The bora ( Greek μπόρα bόra , German 'cold gust of wind, cold rain shower' , from Boreas , literally 'the northern one'; Croatian Bura ; Slovenian Burja ) is generally a meteorological term for cold and gusty winds that occur on different coasts. In particular, Bora is the name of the hurricane-like land wind that occurs between Trieste and the Drimm estuary on the Croatian and Montenegrin Adriatic coast . Bora-type winds are among the strongest in the world, with their frequency and high average speeds. Top speeds of individual gusts reach values of up to 250 km / h at the western foot of the Velebit.
The word Bora is derived from the Greek Boreas , god of the north winds. Since a strong north wind destroyed the Persian fleet off Athens , he was the patron saint here and was worshiped in a separate Athens temple. He was considered the most powerful wind god , which is why he was the godfather of the dreaded hurricane wind Bora. The term originally meant "wind from the mountains" and has to do with the coldness of the downwind. For a long time, the bora was the prototype of katabatic winds .
There are also various special names for a bora. The bora scura in Italy is a bora with cloudy visibility. The Borino is a weak bora in the Trieste area , while the violent shocks of the bora there are called Reffoli . A Borasco is a strong wind, usually accompanied by thunderstorms , over the Mediterranean . A levantera is a bora that comes from the east in Istria and occurs when the weather is cloudy.
In Dalmatia they say: she is born on the Dinara , she is baptized in Makarska and she gets married in Senj . In Istria, however, they say: The bora is born in Senj, rules in Rijeka and dies in Trieste .
Bora winds result from a strong cold air outbreak migrating south from the polar region . On the ground they appear as northerly or northeastern wind currents towards the Adriatic coastal area. From the synoptic point of view , the regional limitation is given by the topographical conditions; (Sub-) polar cold air flows over a mountain threshold before it reaches a relatively warm sea. Only from the mountain obstacle does an acceleration of the air masses result, which take on a hurricane-like form below 1000 m. As a marginal phenomenon of the winter high pressure area over Central Asia , the macroclimatic shape of the hurricane-like cold air discharge in the mountain range of the eastern Adriatic basin in the Dinarides is given.
The bora is very stormy and gusty , with gusts reaching up to 250 km / h in individual cases . The bora blows mainly in winter. In summer it lasts a day or a few hours, while in winter it can blow for up to 14 days. The prevailing wind direction is from east-northeast . The bora has its most drastic effect when very cold continental air or aged polar air reaches the Adriatic . The cold air over the karst valleys of Croatia then rushes down as a fall wind through the alleys of the karst mountains. Despite the dry adiabatic warming when blowing down, the wind is perceived as cold, since the relative height of the littoral Dinarides is too low for a noticeable warming ( Velebit 1756 m, Biokovo 1762 m in Croatia and the Orjen 1894 m in Montenegro ) and the other relative warm Mediterranean air is displaced by the very cold continental or aged polar air.
In 1856 Karl Marx drew a (not entirely) accurate picture of the Bora:
“The Bora, the great troublemaker of this sea, always gets up without the slightest warning sign; with the force of a tornado it attacks the sailors and allows only the boldest to stay on deck. Sometimes it rages for weeks and most violently between the bay of Cattaro and the southern end of Istria . But the Dalmatian is used from childhood to defy her, he becomes hard under her breath and despises the poor winds of other seas. "
Föhn and Bora are the warm and cold downwinds that can be observed around the world in a similar way. The similarity to the hair dryer is through
- Occurrence in relatively limited regions in the lee of a mountain range
- a wall of clouds over the mountain ridge (Föhnwand, Kappa) and
- given a pressure difference between windward and leeward .
In contrast to the foehn, the Bora winds have a cooling character, which also affects a coastal region and has an effect on sea dynamics. In addition, foehn winds are usually short-term phenomena, while long-term occurrence is a primary characteristic of the bora.
It is difficult to differentiate between the foehn in terms of flow dynamics, as the downwind character is clearly pronounced with the hydraulic jump (no katabatic wind in the narrower sense). Therefore, the separation from the hair dryer is problematic from a climatological point of view.
