Bora

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bora

A fall wind with a source so cold that, when the air reaches the lowlands or coast, the dynamic warming is insufficient to raise the air temperature to the normal level for the region; hence it appears as a cold wind.

The terms borino and boraccia denote a weak bora and a strong bora, respectively. The term was originally applied (along with karstbora) to the cold northeast wind on the Dalmatian coast of Croatia and Bosnia in winter when cold air from Russia crosses the mountains and descends to the relatively warm coast of the Adriatic. According to Smith (1987), the bora "has often been considered the prototype fall wind," although recent studies have revealed that some boras have downslope windstorm or hydraulic jump structure. The event often lasts a day or less, although extended events occur with enough frequency that "the longevity of the bora is one of its primary characteristics. A duration of four to six days is not unusual." It is very stormy and squally, the squalls sometimes reaching 50 m s-1 or more. F. Defant (1951) distinguishes between cyclonic bora (bora scura) with clouds and rain, covering the whole Adriatic and occurring with a depression over southern Adria, and the dry anticyclonic bora, with a powerful anticyclone over central Europe extending over Dalmatia; the latter is very violent over the land but extends only a short distance out to sea. A local bora also occurs on the east coast of the Adriatic with a cold anticyclone over the Balkans. The term bora is now applied to similar winds in other parts of the world. Well-known examples occur at Novorossiisk on the northern shore of the Black Sea, and in Novaya Zemlya (islands in the Russian Arctic). A squally katabatic wind at Alme Dagh in the Gulf of Iskenderon (eastern Mediterranean Sea) is termed rageas (also ragut, ghaziyah). The Bulgarian term is buria. In some mountainous regions of the world bora has been further generalized to represent any large mesoscale or synoptic-scale downslope flow of cold air, including post- arctic-frontal fall winds and cold-air downslope windstorms, which may have a hydraulic jump- like character and structure. In the case of downslope windstorms, some authors have used bora for a cold-advection flow (or one that results in cooling to the immediate lee of the mountain barrier), whereas chinook or foehn refer to a warm- or neutral-advection wind (or one that results in warming or no temperature change leeward of the barrier). Those who have attempted to classify downslope windstorms, however, have found that many cases do not fall neatly into one category or the other.
See Boreas, borasca.

Smith, R. B. 1987. Aerial observations of the Yugoslavian bora. J. Atmos. Sci.. 44. 269–297.

Defant, F. 1951. Compendium of Meteorology. 669–670.

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