Bora (wind)

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This article is about the bora wind. For other uses see Bora.
Bora in Croatia.
Bora in Croatia.

Bora (Croatian: bura, Slovenian: burja) is a northern to north-eastern katabatic wind in the Adriatic, Greece and Turkey. Its name derives from the Greek mythological figure of Boreas, the North Wind.

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The changeable Bora can often be felt all over Dalmatia and the rest of the Adriatic east coast. It blows in gusts. The Bora is most common during the winter. It blows hardest, as the meteorologist Baron Ferdinand von Wrangel explained it by extending Julius Hann's explanation of Alpine katabatic winds to the north Adriatic,[1] when a polar high-pressure area sits over the snow-covered mountains of the interior plateau behind the Dinaric coastal mountain range and a calm low-pressure area lies further south over the warmer Adriatic. As the air grows even colder and thus denser at night, the Bora increases. Its initial temperature is so low that even with the warming occasioned by its descent it reaches the lowlands as a cold wind.[2] The wind takes two different traditional names depending on associated meteorological conditions: the "clear bora" (Italian: Bora chiara) is Bora in the presence of clear skies, whereas clouds gathering on the hilltops and moving towards the seaside with rain characterize the "dark bora" (Bora scura).

[edit] Areas hit

The bora on the Kras as depicted in Valvasor's Glory, 17th century.
The bora on the Kras as depicted in Valvasor's Glory, 17th century.

The area where some of the strongest bora winds occur is the Velebit mountain range in Croatia. This seaside mountain chain, spanning 145 kilometers, represents a huge weather and climatic divide between the sharp continental climate of the interior, characterized by significant day/night temperature differences throughout the year, and the Adriatic coast, with a Mediterranean climate. Bora occurs because these two divided masses tend to equalize. Sailing can be extremely dangerous for an unexperienced navigator in the Velebit channel because the wind can start suddenly on a clear and calm day and result in major problems, frequently also affecting road traffic. Near the towns of Senj, Stara Novalja, Karlobag and the southern portal of the Sv. Rok Tunnel in Croatia, it can reach speeds of up to 220 kilometers per hour. On 15 March 2006 the speed of a gust on the Pag Bridge was measured at 235 kilometres per hour.

The wind is also an integral feature of Slovenia's Vipava Valley and Kras region, (known as Carso in Italy), an area of limestone heights over the Trieste Bay stretching towards the Istrian peninsula. Because the region separates the lower Adriatic coast from the Julian Alps range, extreme bora winds often occur there. They have influenced the region's traditional lifestyle and architecture. Towns on the coast, where the Bora also frequently occurs, are built densely with narrow streets in part because of the wind. Buildings in several towns and villages in Slovenia and the Province of Trieste (Italy) have stones on their roofs to prevent the roof tiles from being blown off. Chains and ropes are occasionally stretched along the sidewalks in downtown Trieste, Italy, to facilitate pedestrian traffic.

Strong bora winds also occur in the Tsemes Bay of the Black Sea near the Russian port of Novorossiysk, where they are known as nordost. They can reach speeds of up to 220 kilometres per hour.

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[edit] References

  1. ^ F. von Wrangel, "Die Ursachen der Bora in Noworossisk", Repertorium für Meteorologie 40 (1876:238-40); the Bora of the Karst was described by F. Seidel, "Bermerkungen über die Karstbora", ''M.Z. 8 (1891:232-35), noted by Julius (von) Hann, Handbook of Climatology Robert DeCourcy Ward, tr. (1903): see Peter Seibert, "Hann’s Thermodynamic Foehn Theory and its Presentation in Meteorological Textbooks in the Course of Time".
  2. ^ v. Hann 1903.