Asquall is a sudden, sharp increase inwind speed lasting minutes, as opposed to awind gust, which lasts for only seconds.[1] They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow.[2] Squalls refer to the increase of the sustained winds over that time interval, as there may be higher gusts during a squall event.[3] 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.
There are different versions of the word's origins:
The term "squall" is used to refer to a sudden wind-speed increase lasting minutes. In 1962 theWorld Meteorological Organization (WMO) defined that to be classified as a "squall", the wind must increase at least 8 metres per second (29 km/h; 18 mph) and must attain a top speed of at least 11 metres per second (40 km/h; 25 mph), lasting at least one minute in duration.[1][7] In Australia, a squall is defined to last for several minutes before the wind returns to the long-term mean value.[3] 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 assquall line.[1]
Known locally aspamperos, these are characterized as strong downsloped winds that move across thepampas, eventually making it to the Atlantic Ocean.[8]
In southeastern Australia, the colloquial name for a squall issoutherly buster, which is an abrupt southerly wind change in the southern regions ofNew South Wales andVictoria,Australia, which approaches from the southeast, mainly on a hot day,bringing in cool, usuallysevere weather and a dramatic temperature drop, thus ultimately replacing and relieving the prior hot conditions.[9]
Offshore Central America, agully squall is characterized by strong increases of the wind forced through sharp mountain valleys on the Pacific Ocean side of the isthmus.
Abayamo is a squall emanating from tropical thunderstorms near theBight of Bayamo.[10]
In thePacific Northwest, asquall is a short but furious rainstorm with strong winds, often small in area and moving at high speed, especially as a maritime term. A strongKatabatic outflow occurring in fjords and inlets is referred to by mariners as asquamish.
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.[11]
In most parts of the country, squalls are calledsubasko 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.
"Barat" is a term for a northwest squall in Manado Bay inSulawesi.[11]
"Sumatra squall" is a term used inSingapore and PeninsularMalaysia forsquall lines that form over the island ofSumatra and move east across theStraits of Malacca. Gusts can reach up to 28 m/s (100 km/h).[12]
Asquall line is an organized line ofthunderstorms. 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 withhurricanes or othercyclones, but they can also occur independently. Most commonly, independent squalls occur alongfront lines, and may contain heavyprecipitation,hail, frequentlightning, dangerous straight line winds, and possiblyfunnel clouds,tornadoes andwaterspouts.[13] Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and verticalwind shear from an angle behind the frontal boundary.[14] 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.[15] Significant squall lines with multiple bow echoes are known asderechos.[16]
There are several forms ofmesoscale meteorology, including simplistic isolated thunderstorms unrelated to advancing cold fronts, to the more complex daytime/nocturnalmesoscale convective system (MCS) andmesoscale convective complex (MCC), tosquall line thunderstorms.
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 advancingcold front.
Pressure perturbations within an extent of a thunderstorm are noteworthy. Withbuoyancy 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 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 airoutflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.
The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of anupdraft, rising from ground level to the highest extensions of thetroposphere, condensing water and building a dark, ominous cloud to one with a noticeable overshooting top and anvil (thanks tosynoptic scale winds). Because of the chaotic nature of updrafts anddowndrafts, 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 ashelf cloud – may appear as an ominous sign of potential severe weather.
Beyond the strong winds because of updraft/downdraft behavior, heavy rain (andhail) is another sign of a squall line. In the winter, squall lines can occur albeit less frequently – bringing heavy snow and/orthunder and lightning – usually over inland lakes (i.e.Great Lakes region).
Following the initial passage of a squall line, light to moderatestratiform precipitation is also common. Abow 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 thesquall line, making a "bow" shape. Bow echoes are frequently featured withinsupercell mesoscale systems.
The poleward end of the squall line is commonly referred to as the cyclonic end, with the equatorward side rotating anticyclonically. Because of thecoriolis force, the poleward end may evolve further, creating a "comma shaped" mesolow, or may continue in a squall-like pattern.
Awake 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.[17] 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.[18] 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.
As supercells and multi-cell thunderstorms dissipate due to a weak shear force or poor lifting mechanisms, (e.g. considerableterrain or lack of daytime heating) thesquall line orgust 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 lowCAPE, lowhumidity, insufficient wind shear, or poor synoptic dynamics (e.g. an upper-level low filling) leading tofrontolysis.
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.
Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as athunderstorm's gust front.[19] 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 normally have squalls coincident with spiral bands of greater curvature than many mid-latitude systems due to their smaller size. These squalls can harborwaterspouts andtornadoes due to the significant verticalwind shear which exists in the vicinity of a tropical cyclone's outer bands.
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.
