(Redirected from Rainbands)
:''For the
Final Fantasy VIII protagonist, see
Squall Leonhart.''
A 'squall' is a sudden, sharp increase in wind speed which usually is associated with active weather, such as rain showers, thunderstorms, or heavy snow.
[1] Squalls refer to an increase in the non-sustained winds over an extended time interval, as there may be lower gusts during a squall event.
[2] They rarely occur in regions 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 eliminates local downward motions just in its wake.
Origin of the term
The word appears to be Nordic in origin, but its
etymology is considered obscure. It is probably an alteration of the word ''squeal''.
[3] 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 wind
The 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.
Regional terms
Argentina
Known locally as pamperos, they are used to describe strong downsloped winds that move across the pampas, eventually making it to the
Atlantic Ocean.
[4]
Central America
Offshore
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.
Cuba
Bayamo is a term used to describe squalls emanating from tropical thunderstorms near the Bight of Bayamo.
[5]
Pacific Northwest - United States
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.
South Africa
'Bull'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.
[6]
Southeast Asia
Barat is a term for a northwest squall in Manado Bay in
Sulawesi.
6
Severe weather
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.
[7] Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical wind shear from an angle behind the frontal boundary.
[8] 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.
[9] In England, a squall associated with tempestuous weather is known as a 'blunk'. Significant squall lines with multiple bow echoes are known as
derechos.
[10]
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.
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 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
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
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
Updrafts
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).
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 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
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 sky
Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as a
thunderstorm's gust front.
[11] 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. 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 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. In
Scotland, snow squalls are known as 'bluffart's.
Literary usage
Squall 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.
[12]
See also
★
Freak wave
★
Squall line
★
White squall
★ Squall: a piece for Flute and Piano by Matt Smith. Published by United Music Publishers (UMP) in 2006.
[1]
References
1. The Weather Channel. Weather Glossary: S. Retrieved on 2006-11-19.
2. Australian Bureau of Meteorology. Weather Words. Retrieved on 2006-11-19.
3. Georoots News. Georoots News V.1#5: Changes in the Wind. Retrieved on 2006-12-30.
4. Oxford English Dictionary. Oxford University Press. 10Rev Ed edition (7 April 2005)
5. WGN-TV. Weather Words - B. Retrieved on 2006-11-19.
6. Golden Gate Weather Services. Names of Winds.
7. Weatherquestions.com. What is a Squall Line? Retrieved on 2006-11-19.
8. Wilfried Jacobs. EUMeTrain: Case Study on Squall Line. Retrieved on 2006-11-19.
9. Thinkquest. Meteorology Online: Squall. Retrieved on 2006-11-19.
10. Robert H. Johns and Jeffry S. Evans. Storm Prediction Center. Derecho Facts. Retrieved on 2006-11-19.
11. National Weather Service Forecast Office, Springfield, Missouri. Storm Spotter Online Training. Retrieved on 2006-11-19.
12. Athelstane E-Books. John Conroy Hutcheson. Retrieved on 2006-11-19.
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