Meteorology
Geosciences

Marine Meteorology: Trieste

Port Information

Geographic Location

• Coordinates: 45deg/38min N 13deg/45min E.
• The city of Trieste is located at the head of the Adriatic Sea, at the southern end of a long, narrow, coastal strip of Italy. To the northeast of the city a range of rocky hills rises to heights over 1000 ft and extends as a plateau into Yugoslavia.
• At both anchorages, depths are 59 ft and holding ground is fair to poor.
• The harbor is divided into three large areas: the commercial port, the industrial port, and the oil harbor. The commercial port is formed by four free zones: Porto Franco Vecchio (Old Free Zone), Porto Franco Nuovo (New Free Zone), Scalo Legnami (Timber Dock), and Porto Franco Oli Minerali (Mineral Oil Dock). When entering Porto Franco Vecchio the approximately 518 ft wide south entrance is used. Ships may enter Porto Franco Nuovo through either the north or south entrance.

Harbor as Haven

• The Port of Trieste is an area of frequent Bora wind occurrences and is near the Trieste Gap where winds are frequently over 50 kt during Bora episodes. However, because the winds are from the direction of the land mass, wave heights are not extreme and many port operations can continue except during the most severe Bora outbreaks. The port is also protected from the Sirocco wind (southeasterly) and minimal wave heights occur with Sirocco events.
• The main U.S. Navy berth is at the Stazione Maritima (Maritime Station) as is the fleet landing. Draft on the north side of the pier is 30 ft and 27 ft on the south side. The pier is 825 ft long and is made of concrete. Some of the mooring dolphins have worn caps (August 1987) and lines tend to slip up and off during heavy weather, especially during a Bora outbreak.
• Secondary berths are at the grain elevator pier (Molo VI) and at Scalo Legnami. Molo VII can facilitate large ships (40+ ft draft) but as of August 1987, expansion construction was in progress limiting the pier's use.
• Most of the piers are aligned in the direction of the Bora wind so ships can arrive and depart in winds as high as 40 kts.
• Note that the Maritime Station berth is not aligned with the Bora wind direction (east-northeasterly).

Currents and Tides

• There is a large counterclockwise current gyre in the center of the Adriatic Sea which sometimes breaks into two smaller gyres. In any case, the general current flow is northward along the eastern shores and southward along the western shores.
• Very little surface current is noted in the port area of Trieste. Just south of the port, along the Yugoslavian coast, a one kt current sets northward.
• Astronomical tide range at the port is 2.8 ft.

Visibility

• Visibility is generally good in Trieste. The Bora wind normally brings dry, clean air which can persist for days at a time in the winter.
• However, Trieste is not far from a major source of fog, the Po Valley and the Gulf of Venice. During periods of westerly winds fog will advect in from these areas and reduce the visibility at Trieste to less than one mile, usually one or two days per year in November. Another two or three days per year, visibility will be in the 1 to 3 mile range.
• Although these episodes of reduced visibility are rare, they can last all day when they do occur.

Hazardous Conditions

Spring

• Early spring resembles winter and as spring progresses some summer like days are noted. The strong Bora episodes usually end by April, but milder Boras can occur in any month of the year.
• Some visibility restrictions can occur with fog in the early spring. This is usually due to a fog laden west wind and can last for a day or more.
• Wind chill is still a factor during early spring.

Summer

• The Siberian High is replaced by a large low pressure system extending from Southwest Asia toward Asia Minor. This pressure configuration brings generally warm and dry weather to Trieste. When Bora winds do occur, wind speeds are usually less than 30 kt.
• Thunderstorms are most frequent during the summer months. Due to the hilly topography around Trieste, these storms will occasionally form over the coastal terrain and move over the port area. Generally, thunderstorms are short lived in the Trieste region.

Autumn

• The autumn season in the Adriatic is short, lasting only for the month of October and is characterized by an abrupt change to winter-like weather.
• Wind chill is normally not a factor until late November.