If a three-dimensional hydrodynamic concept is applied to the Föhn and Bora instead of a two-dimensional thermodynamic flow (see Föhn), no physical differences are discernible. Both gravity and potential energy, which is converted into kinetic energy, are essential. If air meets a mountain region, it is slowed down and accumulates like water in front of a dam. After a certain time it falls over lower areas to the other side of the mountains. The kinetic energy increases and a wind appears at the bottom of the lee side. The higher the pressure gradient between leeward and windward, the more pronounced the downwind will be.
Over the mountains this is a ground-level jet stream ( low-level jet stream one, LLJ), which is below km developed by 5th This is quite narrow and relatively strong. In the case of the north Adriatic bora, a hydraulic two-dimensional theory is sufficient to explain the down winds, the process in the south Adriatic region is more complicated, in which the mountain wave theory must be included as an explanation for the bora.
Global distribution of boragen downdrafts
At the 40th north latitude, three quasi-stationary climatic troughs form in the troposphere in winter at longitudes of 20–40 ° East ( Adriatic and Black Sea ), 80 ° West ( Great Plains ) and 140 ° East ( Japan ). This means that polar cold air can easily flow south to areas west of the troughs. Bora regions are therefore bound to these longitudes in a broader context. From the synoptic point of view , the regional narrowing is given by the topography . As a marginal phenomenon of the high above Central Asia, the macroclimatic form of the air discharge is connected to a mountain frame. The bora is known off Dalmatia , the Black Sea , on Novaya Zemlya , in Scandinavia , in the Gulf of Mexico and from the Kantō plain of Japan.
The bora on the Croatian coast
Types of the Bora
For the Dinaric bora - as well as for the boragen winds of North America - it applies that due to a north-south orientation of the mountains ( Dinarides , Cascade Range , Rocky Mountains ) a significant eastern component must prevail over the synoptic mountain winds. The westerly current remains above the downdraft winds flowing to the east. The cooling effect is enhanced by the high wind speed and gustiness. Using the adiabatic warming rate, the temperature between high and low elevations can be calculated, which makes a forecast possible and is necessary for cultures in subtropical climates.
A distinction is made between cyclonic bora ( bora scura 'black bora') with a pressure drop over the sea, which is characterized by low dark clouds and reduced visibility with rain and / or haze, and anticyclonic (with pressure increase over the mainland) bora ( bora chiara ' white bora '), which occurs when the sky is clear and visibility is good. A foehn wall only forms over the Dinarides during the white bora. It serves as an unmistakable indicator for seafarers to call at the protective harbor. The anticyclonic bora is very intense on the coast, but only extends a short distance out at sea. The critical terrain inclination for the occurrence of Bora is 1: 100.
The bora is linked to a strong cold outbreak migrating from the polar region as north or northeast wind currents to the Adriatic coast over the Dinarides. On the ground, the horizontal gradient of the air temperature in the wind direction is 4 to 5 ° C per 100 km, whereas the temperature of the Adriatic Sea is 4 to 10 ° C higher than the air temperature. The bora flows strongly to the coast, especially over mountain passes. The gateways near Trieste , in the Kvarner Bay , in the Velebit Canal (especially notorious near Senj ), in Šibenik , in Makarska and in Risan are known . The colder the incoming air and the lower the mountain height, the less adiabatic warming plays a role. In the low coastal mountains, the adiabatic warming is therefore not sufficient to cause a greater increase in temperature.
The special strength of the bora can be explained by the air pressure gradient and, in the case of a cyclonic case, by the size of the depression. It is similar with boragen winds in the Cascade Mountains in the Pacific Northwest .
It is not uncommon for bridges or coastal roads to be temporarily closed every year (in some cases coaches have already been blown into the sea by the wind) or ferry lines have to be suspended. Wind barriers that are unique in the world are currently being tested on the new A1 Zagreb - Split motorway .
