mesolow squalls updrafts downdrafts mesoscale downdraft stratiform squall squeal bayamo bawl waterspouts updraft supercells perturbations supercell 0581 gust thunderstorms overshooting thunderstorm thinkquest gusts synoptic barat shear 0434 subsiding convective dissipation
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Squall - wikipedia Squall Jump to : navigation , search For other uses , see Squall ( disambiguation ) . A squall is a sudden , sharp increase in wind speed that is usually associated with active weather , such as rain showers , thunderstorms , or heavy snow . Squalls refer to an increase in the sustained winds over a short time interval , as there may be higher gusts during a squall event . They usually occur in a region of strong mid-level height falls , or mid-level tropospheric cooling , which force strong localized upward motions at the leading edge of the region of cooling , which then enhances local downward motions just in its wake . Rainbow after a squall Contents ( hide ) 1 Etymology 2 Character of the wind 3 Regional terms 3.1 Argentina 3.2 Central America 3.3 Cuba 3.4 East Indies 3.5 Pacific Northwest ( North America ) 3.6 South Africa 3.7 Philippines ( West Pacific ) 3.8 South - East Asia 4 Severe weather 4.1 Squall line life cycle 4.1. 1 Formation 4.1. 1.1 Pressure perturbations 4.1. 1.2 Wind shear 4.1. 2 Evolution 4.1. 2.1 Updrafts 4.1. 2.2 Bow echoes 4.1. 2.3 Mesolow 4.1. 3 Dissipation 4.2 Signs in the sky 5 Tropical cyclones 6 Winter weather 7 See also 8 References Etymology ( edit ) 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 an Old Norse word meaning literally , to squeal . By another version , it is an alteration of squeal influenced by bawl . Character of the wind ( edit ) The term `` squall '' is used to refer to a sudden wind - speed increase , both historically and in the present day . In 1962 the World Meteorological Organization ( WMO ) defined that to be classified as a `` squall '' , the wind must increase at least 8 m / s and must attain a top speed of at least 11 m / s , lasting at least one minute in duration . In Australia , a squall is defined to last for several minutes before the wind returns to the long term mean value . 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 . Regional terms ( edit ) Argentina ( edit ) Known locally as pamperos , these are characterized as strong downsloped winds that move across the pampas , eventually making it to the Atlantic Ocean . Central America ( edit ) 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 ( edit ) A bayamo is a squall emanating from tropical thunderstorms near the Bight of Bayamo . East Indies ( edit ) In the East Indies , brubu is a name for a squall Pacific northwest ( north America ) ( edit ) 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 . Such a strong outflow occurring in fjords and inlets is referred to by mariners as a squamish . South Africa ( edit ) 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 . Philippines ( west Pacific ) ( edit ) 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 ( edit ) `` Barat '' is a term for a northwest squall in Manado Bay in Sulawesi . `` Sumatra '' squall is a term used in Singapore and Peninsular Malaysia for squall lines that form over the island of Sumatra and move east across the Straits of Malacca . Gusts can reach up to 28 m / s ( 100 km / h ) . Severe Weather ( edit ) A shelf cloud such as this one can be a sign that a squall is imminent A squall line is an organized line of thunderstorms . 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 hurricanes or other cyclones , but they can also occur independently . Most commonly , independent squalls occur along front lines , and may contain heavy precipitation , hail , frequent lightning , dangerous straight line winds , and possibly funnel clouds , tornadoes and waterspouts . Squall lines require significant low - level warmth and humidity , a nearby frontal zone , and vertical wind shear from an angle behind the frontal boundary . 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 . Significant squall lines with multiple bow echoes are known as derechos . Squall line life cycle ( edit ) 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 ( edit ) 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 ( edit ) 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 ( edit ) 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 leaves the trailing area of the squall line to the mid-level jet , which aids in downdraft processes . Evolution ( edit ) Updrafts ( edit ) 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 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 ( edit ) Following the initial passage of a squall line , light to moderate stratiform precipitation is also common . A Bow echo is frequently seen on the northern and southern most 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 ( edit ) A wake low is a mesolow The northern end of the squall line is commonly referred to as the cyclonic end , with the southern side rotating anticyclonically . Because of the coriolis force , the northern 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 . Due to the subsiding warm air associated with the systems 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 . When the squall line is in the process of decay , heat bursts 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 ( edit ) 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 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 ( edit ) Shelf clouds and roll clouds are usually seen above the leading edge of a squall , also known as a thunderstorm 's gust front . 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 ( edit ) Tropical cyclones 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 ( edit ) Snow squalls can be spawned by an intrusion of cold air aloft over a relatively warm surface layer . Lake effect snows can be in the form of a snow squall . See also ( edit ) Rogue wave Squall line White squall References ( edit ) Wikimedia Commons has media related to Squall lines . Jump up ^ The Weather Channel . Weather Glossary : S. Archived 2008 - 08 - 01 at the Wayback Machine . Retrieved on 2006 - 11 - 19 . Jump up ^ Australian Bureau of Meteorology . Weather Words . Retrieved on 2006 - 11 - 19 . Jump up ^ squall : Definition and Much More from Answers.com Jump up ^ Georoots News . Georoots News V. 1 # 5 : Changes in the Wind . Retrieved on 2006 - 12 - 30 . Jump up ^ Concise Oxford English Dictionary . Oxford University Press . 2011 . p. 1400 . ISBN 978 - 0 - 19 - 960108 - 0 . Retrieved 2014 - 12 - 30 . Jump up ^ Oxford English Dictionary . Oxford University Press . 10Rev Ed edition ( 7 April 2005 ) Jump up ^ `` Weather Words -- B '' . WGN - TV . Archived from the original on 2007 - 03 - 18 . Retrieved 2006 - 11 - 19 . Jump up ^ Wind Names ^ Jump up to : Golden Gate Weather Services . Names of Winds . Jump up ^ National Environment Agency Guide To Singapore 's Weather , archived by archive.org on November 1 , 2013 . Jump up ^ Weatherquestions.com . What is a Squall Line ? Retrieved on 2006 - 11 - 19 . Jump up ^ Wilfried Jacobs . EUMeTrain : Case Study on Squall Line . Archived 2006 - 10 - 19 at the Wayback Machine . Retrieved on 2006 - 11 - 19 . Jump up ^ Thinkquest . Meteorology Online : Squall . Archived 2007 - 02 - 05 at the Wayback Machine . Retrieved on 2006 - 11 - 19 . Jump up ^ Robert H. Johns and Jeffry S. Evans . Storm Prediction Center . Derecho Facts . Retrieved on 2006 - 11 - 19 . Jump up ^ `` Wake Low '' . Glossary of Meteorology . American Meteorological Society . 2009 . Retrieved March 21 , 2018 ... Jump up ^ David M. Gaffin ( October 1995 ) . `` Wake Low Severe Wind Events in the Mississippi River Valley : A Case Study of Two Contrasting Events '' . Weather and Forecasting . AMS. 14 ( 10 ) . doi : 10.1175 / 1520 - 0434 ( 1999 ) 014 < 0581 : WLSWEI > 2.0.CO ; 2 . Retrieved March 22 , 2018 ... Jump up ^ National Weather Service Forecast Office , Springfield , Missouri . Storm Spotter Online Training . Retrieved on 2006 - 11 - 19 . 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