Bora (Wind)
The Bora (Greek μπόρα, "cold gust of wind ", " cold downpour ," Boreas, literally " the North "; Croatian Bura; Slovenian Burja ) is a dry, cold and gusty fall wind between Trieste, the Croatian and Montenegrin Adriatic coast. Winds from Bora type include with their frequency and their high average speeds, especially between Trieste and the northwest coast of Croatia as well as in parts of southern Dalmatia and Montenegro, the strongest in the world. Top speeds gusts reach here values of up to 250 km / h
Bora winds go from a wandering from the polar region, strong cold air outbreak, which occurred on the ground and northern or northeastern wind currents to the Adriatic coast in appearance, forth. From the synoptic point of view, the regional restriction is dictated by the topographical conditions. As a side issue of the winter anticyclone over Central Asia macroclimatic shape of the cold air outflow with the mountain setting in the Adriatic Sea basin through the Dinarides is closely connected.
Bora occurs in similar circumstances, in addition to the eastern coast of the Adriatic, yet at the Russian Black Sea coast near Novorossiysk, on Novaya Zemlya, in Scandinavia and in the Kanto plain of Japan.
- 3.1 Synoptic types of Bora
- 3.2 emergence
Etymology
The word Bora is derived from the Greek Boreas, god of the north wind. Since a strong north wind destroyed the Persian fleet against Athens, he was here and patron saint was venerated in a special Athenian temple. He was considered the most powerful god of the wind, which is why he was the inspiration for the dreaded hurricane wind Bora. The term originally meant " wind from the mountains " and has to do with the cold of the fall wind. Thus, the Bora long time the prototype was katabatic winds.
There are also various special name for a Bora. The Bora Bora at scura is a gloomy view. The Borino is a weak Bora in the area of Trieste, during the violent collisions of the Bora are known there Reffoli. A Borasco is strong wind, usually accompanied by thunderstorms, over the Mediterranean. A Levantera is a Bora that comes in Istria from the east and occurs in cloudy weather.
In Dalmatia it is said, on the Dinara she is born, baptized it is in Makarska and in Senj she marries. In Istria they say dagagen: The Bora comes in Senj at the world, prevails in Rijeka and dies in Trieste.
Character
The bora is very stormy and gusty, with the gusts in some cases reach up to 250 km / hr. The bora blows mainly in winter. In the summer it takes a day or a few hours, while it can blow up to 14 days in winter. Prevailing wind direction is from the east-northeast. The most dramatic effect reaches the Bora, when very cold continental air or aged Arctic air reaches the Adriatic Sea. The cold air over the Croatian karst valleys crashes then shut case as wind through the streets of the karst mountains. Despite the trockenadiabatischen warming when shutting throes of wind is felt as cold as the relative height of the littoral Dinaric a noticeable warming is too low ( Velebit 1 756 m, Biokovo 1762m in Croatia and Orjen 1894m in Montenegro ) and the otherwise relatively warm air of the Mediterranean region is displaced from the very cold continental polar air or aged.
Karl Marx designed in 1856 provides a suitable view of the Bora:
Definition
Hair dryer and Bora are the warm or cold downdrafts that can be so similar also observed worldwide. The similarity to the hair dryer is by
In contrast to the hair dryer, the Bora winds have a cooling nature, which also relates to a coastal region and marine dynamic effect. In addition, foehn winds are usually short-term phenomena, while long-lasting appearance is a primary feature of the Bora.
For hair dryer, a fluid-dynamic distinction is difficult because of the fall wind character with the hydraulic jump is pronounced (no katabatic wind in the narrow sense ). Therefore, the separation from the hair dryer from a climatological point of view is problematic.
Applying to Hair and Bora instead of a two-dimensional thermodynamic flow, a three-dimensional hydrodynamic concept (see hairdryer), so physical differences are not discernible. Both gravity and potential energy is converted into kinetic energy, are essential. Namely, if the air in a mountainous region, it is decelerated and jams like water against a dam. After a certain time it falls on lower areas on the other side of the mountain. Here, the kinetic energy increases and at the base of the wind on the leeward side dips. The higher the pressure gradient between Lee and windward, the more pronounced a fall wind is thereby set.
Over the mountains turns doing a ground-level jet stream ( low-level jet stream, LLJ ) which will, is developed below 5 km. This is pretty strictly limited and relatively strong. In the case of the northern Adriatic Bora a hydraulic two-dimensional theory is sufficient to explain the fall winds, complicated the process the South Adriatic region is where the mountain wave theory must be included as an explanatory cause of the Bora.
Global dissemination boragener downdrafts
At the level of the 40th northern latitude form in winter at longitudes 20-40 ° east ( Adriatic and Black Sea ), 80 ° West ( Great Plains ) and 140 ° East (Japan) three quasi-stationary climatic troughs in the troposphere. This means that cold polar air can easily flow into areas west of the troughs to the south. Bora regions are therefore bound in a broader context to these longitudes. The synoptic position regional concentration is dictated by the topography. As a side issue of the highs over Central Asia macroclimatic shape of the air outflow is connected to a mountain setting. The Bora is famous Dalmatia, the Black Sea, on Novaya Zemlya, in Scandinavia, in the Gulf of Mexico and the Kanto plain of Japan.
The Bora on the Croatian coast
Synoptic types of Bora
For the Dinaric Bora - as well as for the bora own winds of North America - is that due to a north-south orientation of the mountain ( Dinarides, Cascade Range, Rocky Mountains ) must reign a significant component over the eastern mountainous synoptic winds. However, the West flow remains on the east flowing halyard winches. The cooling effect is enhanced by the high speed wind and gusts. About the adiabatic heating rate, the temperature between high and low positions can be calculated, which makes a prediction possible and is necessary for crops in subtropical climates.