Overall, the aerological situation of the bora occurs with high frequency. Characteristics of a bora event are:
- Wind speeds over 15 m / s in winter
- Air temperatures in January and February below 0 ° C
- an interdiural temperature variability of more than −8 to −10 ° C
- relative humidity with anti-cyclonic bora below 40%
- highest wind speeds at night
- Duration usually between 12 and 20 hours (in exceptional cases more than 10 days)
At the beginning of the bora there is a sudden rise in temperature, followed by a clear drop. During the bora, the relative humidity is very low in the anti-cyclonic situation. The height of the bora flow is usually below 3000 m. A strong bora dampens sea breezes during the day, and at night the cold land wind pushes warm sea breezes off the coast. The strongest wind speed occurs in 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 (October 29, 1994), Makarska 69.5 m / s = 250 , 2 km / h (January 26, 1996) and Montenegro 42.9 m / s (January 5, 1978). The average speed of the bora on November 15, 2004 on the Dalmatian coast was 33 m / s = 118.8 km / h, peak gusts recorded 60 m / s.
Effects of the bora on humans and nature
The meteorologist Josef Wasmayer said of the effects of the bora on people's well-being:
"The bora makes people fresh and, apart from the uncomfortable, often cutting cold, does not cause any complaints, such as with the blow dryer with circulatory, heart, muscle and joint problems, nervousness and depression and the like, the case is. On the contrary, it creates liveliness, enterprising and joyful mood. It stimulates people and perks them up. They say: "The bora blows away bad moods". Many sick people feel relieved of their suffering and pain on borate days. "
The Bora is in contrast to the Scirocco, Ostro and Libeccio winds, which cause complaints known as "foehn diseases".
The bora characterizes the completely bare islands of northern Dalmatia, which intensified the deforestation by the Romans and Venetians and the karst character. The bora blew away the exposed ground and rain washed away the bedrock. New tree growth is made more difficult by the mechanical damage. A natural effect of the salinisation of the soil by salt spray is a secondary result of the bora. The regeneration of degraded areas is hardly possible under these conditions.
The bora has always been dangerous for seafaring. The swell at Bora is short, but it reaches a considerable height, the crests of the waves are then whipped into foam, atomized and carried away in clouds of mist ( fumarea ). The height of the swell in bora storms, such as on November 14, 2004 with peak gusts of 60 m / s, increases significantly with the distance from the east coast. Before the first signs of a bora, sailors only have about 30 minutes to call at a safe anchorage.
The bora brings frost and blizzards. Occasionally, even on the coast, there can be a permanent snow cover if, as in the interior of the Bay of Kotor, winds preserve it. In the winter of 1965, for example, there was 93 cm of snow in Risan and in 1983 there was a snow cover here for 43 days (with 19 snowfall days).
A marked contrast of the boragen-shaped coast can be observed in the Kvarner Bay and the Bay of Kotor. Cold-resistant species can be found, for example, on the bora slopes of the Orjens , species-rich and liana-rich, subtropical mixed forests, on more protected slopes. In summary, the regularities in the vertical and horizontal structure of the vegetation in the entire Adriatic coastal area can be seen in general agreement with the Mediterranean climatic conditions, while all natural irregularities and deviations in the characteristic sequence are mostly under the influence of the ecological factor of the bora, i.e. direct reactions of the Plant cover to represent the bora.
The bora has the least effect on forest vegetation. Only in open vegetation types does it intervene more strongly syngenetically . Studies on the island of Pag , the Biokovo Mountains and Istria showed a close connection between the bora and vegetation. Biocenoses on the islands in the Kvarner Bay of the northern Adriatic are characterized by extreme wind erosion and the extremely sparse cover of the cliffs. In addition, halophilic endemic societies have developed from the salty spray. Exposed locations are characterized by salt spray and icing. Glacio-boreal biocenoses of the Mediterranean islands developed on this basis testify to the vehemence of the climate factor bora. Even in higher mountain ranges, the hurricane winds have produced a different plant community than in the alpine setting, as the occurrence of a cryomediterranean stage on the Orjen with representatives of Iranian- turanic thorn cushion formations proves. Snow carried by winter storms also enables typical snow valley communities with characteristic geophytes to be established in the Orjen. Very exposed and wind-exposed locations, which occur in all altitude zones from the outstanding mountain peaks down to the sea coast, promote many specific plant communities with the appearance of endemic species such as the Orjen iris and Crimean peony in the Orjen.
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- Joseph Wasmayer "Weather and Oceanography of the Adriatic" (1976), S. 145th