Warunki pogodowe na Adriatyku

Summer

 

• The summer season produces generally fair weather for Zadar. For weeks at a time weather may be calm, as on a protected lake. The generally hot and humid conditions are relieved by the northwesterly maestral (mistral) wind, which is the dominant wind of summer. The maestral in the Adriatic has been defined as the superposition and enhancement of the etesian (a northerly wind) by the sea breeze. The maestral is a gentle breeze of 3 - 10 kt (1.5 - 5 ms-¹), normally starting by 1000 or 1100 local time and lasting until 1500 or 1600 in the afternoon. Despite the light intensity, a choppy sea condition is sometimes produced that can adversely affect small boating. 
• Onshore the maestral is welcomed for some relief of the hot and humid conditions that generally prevail. Maestral winds quickly diminish after sunset, becoming calm during the evening, and by early morning an offshore northeasterly flow (land breeze) develops, lasting until about 0800 local time. 
• The jugo is occasionally evident during summer, but the intensity is weak and duration only a few hours so that little problem is posed for boating. 
• The tramontana, a sometimes violent wind from the NNW, can raise sea height at the entrance to Luka Zadar. Tramontana squalls are usually associated with cold frontal passage, preceded by a rise in relative humidity, and drop in pressure. The duration of such events is quite brief, but care should be exercised in dispensing small boats to the harbor when frontal passage is anticipated. Small boats within the harbor can move further southeastward in the port, past the Harbor Master's office, for more protection from this effect

Autumn

 

• Quiet summer conditions generally prevail through the months of September and October as temperatures begin to moderate. Jugo and bora wind events occur infrequently, most of them lasting less than 5 hours. The lebic, a southwest wind, also occurs during October, usually following a jugo. The rapidly changing wind direction in a lebic, from SE to S to SW, frequently causes confused, choppy, sea state that can be dangerous for small boats attempting to load or offload personnel at either of the anchorages in Zadarski Kanal. 
• In the month of November, Zadar experiences the second peak frequency (first in March) of severe jugo wind conditions. A severe jugo implies S - SE winds with intensity of greater than 34 kt (17.5 ms-¹) for at least one hour. Jugo conditions, because of the long fetch, can produce very significant swell, with maximum wave heights exceeding 10 m. Winds and swell are best developed over the open sea and coastal island channels, such as Zadarski Kanal, where U. S. Navy ships anchor for entry into Zadar. 
• Strong bora wind conditions (20 -34 kt, 10.3 -17.5 ms-¹ ) are not experienced in the ports and anchorage locations for Zadar. However, they do occur north of Zadar on the south side of Vir Island, and south of Zadar near the southern end of Pasman Island. Ships departing Zadar heading toward Silbanski Kanal should be aware of the potential for severe bora effects, starting near the central portion of Virsko More, in line with the southern coast of Vir Island. The strong bora is normally experienced near (but not in) Zadar a couple of times during the month of November. On rarer occasions the severe bora occurs, with winds in excess of 34 kt (17.5ms-¹).

Winter

 

• Easterly to northeasterly winds predominate during the winter season, reflecting a high frequency of bora events along the Croatian coast. January has the highest annual relative frequency of severe bora winds (mean hourly wind speed > 34 kt; 17.5 ms -¹) and the highest relative frequency of number of hours with the severe bora. However, these effects are experienced only weakly at Zadar, due to the flat topography in the area and the fact that mountains, instrumental in producing such effects, are some distance away. 
• The jugo or scirocco and lebic also have a high frequency during the winter season, and these do raise high seas at anchorage locations in Zadarski Kanal. The winds of a "severe" jugo having speeds of greater than 34 kt (17.5 ms-¹) last only about 5 hours, but on exception, can extend to 25 hours. Strong jugos with winds from 21 -34 kt (10.8 - 17.5 ms-¹) have an average duration of 19 hours, and with exception to 36 hours. The highest mean hourly wind speed for a jugo in the Adriatic, based on data from 1958-1987, was 53 kt (27.3 ms-¹). This occurred on 21 March 1971. 
• Despite the above information concerning high winds, it is important to realize that there are winter periods when high pressure dominates the region, and seas can be essentially calm for periods of several weeks. 
• It is also a statistical fact that the dominant wind speed in the region during winter is <10 kt (5 ms-¹) and that the tendency for higher average wind speeds is during the spring.

