changes in the Wind.]}} Retrieved on 2006-12-30.</ref> <!-- See Discussion The first known use of the term on [[surface weather analysis|surface weather analyses]] was in the [[United States]] in the late Nineteenth Century on U. S. Signal Corp Maps, begun in 1871.-->
* By another version, it is an alteration of ''squeal'' influenced by ''bawl''.<ref>{{cite book |url=https://books.google.com/books?id=4XycAQAAQBAJ&pg=PA1400 |title=Concise Oxford English Dictionary |publisher=Oxford University Press |year=2011 |access-date=2014-12-30 |isbn=978-0-19-960108-0 |page=1400}}</ref>

==Character of the wind==
The term "squall" is used to refer to a sudden wind-speed increase lasting minutes. In 1962 the [[World Meteorological Organization]] (WMO) defined that to be classified as a "squall", the wind must increase at least {{Convert|8|m/s|km/h mph}} and must attain a top speed of at least {{Convert|11|m/s|km/h mph}}, lasting at least one minute in duration.<ref name=AMS/><ref>{{cite web|url=http://www.eumetcal.org/euromet/glossary/squall.htm |archive-url=https://web.archive.org/web/20160303190211/http://www.eumetcal.org/euromet/glossary/squall.htm|archive-date=March 3, 2016|title=Squall|publisher=Eumetcal|author=WMO|access-date=November 15, 2018}}</ref> In Australia, a squall is defined to last for several minutes before the wind returns to the long-term mean value.<ref name=BOM/> In either case, a squall is defined to last about half as long as the definition of sustained wind in its respective country. Usually, this sudden violent wind is associated with briefly heavy precipitation as [[squall line]].<ref name=AMS/>

==Regional terms==

===Argentina===
Known locally as ''[[Pampero (wind)|pamperos]]'', these are characterized as strong downsloped winds that move across the [[pampas]], eventually making it to the Atlantic Ocean.<ref>{{Cite web |title=names of winds |url=https://ggweather.com/winds.html#:~:text=Pampero}}</ref>

===Australia===
In southeastern Australia, the colloquial name for a squall is ''[[southerly buster]]'', which is an abrupt southerly wind change in the southern regions of [[New South Wales]] and [[Victoria (Australia)|Victoria]], [[Australia]], which approaches from the southeast, mainly on a hot day, [[sea breeze|bringing in cool]], usually [[severe weather]] and a dramatic temperature drop, thus ultimately replacing and relieving the prior hot conditions.<ref>{{cite news |url=https://nla.gov.au/nla.news-article138978287 |title=SOUTHERLY BUSTER. |newspaper=[[Wellington Times]] |date=23 December 1901 |accessdate=27 March 2015 |page=2 |publisher=National Library of Australia}}</ref> 

===Central America===
Offshore Central America, a ''gully squall'' is characterized by strong increases of the wind forced through sharp mountain valleys on the Pacific Ocean side of the isthmus.

===Cuba===
A ''bayamo'' is a squall emanating from tropical thunderstorms near the [[Bight (geography)|Bight]] of Bayamo.<ref>{{cite web |publisher=WGN-TV |url=http://wgntv.trb.com/news/weather/wgntv-weatherwords-b,0,1454739.story?coll=wgntv-weather-4 |title=Weather Words – B |access-date=2006-11-19 |url-status=dead |archive-url=https://web.archive.org/web/20070318151426/http://wgntv.trb.com/news/weather/wgntv-weatherwords-b,0,1454739.story?coll=wgntv-weather-4 |archive-date=2007-03-18 }}</ref>

===Pacific Northwest (North America)===
In the [[Pacific Northwest]], a ''squall'' is a short but furious rainstorm with strong winds, often small in area and moving at high speed, especially as a maritime term. A strong [[Katabatic wind|Katabatic]] outflow occurring in fjords and inlets is referred to by mariners as a ''[[squamish (wind)|squamish]]''.

===South Africa===
''Bull's Eye Squall'' is a term used offshore South Africa for a squall forming in fair weather. It is named for the appearance of the small isolated cloud marking the top of the squall.<ref name="ggweather">Golden Gate Weather Services. [http://ggweather.com/winds.html Names of Winds.]</ref>

===Philippines (West Pacific)===
In most parts of the country, squalls are called ''subasko'' and are characterized by heavy rains driven by blustery winds. Local fishermen at sea are often on the lookout for signs of impending squalls on the open water and rush to shore at its early signs.

