View source for Bar (tropical cyclone)

The '''bar''' of a mature [[tropical cyclone]] is a very dark gray-black layer of cloud that appears to be near to the [[horizon]] as seen from an observer preceding the approach of the storm, and is composed primarily of dense [[stratocumulus]] clouds.<ref name="IRBS">{{cite web|last=Bowditch |first=Nathaniel |title=Tropical Cyclones |url=http://www.irbs.com/bowditch/pdf/chapt36.pdf |work=I'd Rather Be Sailing |publisher=The American Practical Navigator |access-date=7 November 2010 |pages=513–515 |url-status=dead |archive-url=https://web.archive.org/web/20110409152823/http://www.irbs.com/bowditch/pdf/chapt36.pdf |archive-date=9 April 2011 }}</ref> [[Cumulus]] and [[cumulonimbus]] clouds bearing [[Precipitation (meteorology)|precipitation]] follow immediately after the passage of the [[wall cloud|wall]]-like bar.<ref name="HurEnc">{{cite book|last=Longshore|first=David|title=Encyclopedia of Hurricanes, Typhoons, and Cyclones.|year=2008|publisher=Checkmark Books|isbn=978-0-8160-7409-9|pages=468}}</ref> [[Altostratus]], [[cirrostratus]] and [[Cirrus cloud|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.

==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]] 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.<ref name="HurEnc"/>

==Inside the bar==
[[File:Hurricane structure graphic.jpg|thumb|right|300px|The basic vertical structure of a mature tropical cyclone. The bar appears as the storm's main disk of initially the upper storm clouds followed by the lower clouds as viewed from a distance, and gaps exist in the rainbands after the bar passes over.]]

When the bar of the storm approaches an observer, it appears stationary in [[azimuth]]al position, whereas a storm moving toward the observer at an angle or perpendicular to the observer will appear to drift along the horizon.<ref name="IRBS"/> A reddish tint sometimes appears toward the top of the bar,<ref>{{cite book|last=Sheets|first=Bob|title=Hurricane watch: Forecasting the deadliest storms on earth|year=2001|publisher=[[Random House]]|isbn=0-375-70390-X|author2=Jack Williams|page=12|chapter=Early History and Science}}</ref> 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 (optics)|prism]]atic effect of water droplets in the cloud. Cumulus clouds first appear at the [[outflow (meteorology)|outflow]] [[outflow boundary|boundary]] of the lower part of the bar.<ref name="HurEnc"/> 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 (meteorology)|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.<ref name="IRBS"/>

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 (cyclone)|eye]], before the eyewall passes over again and the strongest winds arrive from the opposite direction.<ref name="HurEnc"/>

At sea, wind speeds typically reach level 8 on the [[Beaufort scale]] while [[ocean wave|wave]]s become drastically higher when the bar reaches overhead and [[squall line]]s begin to arrive.<ref name="IRBS"/>

==References==
<references/>

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