The Bora Wind of the Adriatic Sea
From the 1911 Encyclopedia Brittanica:
From Treccani Enciclopedia Italiana:
The bora of the Adriatic is a cold, north to northeast katabatic wind. Katabatic is a word derived from the Greek Katabasis which means "an army marching through a country down toward the coast". Such winds are phenomena that originate with a layer of cold air forming near the ground on a night with clear skies and a low pressure gradient. If the ground is sloping, the air close to the ground is colder than air at the same level but at some horizontal distance. The result is downslope gravitational flow of the colder, denser air beneath the warmer, lighter air. This occurs on the largest scale as the outflowing winds from Greenland and Antarctica.
The Adriatic bora flows flow through mountain passages into the Adriatic Sea east of Italy (similar to Bora - Aegean Sea). Bora - Adriatic Sea may be associated with stormy weather, with winds reaching 100 knots or more. Bora - Adriatic Sea develops in the winter when cold polar air builds over the Balkans and flows katabatically through the valleys of the Dinaric Alps and into the Adriatic Sea. As shown, Bora - Adriatic Sea flow is generally confined to the Adriatic Sea area, as the Alps to the north, and the Apennines to the west, channel the flow southwestward. Bora - Adriatic Sea can reach gale to storm-force strengths, and can extend well out to sea, but are usually confined to the coast in the northeast Adriatic. The Bora - Adriatic Sea can also be produced when high pressure over the Balkans interacts with a low in the Ionian Sea. Funneling through the Strait of Otranto can produce storm force northerlies and associated high seas.
The bora, named after Bureas, the God of the North Winds, has been described in many professional treatises, but also in writings expressing admiration or fear of its fury and strength. It is a strong, cold and dry north-easterly wind that is sometimes called impetuous, capricious, and superficial because it blows at intervals and with sudden gusts. But, reaching up to 120 km/hr in volocity, it is can be very dangerous for automobile and boat travel.
The traditional bora is a wind whose source is so cold that when the air reaches the coast, the dynamic warming caused by subsidence is insufficient to raise the air temperature to the level normally experienced. It occurs when cold air accumulates over the Balkan Peninsula to the extent that it spills over coastal maintain ranges and "falls" down the steep slopes to the sea.
The bora is not a sometime wind - it blows for more than 40 days each year. Easterly to northeasterly winds predominate during the winter season, reflecting a high frequency of bora events along the Adriatic coast. January has the highest annual relative frequency of severe bora winds (mean hourly wind speed > 34 knots; 17.5 ms -¹) and the highest relative frequency of number of hours with the severe bora. In the summer it blows as a local wind and then lasts only a few days. In the winter it may continue for six to fourteen days. It blows from the continent, i.e. from the eastern side of the Adriatic towards the open sea and brings bright weather. The bora stores up its violence from the rush of moving air as it falls down the mountains and shoves the lower air along in ever-mounting fury along the narrow valleys and is spit out cold, bleak and strong, into the Adriatic. It starts abruptly and blows in squalls toward the sea and is strongest in the Velebit Channel and the Gulf of Trieste. Picture shows typical "bura" corridors: (1) Golf of Trieste, (2) Kvarner, (3) Velebit Channel, (4) Sibenik, (5) Split - Makarska, (6) Peljesac and (7) Dubrovnik.
The bora is most common in the Adriatic Sea where it flows mainly from the northeast through gaps in the Dinaric Alps. One of these gaps is near Trieste and is known as the Trieste Gap. The gap east of Koper is not as prominent as the Trieste Gap. On occasion, the bora can be very localized, extending only a few miles offshore. At other times, the bora will dominate the entire Adriatic Sea and, when the area of steep pressure gradiant is large enough, the bora can extend as far south as Malta.
Some parts of the coast (including the islands of Lošinj and Hrvar) are protected from the winter bora by chains of mountains on whose lee side the climate is mild. In the southern parts of the Adriatic coast, from Dubrovnik to Ulcinj, the bora is moderate and the climate genial. At Zadar, the bora effects are experienced only weakly due to the flat topography in the area and the fact that mountains, instrumental in producing such effects, are some distance away. It is rare for the temperature in sheltered places around Zadar to drop below zero even in the most severe winter conditions (a maximum of 1-3 days throughout the winter).
There are two primary weather patterns associated with the bora:
1. Anticyclonic (white) pattern
In this pattern, a large high pressure cell is present over central Europe without a well defined low pressure center to the south. The so-called white bora is most intense to the north, decreasing somewhat moving southward. With the anticyclonic pattern, the bora is basically a dry wind due to its katabatic nature. Clear skies and good visibilities are found in the lee of the mountains while thick clouds associated with up-slope motions are found on the mountain crests. These clouds subsequently dissipate in the descending air on the lee side, and appear as "cap clouds" to an observer on their west side.
2. Cyclonic (black) pattern
In this pattern, a low pressure center is present in the southern Adriatic Sea or in the Ionian Sea. In either case, the pressure is higher on the European side of the mountains and lower on the Mediterranean side. With the cyclonic pattern, the so-called black bora is often accompanied by low clouds and reduced visibilities associated with rain and/or drizzle. These conditions are more noticeable over the open water areas than along the coastal zone.