Winter

• Bora winds are common in Trieste during wintertime. Winds of 50 kt with occasional gusts to 80 kt are not uncommon. In 1956 a gust of 125 kt was recorded. However, in more recent years the highest gust has been 95 kt. Peak month for Bora occurrence is February while strong winds associated with the Bora can occur in any month. Other strong winds, usually from the south, can occur prior to cold frontal passage associated with a transitory low pressure system from the Gulf of Genoa.
• Below freezing temperatures are common during winter. Wind chill factors can be dangerous when cold temperatures occur with high winds, quite common in a Bora outbreak.
• Trieste normally experiences good visibility year-round. However, one or two days per year, usually in November, expect visibility to be near zero in dense fog. The fog is associated with a west wind and can last the entire day. On another two or three days of the year, visibility will be in the 1 to 3 mile range, again associated with fog and a west wind.

Protective or Mitigating Measures

Moving to a New Anchorage

• In most bad weather instances in Trieste, wind will be the dominating factor rather than waves although some local seas will build even in a limited fetch. Most of the harbor area is protected by the local terrain. During an intense Bora event, if at anchorage, it is best to go to sea. If berthed--add lines.
• One maneuver to decrease the effect of the local seas during a severe Bora is to get as close to the coastline as possible, in the lee of the high terrain. However, south of the port is the Yugoslavian coast with a two-mile territorial limit. The recommended location is north of Castella Miramare, moving to within one-half mile of the coast. Consult charts as there are mussel farms in this area. This maneuver will decrease seas substantially and decrease winds slightly.

Sortie/Remain in Port

There is no sortie information available for this port.

Scheduling

There is no scheduling information available for this port.

