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Mediterranean Pilot Charts The winds

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EXPLANATION OF WIND ROSES

PREVAILING WINDS AND CALMS.- The wind rose in blue color is located in the centre of each 5 deg square where there was sufficient data. The rose shows the distribution of the winds that have prevailed in the area over a considerable period. The wind percentages are summarized for the eight points and calm. The arrows fly with the wind indicating the direction from which the wind blew. The length of the shaft, measured from the outside of the circle gives the percentage of the total number of observations in which the wind has blown from that direction. The number of feathers the average force of the wind on the Beaufort Scale. The figure in the Centre of the circle gives the percentage of calms. When the arrow is too long to fit conveniently into the 5 deg square, anything over 29 %, the shaft is broken and the percentage is indicated by numerals.

Gales are most likely in the Golfe du Lion, the Adriatic Sea, and the Aegean Sea. There is a general decrease toward the S and E, but gale frequency increases in the main straits, such as the Strait of Gibraltar and the Alboran Sea, due to funnelling.

Winter is the primary season for strong winds. The highest frequency of gales is the Golfe du Lion, where they occur greater than 20 per cent of the time near the head, decreasing to 10 per cent in the open seas. Both the Adriatic Sea and the Aegean Sea show a 10 per cent frequency of gales in this season.

The extreme winds are most likely from September through March in the Mediterranean Sea. The maximum recorded wind was 65 knots, at Iskenderun in March. Izmir has recorded 61 knots in that month and has a summer record of 50 knots (June). Split has recorded a 58 knot NNE gale during the month of December. Very few stations have recorded winds greater than 30 knots during June, July, and August.

The diurnal alternation of land and sea breezes caused by the differential heating of land and sea is pronounced in the warm season and sometimes noticeable in the cool season. During daylight hours, the land warms up much more rapidly than the sea, causing air near the surface to rise. Air flowing in from seaward to replace this rising air forms the sea breeze. At night the reverse action takes place.

Regular sea breezes prevail from April to October, beginning at 0700 or 0800, reaching a maximum about 1300 or 1400, and continuing until about 1800 or 1900. In the spring and autumn, the sea breeze begins later in the morning; in the winter, when it occurs, its onset may be delayed until noon. The extent of land-sea breezes is about 10 to 20 miles from the coast. The sea breezes usually reach 11 to 16 knots, while the land breezes are weaker, averaging 5 to 9 knots.

Tides

The tide in the Mediterranean Sea is typically semidiurnal (two high and two low waters each tidal day, with little or no inequality between the heights of successive high or successive low waters) except for parts of the Adriatic Sea where the tide is mixed (two high and two low waters each day, with marked inequality between successive high and successive low waters) or where the tide at times becomes diurnal (one high and one low water each day). There is an amphidromic point (where the tidal range is zero) in the Strait of Messina, the Strait of Sicilia, and the Adriatic Sea. The mean and spring tide ranges are small and average less than 0.6m. In the Strait of Gibraltar, the range of the tide is about 0.9m. The range decreases farther E until, along the coast of Spain near Cartagena, in the Islas Balereas, and along the coast of Algeria from Tenes to Djidjelli, there is very little tidal range at all. Elsewhere the range is less than 0.3m., except in the Gulf of Gabes, where the range increases to nearly 1.8m at Gabes. The tidal range in the Mediterranean Sea and the Ionian Sea is only a few centimeters.

Meteorological conditions may influence water level heights more than astronomical forces. In the western Mediterranean Sea near Sicilia, sudden strong winds (marrobbio) may increase the water level about 0.9m above mean sea level; rapid changes of 0.3m may occur within a few minutes and cause problems for small boats in the area. In the central Mediterranean Sea, from February through April, the average sea surface height may be 0.5m below mean sea level. In the Adriatic Sea, a strong S or SE wind at Venezia will raise the water level 1.8m. Offshore winds frequently lower water levels along the E shore of the Adriatic Sea and along the leeward side of the islands in that region.

Credits:

Created with images by philippeschneider - "greece sea mediterranean sea" • juliacasado1 - "sea boats mediterranean"

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