The bora is a cold and typically very dry and often gusty katabatic wind (fall-wind) from the north-east. Bora winds can occur anytime during the year. However, the peak frequency occurs in the cold season (November - March). In general, the frequency of gale force bora winds varies from one day per month, or less, in the summer to six days per month during winter months. The term bora derives from boreas, the north. In other areas it is used as a generic term for cold squalls moving downhill from uplands.
The bora is most common blowing down from the mountains on the eastern side of the Adriatic Sea where it flows mainly from the northeast through gaps in the Dinaric Alps. 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. The strongest winds occur along the eastern shores of the Adriatic Sea from Trieste to the Albanian border
There are two primary weather patterns associated with the bora and both are locally classified as being a white bora and a black bora. In either case, the pressure is higher on the European side of the mountains and lower on the Mediterranean side. However, it always takes a cold pool of air accumulating over the Balkan Peninsula at first. When the depth of the cold air pool reaches the height of mountain passes, the bora will commence breaking through the numerous passes that lie along the mountain barrier and sweeping westwards towards the coast. Enhanced by funneling and topography.
The anticyclonic pattern - or white bora: A large high pressure cell is present over central Europe and generally lower pressure without a well defined low pressure center to the south over the Mediterranean Sea. The white bora is basically a dry and very gusty wind due to its katabatic nature. A white bora is characterized by cold, clear weather and good visibilities in the lee of the coastal mo untains, 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.
The cyclonic pattern - or black bora: A low pressure center (sometimes a so called Genoa Low) is present in the southern Adriatic Sea or in the Ionian Sea so that the weather across the area is cloudy and rainy. Black bora winds are less gusty than those in a white bora, but might bring substantial amounts of rain or snow to the region. With the cyclonic pattern, the 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.
The greatest intensity of the bora occurs where the mountain peaks are at least 600 m (2000 ft) above sea level and not more than two or three miles inland. Anticyclonic pattern white bora winds are most intense to the north, decreasing somewhat moving southward, while during the cyclonic pattern, the strongest black bora winds are usually found in the southern Adriatic Sea. In 1956 a gust of 125 kt was recorded at Trieste.
The bora does not usually start with a sudden blast but will build up at a relatively moderate pace. A 60 kt 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. 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 average duration of a Bora that reaches gale force some time during its history is 40 hours with a maximum duration of 5 days.
Winds are usually less intense over the open water of the Adriatic Sea, but gale force winds (greater than or equal to 34 kt) 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. 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 noon and 6pm local time.