Marine Meteorology: Trieste
- Coordinates: 45deg/38min N 13deg/45min
- 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
- 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
- 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
- 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
- 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.
- 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
- 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
- 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.
- 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.
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.
There is no scheduling information available
for this port.
- 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:
- Anticyclonic Pattern: A large high
pressure center is present over central Europe without a well
defined low to the south.
- 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
- 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
- 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
- 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
- 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