Bora winds are most common during the cool season (November through March). In general, the frequency of gale force winds varies from one day per month, or less, in the summer to six days per month during winter months.
In the northern Adriatic Sea, the wind direction associated with the bora is generally northeasterly but can vary in local areas due to the terrain. The bora at Koper and Trieste is east-northeasterly. It is more northerly farther south and even northwesterly along Italy's southeast coast. The strongest winds occur along the eastern shore of the Adriatic Sea from Trieste to the Albanian border. In 1956 a (record?) gust of 125 knots (or 231.5 kilometers/hr (1 knot = 1 nautical mi/hr = 1.852 kilometers/hour) was recorded at Trieste.
The greatest intensity of the bora occurs where the mountain peaks are at least 2000 ft above sea level and not more than two or three miles inland. Winds are usually less intense over the open water of the Adriatic Sea, but gale force winds (greater than or equal to 34 knots) are common. The frequency of the gale force bora in the open sea is greater for the cyclonic type of pattern than for the anticyclonic pattern.
The average duration of a bora that reaches gale force some time during its history is 40 hours with a maximum duration of 5 days. The average duration of a continuous gale force bora over the Adriatic Sea is about 12 hours but the winds sometimes will last up to two days.
The bora does not usually start with a sudden blast but will build up at a relatively moderate pace. A 60 knot bora will not reach peak intensity during the first 3 or 4 hours. This may allow time for some protective measures to be assessed and conducted.
There is a noticeable diurnal variation at stations along the eastern coast of the Adriatic Sea during bora conditions. During the day, the sea breeze counteracts the offshore flow of the bora, which decreases the strength of the bora between 1200L and 1800L.
Koper's climate is dominated by the bora wind. Due to topography differences, these winds can occur anytime during the year, with the peak frequency occurring in the cold season (November to March) but are not as strong as those experienced at Trieste. Nevertheless, sustained wind speeds of 54 kt (100 kph) have been observed at the Port. Bora events may last two to three days.
The white bora (anticyclonic) of Koper results when a strong pressure gradient exists between a cold high pressure area north of Slovenia and a low-pressure area over the Mediterranean Sea. It is characterized by cold, clear weather. The so-called black bora (cyclonic) results when the low-pressure area is located over the Adriatic Sea resulting in cloudy and rainy weather over Slovenia.
Due to the instability of the cold air mass, winds in a white bora are gustier than those in a black bora. The gusts can be more than twice the sustained wind speed. The direction of the wind from either bora is approximately 060° in the entrance channel (30° off the port bow of an incoming vessel) and in Basin 3, the worst affected area of the port. The direction becomes somewhat more easterly in the southern parts of the harbor, including Basins 1 and 2. bora events are normally strongest and most frequent during the months of December through March, but can occur outside that four-month period. They are uncommon during the summer season, but weak boras of short duration may occur. Bora winds do not normally cause problems for ships in port, but additional mooring lines are deployed when necessary to keep a ship safe at her berth. Wave heights near the port of Koper are normally not high with a bora because they are fetch limited. The safest anchorage during a bora is located in Koprski Bay just south of Debeli Point.
At Koper, the average duration of a gale force bora varies from three days in winter to one day in summer. Local mariners state that the bora will last an odd number of days; 1, 3, 5, etc., unlike Trieste where the bora has been known to last for up to 30 days.
The sirocco (see also scirocco) wind affects Koper to a lesser extent, but is not nearly as strong or as frequent as the bora.
The sirocco is a southeasterly to southwesterly wind over the Mediterranean originating over North Africa and sometimes affecting the Adriatic Sea area. The sirocco normally occurs within the warm sector of a cyclone passing either north or west of the region. These cyclones originate either over North Africa or south of the Alps, primarily in the Gulf of Genoa. Sirocco conditions occur in the latter case when the circulation extends far enough southward to draw air from the North African region. The onset of the sirocco is more gradual than the onset of a bora. It occurs more frequently in the southern part of the Adriatic with a decrease in frequency northward. Although the airocco is not as strong as the bora, winds can reach gale force (30+ kt), especially in winter and spring. The average duration of continuous gale force winds during a sirocco is 10 to 12 hours with rare occurrences as long as 36 hours. The maximum wind speed likely during a sirocco is about 55 kt.
Although Sirocco winds occur year-round the favored months are October through January. These winds usually last one or two days and often bring rain, (sometimes mixed with Saharan dust) or fog. Wave heights can reach 4 m (13 ft) at the anchorage.
Gulf of Genoa lows have an influence on weather in the northern Adriatic Sea as they either move toward Koper causing stormy weather with clouds and rain, or they move southeastward causing a steep pressure gradiant over at Koper, initiating a bora outbreak.
Genoa Lows are described as low-pressure systems which develop to the south of the Alps in the region incorporating the Gulf of Genoa, Ligurian Sea, Po Valley, Gulf of Venice and northern Adriatic Sea. Although several factors are important in cyclogenesis, the development of the cyclone near the Gulf of Venice - as opposed to the west near the Gulf of Genoa - depends on the amount of cold air penetrating the Po Valley from the northeast. If there is little or no cold air entering the Po Valley, the low will probably form in the Gulf of Venice; otherwise, cyclogenesis will occur to the west.
This page compliments of Marisa Ciceran