Squall

Last updated December 17, 2023

A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind 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.

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^[4] an Old Norse word meaning literally to squeal.^[5]
By another version, it is an alteration of squeal influenced by bawl.^[6]

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 8 m/s and must attain a top speed of at least 11 m/s, 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 as squall line.^[1]

Regional terms

Argentina

Known locally as pamperos, these are characterized as strong downsloped winds that move across the pampas, eventually making it to the Atlantic Ocean.^[8]^{[ verification needed ]}

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, which approaches from the southeast, mainly on a hot day, bringing in cool, usually severe weather and a dramatic temperature drop, thus ultimately replacing and relieving the prior hot conditions.^[9]

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 of Bayamo.^[10]

East Indies

In the East Indies, brubu is a name for a squall^[11]

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 outflow occurring in fjords and inlets is referred to by mariners as a 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.^[12]

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.^[12]

"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).^[13]

Severe weather

A shelf cloud such as this one can be a sign that a squall is imminent DangerousShelfCloud.jpg — 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.^[14] Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical wind shear from an angle behind the frontal boundary.^[15] 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.^[16] Significant squall lines with multiple bow echoes are known as derechos.^[17]

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

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

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 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 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

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.^[18] 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.^[19] 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

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

Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as a thunderstorm's gust front.^[20] 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 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

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.

Related Research Articles

<span class="mw-page-title-main">Thunderstorm</span> Type of weather with lightning and thunder

A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

A supercell is a thunderstorm characterized by the presence of a mesocyclone; a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms, supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2–4 hours.

A mesocyclone is a meso-gamma mesoscale region of rotation (vortex), typically around 2 to 6 mi in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank of a supercell, or often on the eastern, or leading, flank of a high-precipitation variety of supercell. The area overlaid by a mesocyclone’s circulation may be several miles (km) wide, but substantially larger than any tornado that may develop within it, and it is within mesocyclones that intense tornadoes form.

<span class="mw-page-title-main">Squall line</span> Line of thunderstorms along or ahead of a cold front

A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.

<span class="mw-page-title-main">Cyclogenesis</span> The development or strengthening of cyclonic circulation in the atmosphere

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

An anticyclonic storm is a storm with a high-pressure center, in which winds flow in the direction opposite to that of the flow above a region of low pressure. These storms can create powerful mesoanticylonic supercell storms that can generate anticyclonic tornadoes. Examples include the anticyclonic blizzard of 2018, Hartmut, Jupiter, and Neptune's anticyclonic cloud system.

A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell thunderstorms, and are often, but not always, a visual precursor to tornadoes. Funnel clouds are visual phenomena, these are not the vortex of wind itself.

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.

Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX, tornadogenesis is a volatile process and the intricacies of many of the mechanisms of tornado formation are still poorly understood.

An air-mass thunderstorm, also called an "ordinary", "single cell", "isolated" or "garden variety" thunderstorm, is a thunderstorm that is generally weak and usually not severe. These storms form in environments where at least some amount of Convective Available Potential Energy (CAPE) is present, but with very low levels of wind shear and helicity. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by weather fronts and other low-level boundaries associated with wind convergence. The energy needed for these storms to form comes in the form of insolation, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, as well as showery light, moderate, or heavy rainfall. Heavy rainfall can interfere with microwave transmissions within the atmosphere.

Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.

Convective storm detection is the meteorological observation, and short-term prediction, of deep moist convection (DMC). DMC describes atmospheric conditions producing single or clusters of large vertical extension clouds ranging from cumulus congestus to cumulonimbus, the latter producing thunderstorms associated with lightning and thunder. Those two types of clouds can produce severe weather at the surface and aloft.

A storm cell is an air mass that contains up and down drafts in convective loops and that moves and reacts as a single entity, functioning as the smallest unit of a storm-producing system. An organized grouping of thunder clouds will thus be considered as a series of storm cells with their up/downdrafts being independent or interfering one with the other.

