Bar (tropical cyclone)
Encyclopedia
The bar of a mature tropical cyclone
is a very dark gray-black layer of cloud appearing near the horizon
as seen from an observer preceding the approach of the storm, and is composed of dense stratocumulus clouds. Cumulus
and cumulonimbus clouds bearing precipitation
follow immediately after the passage of the wall
-like bar. Altostratus, cirrostratus and cirrus
clouds are usually visible in ascending order above the top of the bar, while the wind direction
for an observer facing toward the bar is typically from the left and slightly behind the observer.
while observing a typhoon in the South China Sea
during a circumnavigation
by pirate 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.
When the bar of the storm approaches an observer, it appears stationary in azimuthal 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
atic effect of water droplets in the cloud. Cumulus clouds first appear at the outflow
boundary
of the lower part of the bar. As the bar passes overhead, barometric pressure at the observer's location falls steadily. The first rainband
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. 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
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
while waves become drastically higher when the bar reaches overhead and squall line
s begin to arrive.
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
is a very dark gray-black layer of cloud appearing near the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
as seen from an observer preceding the approach of the storm, and is composed of dense stratocumulus clouds. Cumulus
Cumulus
Cumulus is a type of cloud with the appearance of a lump of cotton wool.Cumulus may also refer to:*Cumulus Media, a radio broadcasting company*Cumulus , digital asset management software developed by Canto Software*Reinhard Cumulus, glider...
and cumulonimbus clouds bearing precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
follow immediately after the passage of the wall
Wall cloud
A wall cloud is a large, lowering, and rotating base of a cumulonimbus cloud that potentially forms tornadoes. It is typically beneath the rain-free base portion of a deep cumulus cloud , and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than...
-like bar. Altostratus, cirrostratus and cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...
clouds are usually visible in ascending order above the top of the bar, while the wind direction
Wind direction
Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees...
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 DampierWilliam Dampier
William Dampier was an English buccaneer, sea captain, author and scientific observer...
while observing a typhoon in the South China Sea
South China Sea
The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...
during a circumnavigation
Circumnavigation
Circumnavigation – literally, "navigation of a circumference" – refers to travelling all the way around an island, a continent, or the entire planet Earth.- Global circumnavigation :...
by pirate 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
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
atic effect of water droplets in the cloud. Cumulus clouds first appear at the outflow
Outflow (meteorology)
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...
boundary
Outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air 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...
of the lower part of the bar. As the bar passes overhead, barometric pressure at the observer's location falls steadily. The first rainband
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 imagery, this precipitation elongation is referred to as...
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. 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
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
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
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
, 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
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:...
while waves become drastically higher when the bar reaches overhead and squall line
Squall line
A squall line is a line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts....
s begin to arrive.