A distinction is made between cyclonic Bora ( Bora scura = black Bora ) with pressure drop across the sea, marked by low dark clouds and reduced visibility with rain and / or fog, and anticyclonic ( with pressure rise across the mainland) Bora ( Bora chiara = white Bora ) which occurs under clear skies and good visibility. About the Dinarides a Föhnwand forms only during the white Bora. It serves as an unmistakable sign seafarers to start off the protective harbor. The anticyclonic Bora is very intense on the coast, but extends only a short distance out to sea. The critical terrain slope for the occurrence of Bora is 1:100.
Formation
The Croatian Bora is bound to the Adriatic coast, through the Dinarides to a wandering from the polar region, strong cold air outbreak as a northern or northeastern wind currents. At the bottom is the horizontal gradient of air temperature in wind direction 4 to 5 ° C per 100 km, where the temperature of the Adriatic Sea by 4 to 10 ° C higher than the air temperature. The Bora flows mainly through mountain passes sharply to the coast. Known are the gateways at Trieste, in the Kvarner Bay, in the Velebit Channel ( particularly notorious in Senj ), in Sibenik, Makarska and in Risan. The colder the inflowing air and the lower the mountain height, the less adiabatic heating also plays a role. At the low coastal mountains, the adiabatic heating therefore not sufficient to elicit a greater increase in temperature.
The particular strength of Bora can be explained by the pressure gradient and the cyclonic case by the size of the lows. This is similar in bora own winds in the Cascade Mountains in the Pacific Northwest.
It is not uncommon that each year, bridges or coastal roads temporarily blocked or ferry (some were already blown by the wind into the sea coaches ) must be set. At the new highway A1 Zagreb- Split unique wind barriers are currently being tested in the world.
Aerological situation
- Wind speeds above 15 m / s in the winter
- Air temperatures in January and February is below 0 ° C
- A interdiurne temperature variability of more than -8 to -10 ° C
- Relative humidity below 40% anticyclonic Bora
- Highest wind speeds at night
- Duration usually 12 to 20 h (in exceptional cases more than 10 days )
- In January are usually 15 days Bora recorded.
At the beginning of the Bora is a sudden rise in temperature to determine with then clear waste. During the Bora the relative Humidität is very low in anticyclonic situation. The height of the Bora flow is usually less than 3000 m. Strong Bora contained daytime sea breezes at night, urging the country cold wind warm sea breezes far off the coast from. The strongest wind speed occurs in the deep layers.
Maximum values were in Trieste with 231.5 km / h, Krk 69.0 m / s = 248.4 km / h, Split 174.6 km / h (29 October 1994), Makarska 69.5 m / s = 250.2 km / h (26 January 1996) and Montenegro 42.9 m / s (5 January 1978) measured. The average speed of the Bora 15 November 2004, on the Dalmatian coast 33 m / s = 118.8 km / h, peak gusts recorded 60 m / s
Effects of Bora on people and nature
The Bora characterizes the completely barren islands of northern Dalmatia, what deforestation by Romans and Venetians and the Karst character even stronger. The Bora blew away the denuded soil and rain washed the rock reason. New tree growth is hindered by the mechanical damage. A nature -related effect in the salinization of the soil by salt spray is a secondary result of the Bora. The regeneration of degraded areas is hardly possible under these conditions.
For the maritime field, the Bora has always been dangerous. The sea at Bora is short, but it reaches a considerable height, the wave crests are then lashed into foam, atomized and carried away in clouds of vapor ( fumarea ). The height of the swell at Bora storms, as on 14 November 2004 peak gusts of 60 m / s, decreases with distance from the east coast too important. Before the first signs of Bora seafarers remain only about 30 minutes to run a safe anchorage.
The Bora brings frost and snow storms. Occasionally it may even come to the coast to continuous snow cover, where, as in the interior of the Bay of Kotor, winch preserve these. In the winter of 1965, for example, fell in Risan 93 cm of snow and 1983 stayed here a snowpack 43 days (at 19 snow days ).
Furthermore, the speech will be on the effects on plants and plant communities by Bora. Boragen is a sharp contrast embossed coast can be observed in the Kvarner Bay and the Bay of Kotor. Cold Resistant species are found for example in Bora slopes of Orjens, species and liana- rich, subtropical mixed forests on sheltered slopes. In summary, the regularities in the vertical and horizontal structure of the vegetation in the whole Adriatic coast are available in general accordance with the climatic conditions, all natural irregularities and deviations of the characteristic sequence to see and thus immediate reactions of vegetation mostly under the influence of the environmental factor of the Bora.
In the forest vegetation, the Bora has it the slightest effect. Only in open vegetation types, it uses a more syngenetic. Studies on the island of Pag, Biokovo Mountains and Istria showed a close connection between Bora and vegetation. Biological communities in the Kvarner Gulf in the northern Adriatic, are marked by extreme wind erosion and the very sparse coverage of the cliffs. In addition, developed by the salty sea spray halophilic endemic companies. Exposed layers are dominated by salt spray and icing. Then developed glacio - boreal biotic communities of the Mediterranean islands attest to the vehemence of the climate factor. Also in higher mountain stages, the gale-force winds have a different type of plant association, as they are developed in alpine frame, produced as a kryomediterranen stage presence on the Orjen, Irano - Turanian with representatives thorn cushion formations proves. From winter storms verfrachteter snow, about allowing the Orjen addition, typical Schneetälchengesellschaft with characterizing geophytes. Very exposed, wind -exposed sites, which occur in all elevation zones of the outstanding mountain peaks down to the seashore, promote many specific plant communities occur with endemic species such as the Orjen iris and Crimean peony in Orjen.