A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone, at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak, a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system, a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to a mile or less and can be immensely intense.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

1 2 3 "Squall". Glossary. American Meteorological Society. Retrieved November 15, 2018.
↑ "Weather Glossary: S." The Weather Channel. Archived from the original on August 1, 2008. Retrieved November 15, 2018.
1 2 "Weather Words". Australian Bureau of Meteorology. Retrieved November 15, 2018.
↑ squall: Definition and Much More from Answers.com
↑ Georoots News. Georoots News V.1#5: Changes in the Wind. Retrieved on 2006-12-30.
↑ Concise Oxford English Dictionary. Oxford University Press. 2011. p. 1400. ISBN 978-0-19-960108-0 . Retrieved 2014-12-30.
↑ WMO. "Squall". Eumetcal. Archived from the original on March 3, 2016. Retrieved November 15, 2018.
↑ Oxford English Dictionary. Oxford University Press. 10Rev Ed edition (7 April 2005)
↑ "SOUTHERLY BUSTER". Wellington Times . National Library of Australia. 23 December 1901. p. 2. Retrieved 27 March 2015.
↑ "Weather Words – B". WGN-TV. Archived from the original on 2007-03-18. Retrieved 2006-11-19.
↑ Wind Names
1 2 Golden Gate Weather Services. Names of Winds.
↑ National Environment Agency Guide To Singapore's Weather, archived by archive.org on November 1, 2013.
↑ Weatherquestions.com. What is a Squall Line? Retrieved on 2006-11-19.
↑ Wilfried Jacobs. EUMeTrain: Case Study on Squall Line. Archived 2006-10-19 at the Wayback Machine Retrieved on 2006-11-19.
↑ Thinkquest. Meteorology Online: Squall. Archived 2007-02-05 at the Wayback Machine Retrieved on 2006-11-19.
↑ Robert H. Johns and Jeffry S. Evans. Storm Prediction Center. Derecho Facts. Archived 2010-05-27 at the Wayback Machine Retrieved on 2006-11-19.
↑ "Wake Low". Glossary of Meteorology. American Meteorological Society. 2009. Retrieved March 21, 2018..
↑ David M. Gaffin (October 1999). "Wake Low Severe Wind Events in the Mississippi River Valley: A Case Study of Two Contrasting Events". Weather and Forecasting . AMS. 14 (10): 581–603. Bibcode:1999WtFor..14..581G. doi: 10.1175/1520-0434(1999)014<0581:WLSWEI>2.0.CO;2 ..
↑ National Weather Service Forecast Office, Springfield, Missouri. Storm Spotter Online Training. Retrieved on 2006-11-19.

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[AMS-1] 1 2 3 "Squall". Glossary. American Meteorological Society. Retrieved November 15, 2018.

[2] "Weather Glossary: S." The Weather Channel. Archived from the original on August 1, 2008. Retrieved November 15, 2018.

[BOM-3] 1 2 "Weather Words". Australian Bureau of Meteorology. Retrieved November 15, 2018.

[4] squall: Definition and Much More from Answers.com

[5] Georoots News. Georoots News V.1#5: Changes in the Wind. Retrieved on 2006-12-30.

[6] Concise Oxford English Dictionary. Oxford University Press. 2011. p. 1400. ISBN 978-0-19-960108-0 . Retrieved 2014-12-30.

[7] WMO. "Squall". Eumetcal. Archived from the original on March 3, 2016. Retrieved November 15, 2018.

[8] Oxford English Dictionary. Oxford University Press. 10Rev Ed edition (7 April 2005)

[9] "SOUTHERLY BUSTER". Wellington Times . National Library of Australia. 23 December 1901. p. 2. Retrieved 27 March 2015.

[10] "Weather Words – B". WGN-TV. Archived from the original on 2007-03-18. Retrieved 2006-11-19.

[11] Wind Names

[ggweather-12] 1 2 Golden Gate Weather Services. Names of Winds.

[sumatras-13] National Environment Agency Guide To Singapore's Weather, archived by archive.org on November 1, 2013.

[14] Weatherquestions.com. What is a Squall Line? Retrieved on 2006-11-19.

[15] Wilfried Jacobs. EUMeTrain: Case Study on Squall Line. Archived 2006-10-19 at the Wayback Machine Retrieved on 2006-11-19.

[16] Thinkquest. Meteorology Online: Squall. Archived 2007-02-05 at the Wayback Machine Retrieved on 2006-11-19.

[17] Robert H. Johns and Jeffry S. Evans. Storm Prediction Center. Derecho Facts. Archived 2010-05-27 at the Wayback Machine Retrieved on 2006-11-19.

[18] "Wake Low". Glossary of Meteorology. American Meteorological Society. 2009. Retrieved March 21, 2018..

[Gaffin-19] David M. Gaffin (October 1999). "Wake Low Severe Wind Events in the Mississippi River Valley: A Case Study of Two Contrasting Events". Weather and Forecasting . AMS. 14 (10): 581–603. Bibcode:1999WtFor..14..581G. doi: 10.1175/1520-0434(1999)014<0581:WLSWEI>2.0.CO;2 ..

[20] National Weather Service Forecast Office, Springfield, Missouri. Storm Spotter Online Training. Retrieved on 2006-11-19.

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