===South-East Asia===
"Barat" is a term for a northwest squall in Manado Bay in [[Sulawesi]].<ref name="ggweather" />

"[[Sumatra squall]]" is a term used in [[Singapore]] and Peninsular [[Malaysia]] for [[squall line]]s that form over the island of [[Sumatra]] and move east across the [[Straits of Malacca]]. Gusts can reach up to 28&nbsp;m/s (100&nbsp;km/h).<ref name="sumatras">National Environment Agency [https://web.archive.org/web/20131101173456/http://app.nea.gov.sg/data/mss/pdf/26March07.pdf Guide To Singapore's Weather, archived by archive.org on November 1, 2013.]</ref>

==Severe weather==

[[File:DangerousShelfCloud.jpg|thumb|299px|A [[shelf cloud]] such as this one can be a sign that a squall is imminent]]
A ''[[squall line]]'' is an organized line of [[thunderstorm]]s. It is classified as a multi-cell cluster, meaning a thunderstorm complex comprising many individual updrafts. They are also called multi-cell lines. Squalls are sometimes associated with [[hurricane]]s or other [[cyclone]]s, but they can also occur independently. Most commonly, independent squalls occur along [[Weather front|front lines]], and may contain heavy [[precipitation (meteorology)|precipitation]], [[hail]], frequent [[lightning]], dangerous straight line winds, and possibly [[funnel cloud]]s, [[tornado]]es and [[waterspout]]s.<ref>Weatherquestions.com. [http://www.weatherquestions.com/What_is_a_squall_line.htm What is a Squall Line?] Retrieved on 2006-11-19.</ref> Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical [[wind shear]] from an angle behind the frontal boundary.<ref>Wilfried Jacobs. [http://www.zamg.ac.at/eumetrain/EUMeTrain2005/Squall/intro.htm EUMeTrain: Case Study on Squall Line.] {{webarchive|url=https://web.archive.org/web/20061019140100/http://www.zamg.ac.at/eumetrain/EUMeTrain2005/Squall/intro.htm |date=2006-10-19 }} Retrieved on 2006-11-19.</ref> The strong winds at the surface are usually a reflection of dry air intruding into the line of storms, which when saturated, falls quickly to ground level due to its much higher density before it spreads out downwind.<ref>Thinkquest. [http://library.thinkquest.org/C0112425/stu_thundertornado.htm#2 Meteorology Online: Squall.] {{webarchive|url=https://web.archive.org/web/20070205035130/http://library.thinkquest.org/C0112425/stu_thundertornado.htm |date=2007-02-05 }} Retrieved on 2006-11-19.</ref> Significant squall lines with multiple bow echoes are known as [[derecho]]s.<ref>Robert H. Johns and Jeffry S. Evans. Storm Prediction Center. [http://www.spc.ncep.noaa.gov/misc/AbtDerechos/derechofacts.htm Derecho Facts.] {{Webarchive|url=https://web.archive.org/web/20100527085203/http://www.spc.ncep.noaa.gov/misc/AbtDerechos/derechofacts.htm |date=2010-05-27 }} Retrieved on 2006-11-19.</ref>

===Squall line life cycle===
There are several forms of [[mesoscale meteorology]], including simplistic isolated thunderstorms unrelated to advancing cold fronts, to the more complex daytime/nocturnal [[mesoscale convective system]] (MCS) and [[mesoscale convective complex]] (MCC), to [[squall line]] thunderstorms.

====Formation====
The main driving force behind squall line creation is attributed to the process of in-filling of multiple thunderstorms and/or a single area of thunderstorms expanding outward within the leading space of an advancing [[cold front]].

=====Pressure perturbations=====
Pressure perturbations within an extent of a thunderstorm are noteworthy. With [[buoyancy]] rapid within the lower and mid-levels of a mature thunderstorm, one might believe that low pressure dominates in the mesoscale environment. However, this is not the case. With downdrafts ushering colder air from mid-levels, hitting ground and propagating away in all directions, high pressure is to be found widely at surface levels, usually indicative of strong (potentially damaging) winds.

=====Wind shear=====
[[File:Squall line arthur ontario 2003.jpg|thumb|A summer squall line in Southern Ontario, producing lightning and distant heavy rains.]]

[[Wind shear]] is an important aspect to measuring the potential of squall line severity and duration. In low to medium shear environments, mature thunderstorms will contribute modest amounts of downdrafts, enough to turn will aid in create a leading edge lifting mechanism – the gust front. In high shear environments created by opposing low level jet winds and synoptic winds, updrafts and consequential downdrafts can be much more intense (common in supercell mesocyclones). The cold air [[outflow (meteorology)|outflow]] leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.