Spring

• Zadar, unlike many other locations along the Croatian coast, does not experience severe effects of bora winds. This is primarily a result of the local, low lying, topography, with no mountains in the immediate vicinity. Luka Gazenica experiences no bora effects and Luka Zadar only weak effects of bora winds. The jugo or scirocco, however, blows fiercely in Zadarski Kanal, raising high seas at the designated anchorage locations. Movement of the ship across Zadarski Kanal for anchorage as close to Ugljan Island as possible can provide some protection from these effects.

• The jugo or scirocco is a south to southeasterly wind which affects Zadar during the spring, having a peak frequency in March and November. The jugo is generally not a scirocco, although the terms are often used interchangeably. The term "jugo" is applied to southerly winds that occur in advance of a low moving across the northern Adriatic. Only when this low expands adequately to draw dry and dusty air from North Africa into the Adriatic does the term "scirocco" really apply. 
• The jugo can be as as dangerous as the bora, and particularly affects anchorages in Zadarski Kanal, where winds of 40 - 50 kt (20.6 -25.7 ms-¹), and seas raised to 3 -4 m are not uncommon. Unlike the bora, the winds and seas during a jugo build gradually, in a predictable fashion, and there is usually adequate time to take defensive measures. 
• The winds of a "severe" jugo having speeds of greater than 34 kt (17.5 ms-¹) last only about 5 hours, but on exception, can extend to 25 hours. Strong jugos with winds from 21 -34 kt (10.8 - 17.5 ms-¹) have an average duration of 19 hours, and with exception to 36 hours. The highest mean hourly wind speed for a jugo, based on data over the Adriatic from 1958-1987, was 53 kt (27.3 ms-¹). This occurred on 21 March 1971. 
• It is often observed at Zadar, and other locations along the Croatian coast, that there can be a rapid change from strong southeasterly winds (jugo) to equally strong northeasterly winds (bora) in as little as one hour. This is a natural consequence of an intense low pressure system moving rapidly southeastward through the region. 
• The lebic often follows the jugo in springtime as a violent southwesterly wind in most regions along the Croatian coast. Wave action is somewhat heightened in Luka Gazenica when such winds occur along the north pier of the port, where U. S. vessels tie up. Luka Zadar is completely protected from lebic wind and sea state effects. The lebic occurs as a low pressure center moves on a track across the northern Adriatic, north of Zadar. The changing wind direction from SE to S to SW does produce choppy seas at the anchorage locations in Zadarski Kanal, which can pose problems for small boats arriving or departing the anchorage area. 
• The tramontana, a sometimes violent wind from the NNW, can raise sea height at the entrance to Luka Zadar. Tramontana squalls are usually associated with cold frontal passage, preceded by a rise in relative humidity, and drop in pressure. The duration of such events is quite brief, but care should be exercised in dispensing small boats to the harbor when frontal passage is anticipated. Small boats within the harbor can move further southeastward in the port, past the Harbor Master's office, for more protection from this effect. 
• Thunderstorms sometimes occur during springtime at Zadar, as dry, cool air, swinging around a low to the north, encounters a current of warm, moist air, flowing northward over the Adriatic from the south. The condition commonly is facilitated by upper level divergence, found in advance of an upper level trough.

