A squall is a sudden, sharp increase in wind speed which 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.
Origin of the termThe 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. The first known use of the term on surface weather analyses was in the United States in the late Nineteenth Century on U. S. Signal Corp Maps, begun in 1871.
Character of the windThe term "squall" is universally used to refer to a sudden wind-speed increase, both historically and in the present day. To be called a squall in the United States, the wind must increase at least 18 miles per hour (8 meters per second) and must attain a top speed of at least 25 miles per hour (11 meters per second), lasting at least a full 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 often associated with briefly heavy precipitation.
ArgentinaKnown locally as pamperos, they are used to describe strong downsloped winds that move across the pampas, eventually making it to the Atlantic Ocean.
Central AmericaOffshore Central America, the term gully squall is used to describe strong increases of the wind forced through sharp mountain valleys on the Pacific Ocean side of the isthmus.
CubaBayamo is a term used to describe squalls emanating from tropical thunderstorms near the Bight of Bayamo.
East IndiesIn the East Indies, brubu is a name for a squall
Pacific Northwest - United StatesIn 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 AfricaBull's Eye Squall is a term used offshore South Africa, describing a squall forming in fair weather. It is named for the appearance of the small isolated cloud marking the top of the squall.
Southeast AsiaBarat is a term for a northwest squall in Manado Bay in Sulawesi. 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. In England, a squall associated with tempestuous weather is known as a blunk. Significant squall lines with multiple bow echoes are known as derechos.
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.
Squall Line 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.
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 extend of the troposhere, condensing water and building a dark, ominous clouds 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.
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 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 air outflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.
Squall Line Evolution
As thunderstorms fill into a distinct line, strong leading-edge updrafts - occasionally visible to a ground observer in the form of a shelf cloud, 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).
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.
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.
Squall Line Dissipation
As supercell or multi-cell thunderstorms disappate because of a weak shear, poor lifting mechanisms: (e.g. considerable terrain or lack of daytime heating. The squall line associated gust front may outrun the squall line, the synoptic scale low may fill - leading to a weaking of a cold front, or the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system. The areas of disappating 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: within, winds decaying with time, outflow boundaries weakening updrafts substantially, and clouds losing their thickness.
Signs in the skyShelf 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 cyclonesTropical cyclones normally have squalls coincident with spiral bands of greater curvature than many mid-latitude systems due to their smaller size. This 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 weatherSnow 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. In Scotland, snow squalls are known as bluffarts.
Literary usageSquall is a vocabulary word in the book Sarah, Plain and Tall in the title of the book "White Squall", written by John Conroy Hutcheson in 1900.
rainband in Bulgarian: Шквал
rainband in Catalan: Torbonada
rainband in German: Bö
rainband in Spanish: Turbonada
rainband in French: Grain (météorologie)
rainband in Korean: 스콜
rainband in Italian: Groppo
rainband in Dutch: Bui
rainband in Japanese: スコール
rainband in Norwegian: Bygelinje
rainband in Norwegian Nynorsk: Bygelinje
rainband in Polish: Szkwał
rainband in Portuguese: Aguaceiro
rainband in Russian: Шквал
rainband in Sicilian: Gruppu (nudu ntricatu)