Bar (tropical cyclone)
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The bar of a mature
tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
is a very dark gray-black layer of cloud that appears to be near to the
horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
as seen from an observer preceding the approach of the storm, and is composed primarily of dense
stratocumulus A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and th ...
clouds.
Cumulus Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
and
cumulonimbus Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful Buoyancy, buoyant air ...
clouds bearing
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
follow immediately after the passage of the
wall A wall is a structure and a surface that defines an area; carries a load; provides security, shelter, or soundproofing; or, is decorative. There are many kinds of walls, including: * Walls in buildings that form a fundamental part of the supe ...
-like bar.
Altostratus Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usuall ...
,
cirrostratus Cirrostratus is a high-level, very thin, generally uniform ''stratiform'' genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes ...
and
cirrus Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal *Cirrus (botany), a tendril *Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a type ...
clouds are usually visible in ascending order above the top of the bar, while the
wind direction Wind direction is generally reported by the direction from which it originates. For example, a ''north'' or ''northerly'' wind blows from the north to the south. The exceptions are onshore winds (blowing onto the shore from the water) and offsho ...
for an observer facing toward the bar is typically from the left and slightly behind the observer.


History

The dark layer of clouds on the horizon seen prior to a tropical cyclone's passage over a location was first described in 1687 and the observed phenomenon later published in 1697 by
William Dampier William Dampier (baptised 5 September 1651; died March 1715) was an English explorer, pirate, privateer, navigator, and naturalist who became the first Englishman to explore parts of what is today Australia, and the first person to circumnav ...
while observing a typhoon in the
South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phil ...
during a
circumnavigation Circumnavigation is the complete navigation around an entire island, continent, or astronomical object, astronomical body (e.g. a planet or natural satellite, moon). This article focuses on the circumnavigation of Earth. The first recorded circ ...
by
pirate Piracy is an act of robbery or criminal violence by ship or boat-borne attackers upon another ship or a coastal area, typically with the goal of stealing cargo and other valuable goods. Those who conduct acts of piracy are called pirates, v ...
ship. These observations led to an improved understanding of the nature of tropical cyclones. The use of "bar" as a term to describe this cloud layer first appeared in the 19th century.


Inside the bar

When the bar of the storm approaches an observer, it appears stationary in
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematicall ...
al position, whereas a storm moving toward the observer at an angle or perpendicular to the observer will appear to drift along the horizon. A reddish tint sometimes appears toward the top of the bar, while the darkness often varies depending on the storm's intensity. Tints of indigo, green, yellow or violet may also be present depending on the time of day due to the
prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...
atic effect of water droplets in the cloud. Cumulus clouds first appear at the
outflow Outflow may refer to: *Capital outflow, the capital leaving a particular economy *Bipolar outflow, in astronomy, two continuous flows of gas from the poles of a star *Outflow (hydrology), the discharge of a lake or other reservoir system * Outflow ...
boundary Boundary or Boundaries may refer to: * Border, in political geography Entertainment *Boundaries (2016 film), ''Boundaries'' (2016 film), a 2016 Canadian film *Boundaries (2018 film), ''Boundaries'' (2018 film), a 2018 American-Canadian road trip ...
of the lower part of the bar. As the bar passes overhead,
barometric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
at the observer's location falls steadily. The first
rainband A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar ima ...
arrives, the clouds moving from left to right roughly along isobar lines, but this is often followed by short periods of relative calm. During the onset of the bar's arrival, winds continuously increase in intensity leading toward the
eyewall The eye is a region of mostly calm weather at the center of tropical cyclones. The eye of a storm is a roughly circular area, typically in diameter. It is surrounded by the ''eyewall'', a ring of towering thunderstorms where the most severe weat ...
. Navigators of ships at sea often use the first appearance of a bar to steer clear of the approaching tropical cyclone. Closer to the center of tropical cyclone, the eyewall also exhibits the appearance of a bar, and high
lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
activity occurs within this central bar. After the bar passes through, the strongest winds and often the heaviest precipitation abruptly changes to calmer conditions within the eye, before the eyewall passes over again and the strongest winds arrive from the opposite direction. At sea, wind speeds typically reach level 8 on the
Beaufort scale The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort wind force scale. History The scale was devised in 1805 by the Irish hydrographer Francis Beaufort ...
while
wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (res ...
s become drastically higher when the bar reaches overhead and
squall line 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 (which often are accompa ...
s begin to arrive.


References

{{DEFAULTSORT:Bar (Tropical Cyclone) Clouds Marine meteorology Navigation Severe weather and convection Tropical cyclones Weather hazards Wind