Local Hazardous Weather Conditions

• Warning information. Weather forecasts for the Adriatic coast are prepared by the Marine Meteorological Center in Split. These are updated throughout the day and broadcast periodically in Croatian, English, French, German, and Italian. Forecasts in English are broadcast at 0700, 1100, and 1800, local time. Reception of such forecasts is generally on VHF channels 4 and 7. If difficulty is encountered, the Harbor Master's office may be contacted on channel 16 for the most recent information, and possible channel changes. NAVTEX marine weather forecasts are also available. The Harbor Master's office also receives daily, via facsimile from Split, a one page summary of weather information, including a synoptic surface chart, valid for 0300 local, specific port warnings in effect, a 3-day outlook, and local weather conditions for 14 ports along the Croatian coast. This report is sent separately to each major port subscribing to the service. 24 hour forecasts are considered quite accurate, employing the latest techniques in mesoscale modeling. 
• The three most severe wind systems affecting the Zadar region are the jugo, or scirocco, the bora, and the lebic. The jugo or scirocco adversely affects anchorage locations in Zadarski Kanal and raises sea height in Virsko More. The passenger port (Luka Zadar) is well-protected from all three of these winds. The commercial port (Luka Gazenica) is also protected against the jugo (scirocco) and the bora, but experiences some effects of the lebic. Ships at anchor in Zadarski Kanal should consider moving to a pier location within Luka Gazenica when jugo or sirocco winds are experienced or are forecast. An alternate potential for ships who may desire to "ride out" the jugo, is to move to an anchorage position off the north coast of Ugljan Island, as close to shore as possible. Water is deep, even very near the shore of the island. The island provides protection from southerly winds, and there are a number of bays that may offer protection, also, if the wind is from the southeast. 
• Bora winds occur only weakly in the ports and anchorage locations for Zadar and do not pose a weather problem for ships at those locations. 
• The lebic often follows the jugo in springtime as a violent southwesterly wind in most regions along the Croatian coast. Wave action is somewhat heightened in Luka Gazenica when such winds occur along the north pier of the port, where U. S. vessels tie up. Luka Zadar is completely protected from lebic wind and sea state effects. The lebic occurs as a low pressure center moves on a track across the northern Adriatic, north of Zadar. The changing wind direction from SE to S to SW does produce choppy seas at the anchorage locations in Zadarski Kanal, which can pose problems for small boats arriving or departing the anchorage area. 
• The bora causes problems only for ships enroute to Zadar through Virsko More. Ships should be especially alert to a notorious strong bora region at the latitude of the southern end of Vir Island. Here the the bora funnels through valley regions as a mountain gap wind. Local fisherman and boat owners watch for signs of a transition to white, foam-covered water ahead, and take appropriate precautionary measures (stopping, coming in closer to shore, etc.).
• The jugo is commonly terminated and followed by southwesterly winds called lebic. The sudden change from SE to S to SW, as low pressure, responsible for such winds, moves eastward, produces choppy, confused seas, and is especially dangerous for small boating. Many accidents involving broken arms and legs, while attempting to embark or debark into small boats, have occurred under such conditions. The southwesterly lebic sometimes affects the commercial port (Luka Gazenica), mainly at the northern pier where U. S. Naval vessels tie up. Although winds can be fairly strong, wave height is not much of a problem because of the limited fetch provided in Zadarski Kanal. Ships can move to the south side of the port for greater protection. Fortunately, the lebic, as a strong wind, normally only occurs only 1 or 2 times a year, and its duration is short, with seas abating rapidly in the aftermath.
• Statistics show two peak periods for jugo or scirocco winds, March and November, with very few occurrences during the summer. The average duration of the jugo (based on statistics from 1958-1987) was about 36 hours; however, severe wind strength (> 34 kt; 17.5 ms-¹) normally was sustained for only 5 hours. The winds are best developed over the open seas and island regions, and in the Adriatic, have reached a maximum hourly wind speed of 53 kt (27.3 ms-¹). 
• The scirocco, which taps dry, dusty, air from N. Africa, generally occurs in two phases. The first phase, called "fresh", "new", or "clear", scirocco, is denoted by a gradual strengthening of the wind, and a rise in water level, noticeable at port locations . Skies remain clear during this phase. Local fisherman take these conditions during winter and spring as enough of a sign of deteriorating weather conditions to return to port. The second phase occurs as winds continue to increase and clouds are observed, gradually moving over the area from the south. Light to moderate rain often accompanies the strong sustained winds of the second phase, leaving a dusty residue over exposed outside objects in the aftermath. Sea height builds gradually during scirocco conditions, reaching extremes of over 10 m in the open sea regions.