====Evolution====

=====Updrafts=====
The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of an [[updraft]], rising from ground level to the highest extensions of the [[troposphere]], condensing water and building a dark, ominous cloud to one with a noticeable overshooting top and anvil (thanks to [[synoptic scale meteorology|synoptic scale]] winds). Because of the chaotic nature of updrafts and [[downdrafts]], pressure perturbations are important.

As thunderstorms fill into a distinct line, strong leading-edge updrafts – occasionally visible to a ground observer in the form of a [[shelf cloud]] – may appear as an ominous sign of potential severe weather.

Beyond the strong winds because of updraft/downdraft behavior, heavy rain (and [[hail]]) is another sign of a squall line. In the winter, squall lines can occur albeit less frequently – bringing heavy snow and/or [[thunder]] and lightning – usually over inland lakes (i.e. [[Great Lakes]] region).

=====Bow echoes=====
Following the initial passage of a squall line, light to moderate [[Stratus cloud|stratiform]] precipitation is also common. A [[bow echo]] is frequently seen on the northern and southernmost reaches of squall line thunderstorms (via satellite imagery). This is where the northern and southern ends curl backwards towards the middle portions of the [[squall line]], making a "bow" shape. Bow echoes are frequently featured within [[supercell]] mesoscale systems.

=====Mesolow=====
[[File:Wakelowimage zoom.png|thumb|upright=0.65|A [[wake low]] is a mesolow]]
The poleward end of the squall line is commonly referred to as the cyclonic end, with the equatorward side rotating anticyclonically. Because of the [[coriolis force]], the poleward end may evolve further, creating a "comma shaped" mesolow, or may continue in a squall-like pattern.

A [[wake low]] is another kind of mesoscale low-pressure area to the rear of a squall line near the back edge of the stratiform rain area.<ref>{{cite web |work=Glossary of Meteorology |date=2009 | url=http://glossary.ametsoc.org/wiki/Wake_low | title=Wake Low | publisher=[[American Meteorological Society]] |access-date= March 21, 2018}}.</ref> Due to the subsiding warm air associated with the system's formation, clearing skies are associated with the wake low. Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough.<ref name="Gaffin">{{cite journal|author1=David M. Gaffin |date=October 1999|doi=10.1175/1520-0434(1999)014<0581:WLSWEI>2.0.CO;2|title= Wake Low Severe Wind Events in the Mississippi River Valley: A Case Study of Two Contrasting Events.|journal=[[Weather and Forecasting]]|volume=14|issue=10|pages= 581–603|publisher=[[American Meteorological Society|AMS]]|bibcode=1999WtFor..14..581G|doi-access=free}}.</ref> When the squall line is in the process of decay, [[heat burst]]s can be generated near the wake low. Once new thunderstorm activity along the squall line concludes, the wake low associated with it weakens in tandem.

====Dissipation====
As supercells and multi-cell thunderstorms dissipate due to a weak shear force or poor lifting mechanisms, (e.g. considerable [[terrain]] or lack of daytime heating) the [[squall line]] or [[gust front]] associated with them may outrun the squall line itself and the synoptic scale area of low pressure may then infill, leading to a weakening of the cold front; essentially, the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system. The areas of dissipating squall line thunderstorms may be regions of low [[Convective available potential energy|CAPE]], low [[humidity]], insufficient wind shear, or poor synoptic dynamics (e.g. an upper-level low filling) leading to [[frontolysis]].

From here, a general thinning of a squall line will occur: with winds decaying over time, outflow boundaries weakening updrafts substantially and clouds losing their thickness.

===Signs in the sky===
Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as a [[thunderstorm]]'s gust front.<ref>National Weather Service Forecast Office, Springfield, Missouri. [http://www.crh.noaa.gov/sgf/?n=spotter_squall_lines Storm Spotter Online Training.] Retrieved on 2006-11-19.</ref> From the time these low cloud features appear in the sky, one can expect a sudden increase in the wind in less than 15 minutes.

==Tropical cyclones==
[[Tropical cyclone]]s normally have squalls coincident with spiral bands of greater curvature than many mid-latitude systems due to their smaller size. These squalls can harbor [[waterspouts]] and [[tornadoes]] due to the significant vertical [[wind shear]] which exists in the vicinity of a tropical cyclone's outer bands.

==Winter weather==
[[Snow squalls]] can be spawned by an intrusion of cold air aloft over a relatively warm surface layer. [[Lake-effect snow]]s can be in the form of a snow squall.

==See also==
* [[Rogue wave]]
* [[Squall line]]
* [[White squall]]

==References==
{{Commons category|Squall lines}}
{{Reflist}}

{{Authority control}}

[[Category:Storm]]
[[Category:Wind]]
[[Category:Rain]]