Harbor Protection

• Wind and Weather: Trieste's climate is dominated by the Bora wind which can occur anytime during the year. However, the peak frequency occurs in the cold season (November - March). To a lesser extent, the Sirocco wind affects Trieste but is not nearly as strong or as frequent as the Bora. Gulf of Genoa lows have an influence on weather in the northern Adriatic Sea as they either move toward Trieste causing stormy weather with clouds and rain, or they move southeastward causing a pressure differential at Trieste and trigger a Bora outbreak.
• Bora: The Bora occurs when cold air accumulates over the Balkan Peninsula, especially Yugoslavia. When the depth of the cold air pool reaches the height of the mountain passes, the Bora will commence. There are two primary weather patterns associated with the Bora:
  1. Anticyclonic Pattern: A large high pressure center is present over central Europe without a well defined low to the south.
  2. Cyclonic 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.
• 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. 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 pressure differential is large enough, the Bora can extend as far south as Malta.
• In the northern Adriatic, the wind direction associated with the Bora is generally northeasterly but can vary in local areas due to the terrain. The Bora at Trieste is east-northeasterly. It is more northerly further south and even northwesterly along Italy's southeast coast.
  • The strongest winds occur along the eastern shore of the Adriatic from Trieste to the Albanian border.
  • It is most intense to the north, decreasing somewhat moving southward.
  • 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.
  • Over the open water of the Adriatic, winds are usually less intense, but gale force winds (30+ 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.
    • During the cyclonic pattern, the strongest winds are usually found in the southern Adriatic.
• Bora winds are most common during the cool season (November through March). In Trieste, the highest frequency of occurrence and strongest winds are in February. 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.
  • The average duration of a continuous gale force Bora over the Adriatic 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.
• At Trieste, 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. However, the Bora has been known to last for up to 30 days at Trieste without a significant lull.
• In 1956 a gust of 125 kt was recorded at Trieste. However, in a recent 10 year period from the mid-70's to the mid-80's the highest gust recorded was 95 kt.
• In the Trieste area, 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.
• Wave heights near the port of Trieste are normally not high with a Bora as the terrain limits the fetch. Because most of the piers are aligned with the direction of the Bora, certain ships can be berthed during a Bora episode, even with winds of 40 kt. Note that the primary U.S. Navy berth, the Maritime Station Pier, is not aligned with the Bora direction.
• There is a noticeable diurnal variation at coastal Adriatic stations during Bora conditions. During the day, along the eastern shore, the sea breeze counteracts the offshore flow of the Bora which leads to a decrease in the strength of the Bora between 1200L and 1800L. In Trieste, winds are weakest at noon and strongest at sunrise and sunset.
• 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 upslope motions are found on the mountain crests. These clouds subsequently dissipate in the descending air on the lee side.
• With the cyclonic pattern, the 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.
• Sirocco: The Sirocco is a southeasterly to southwesterly wind over the Mediterranean originating over North Africa, sometimes affecting the Adriatic Sea area.
• The Sirocco tends to occur year-round without a favored month or season.
• 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 in the latter case.
  • Sirocco conditions occur in the Gulf of Genoa 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 Sirocco 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 and occasionally as long as 36 hours. The maximum wind speed likely during a Sirocco is about 55 kt.
• At Trieste, local terrain features alter the effect of the Sirocco. Winds will parallel the coast in general and sometimes reach the port through a small river valley which terminates just south of the port area in the Baia de Muggia. The valley winds are normally stronger than the coastal winds.
• In August 1985, a gust of 90 kt was recorded as a 1003 mb low tracked across Italy into the northern Adriatic. This high wind was most likely due to a combination of the Venturi effect from the valley and the effects of a barrier such as an elevated land mass. As the front approaches from a relatively flat, low surface, such as the ocean, toward an elevated land mass, supergradient winds will occur, again due to the Venturi effect. These winds are restricted to a narrow band between the front and the landmass. Seas are usually not high with strong southeasterlies due to protection from the terrain. Also, these winds do not normally sustain for long periods of time.
• Genoa Lows: Genoa lows are 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.
• Genoa cyclones usually remain stationary (or at least leave a residual trough) south of the Alps throughout their life history. If the lows do move, they generally follow one of two tracks.
  • The first track, common for cyclones developing in the Gulf of Venice, is a northeasterly to north-northeasterly direction across the Alps. This track is associated with strong southwesterly flow aloft. In this case, Sirocco conditions are likely if the circulation of the low extends southward into North Africa, allowing air from the desert source to move northward.
  • The second track, associated with a strong anticyclone over the Balkans, Turkey and the Black Sea is in a southeasterly direction from the Gulf of Genoa towards the Ionian Sea. In this case, a gale force Bora is extremely likely by the time the depression moves into the Ionian Sea.

Local Hazardous Weather Conditions

• There are few local indicators of the Bora. Because the wind is usually dry, there are no cloud patterns occurring at Trieste prior to a Bora onset. However, there are often clouds atop the mountains to the north before a Bora event. These clouds will have an east-to-west movement which precedes the Bora onset by an hour or two. Another tip-off used by local mariners is that after a solid day of southeasterlies, they expect a Bora wind the next day. Unfortunately, most of the time when a brisk, cold wind is experienced, the Bora has already started without much warning. The strongest winds, however, are usually not in the beginning stages of the Bora event so there may be time for protective measures to be taken. Also, there are some general guidelines to use when other than local observations are available.
• Expect Bora conditions in the Adriatic Sea when high pressure is forecast to build over the Balkans and/or a low pressure system is expected to move into the Ionian Sea, especially from the Gulf of Genoa.
• When Bora conditions are occurring, a well-defined foehn wall cloud over the Dinaric Alps can be seen in satellite imagery. Also, cumulus cloud streaks over the water will indicate gale force (30+ kt) Bora winds are present.
• Likewise, there are very few hints available for predicting the onset of a Scirocco. However, the Scirocco's onset is much more gradual than the Bora and it is usually not as intense. One rule, almost foolproof, is that the Scirocco is normally associated with a depression or cyclone which approaches the northern Adriatic Sea from the west or south.

Heat Index and Windchill Charts

Source:

• Naval Research Laboratory Marine Meteorology Division - http://www.nrlmry.navy.mil/~cannon/medports/Trieste/index.html