• During bora conditions sustained winds of 20 - 34 kt (10 - 17.5 ms-¹) are common. The bora is a noticeably gusty wind; gusts of 40 - 70 kt (20.6 - 36.0 ms-¹) are common. The onset of a bora is frequently associated with frontal passage.
• The bora has generally been described as a wind of two types; the anticyclonic (also called "white" or "clear") bora, and the cyclonic (also called "dark" or "black") bora. At some locations they sometimes speak of a third type, simply called "bora."
• The anticyclonic bora or clear bora is the most intense configuration, producing the highest sustained wind speeds, and the highest wind speed gusts. Synoptically, the pattern can be identified as one evolving from an intense high pressure system over eastern Europe, with a low to the south over the Ionian Sea, and the southern Adriatic Sea. A frontal boundary across the northern Adriatic separates the two systems, where extremely cold air coming around the east side of the high merges with moist, southerly air, coming around the east side of the low. High pressure tends to change from a meridionally-oriented configuration, to a latitudinally-oriented configuration, as the pressure gradient between the high and the low suddenly intensifies north of the frontal boundary. This produces a strong northeasterly to easterly flow over the Dinaric Alps, north of the front, heading into the Adriatic Sea. Cap clouds are sometimes noted on the mountain crests, giving alert mariners 2-3 hours of advance warning that strong winds are coming. Knowledge that it is raining on the east side of the mountains is often an excellent additional indicator of impending strong bora winds. Data obtained from research aircraft in the region show that winds in the strong pressure gradient region behind the front actually accelerate upstream of the Alpine mountain ridge before descending as cold "fall" winds onto the coastal regions of the eastern Adriatic. The subsiding motion of the cold dry air along the coast generally clears out all cloudiness in this region - hence, the name "clear" or "white" bora. A key feature for Navy meteorologists and ship captains to understand is that the onset of strong bora winds normally commences following frontal passage in the strong pressure gradient behind the front. Equally important is the established fact that highest maximum gusts, often approaching or exceeding hurricane force, usually occur several hours after frontal passage and initial bora onset. 
• The cyclonic bora, often called the "black" or "dark" bora, is often an evolution of the anticyclonic bora, as high pressure to the north begins to weaken and the frontal boundary moves southward toward Greece. The residual low continues to spin warm, moist air over countries bordering the eastern Adriatic, producing low and mid-level cloudiness with rain over the coastal regions (hence the term "dark" or "black" bora). At the same time the cyclonic motion taps cold air from high pressure to the north, forcing this air over the mountain ridges and into the valleys, producing generally moderate speed (21 - 34 kt; 10.8 - 17 ms-¹) bora winds at coastal locations. Gusts within the cyclonic bora are often much greater than the sustained wind speed, making them of obvious concern to Naval operations. 
• Both the anticyclonic bora and the cyclonic bora result from pressure patterns obvious in synoptic analyses and forecasts. With present day mesoscale analysis, weather centers, including that at Split, in Croatia, have had great success in predicting these events. They are less successful, however, in predicting the normal bora, the final type of bora, which frequently catches weather centers, as well as ships and small boats off-guard. This is probably the result of an inadequate number of observing stations, having vertical sounding capabilities, in the region. This type of bora, similar to the anticyclonic and cyclonic bora, depends on high pressure and extremely cold air to the east. This cold air, normally, is capped by an inversion, whose height is lower than that of ridgelines of the Dinaric Alps, so that the cold air cannot reach coastal locations. When this inversion is lifted, for whatever reason, the north-south pressure gradient may be strong enough to force cold air over the ridgelines so that it can spill down toward the coastal area, gathering speed in the process. Cap cloud formation along the ridge line of the mountains is taken as an important harbinger that strong bora winds may occur within 2 -3 hours over the coastal regions. 
• Thunderstorms may occur, infrequently, anytime during late fall, winter or spring. They are a usual result of low pressure moving across the northern Adriatic, bringing cold air around the southern circumference, converging with a current of warm, moist air, coming up from the Ionian Sea. A necessary corollary condition is that higher level divergence be present over the lower level convergent area (normally the divergent area in advance of an upper level trough). Strong gusty wind speeds, especially dangerous to small boats can occur under such conditions. Waterspouts have infrequently been observed in the Zadar area, with mammatus cloud conditions signaling extreme instability aloft.