The other day I opened a book on Mediterranean weather and it started with the words ‘light and variable with possible gales later, returning light and variable’, a forecast that covers just about everything and one that is often the experience of sailors in the Mediterranean. Even through the summer months, idyllic conditions can turn rapidly nasty and in the winter some areas have such strong winds that they have even been known to de-rail trains.
However, for many of us the Mediterranean gives some great cruising in balmy conditions with light winds that fade away at night leaving us in quiet, unspoilt anchorages (if only the taverna would stop the music). This has led to an array of flotillas and charter operations where most holidays go off without a hitch, but in some areas a rapid increase in wind can lead to an exciting sail and days lost to the weather. In the UK our weather mainly comes from the west across the Atlantic, associated with depressions, and although there are some fairly localised conditions, the whole country is generally under the influence of one system. The systems are large, be they depressions or anticyclones. In contrast, within the Mediterranean area, there are often much smaller scale features. Small lows can often develop and quickly move off. If we look at Mediterranean weather charts there are often lows somewhere and they may have associated fronts, but will sometimes be without. On the charts, the difference between the low and high pressure can be small and it is the pressure relative to the general area that is important.
The causes of the lows are also important. The ones we get in the UK are generally formed along the polar front (where warm moist air from the south meets cold dry air from the north, way out in the Atlantic), but wherever we get strong heating of a land mass, moisture and air rises and the surface pressure falls. This is a feature often seen during the summer months over Spain. Referred to as the Iberian low, it can stay for days or weeks at a time and gives light winds on the Spanish coast in the Mediterranean with daily sea breezes and calms at night, but it also helps drive the Portuguese trade winds that will regularly reach Force 7 or more along the coast of Portugal. It is not just Spain though that generates semi permanent low pressure. The eastern Mediterranean is also affected by low pressure over Turkey and further east over Asia. This is the Meltemi (Turkish) or Etesian (Greek) wind that blows through the Aegean as a strong northerly wind through the summer months often reaching Force 7 or 8 depending where in the Aegean you are. It makes for fantastic pictures, but the sailing can be tough and yachts are best to avoid it if possible.
If we look at a map of the Mediterranean Sea it is largely surrounded by mountains and it is these mountains along with the intense heating and cooling throughout the day’s cycle, that leads to the rapid changes that can be experienced and the local nature of the winds. As these winds over the Mediterranean are very localised there are over 40 different local names for winds. Some are just different names for the same wind, depending on the language of the country, but there are numerous different winds.
Such localised winds occur because mountains affect the synoptic wind – the wind created by the pressure gradient. The wind would blow directly from high to low pressure, but because of the spinning earth, it blows anti-clockwise around low pressure and clockwise around high pressure. There is also a slight outflow from high pressure and inflow to low pressure, so the wind crosses the isobars by around 15°.
The mountains, however, affect this wind. In a mechanical way, they block the wind and tend to funnel it down valleys and through gaps. The best example of a gap wind is through the Straits of Gibraltar. The wind is said to be either easterly, Levanter, or westerly, Vendaval, for 98 per cent of the time. This wind is then strongly accelerated and can be blowing gale force through the Straits whilst just a few miles away there is only a F4. Strong currents along with a large amount of shipping makes the straits a difficult place to sail and often holds up yachts entering or leaving the Med. Mountains can also create katabatic or downslope winds. As the mountains cool, the air over them becomes colder and denser, then moves downhill under the force of gravity until it bursts down a valley and out over the sea. This can be anything from a light early morning breeze that is refreshingly cool to a strong wind upsetting our anchorage. Because these winds are local and fairly regular, they are not always forecast in general forecasts, but are known locally and a good pilot book will give warning of any dangers for local anchorages.
MOUNTAINS AND THE MISTRAL
High altitude mountains will also create an area of low pressure in their lee, so anywhere downwind of mountains will experience a lowering of pressure known as a lee trough. In some cases this will develop into a full depression, as is sometimes the case in the Gulf of Genoa. It is therefore clear that mountains do affect the wind greatly.
One of the best known of all the Mediterranean winds is the Mistral. This tends to have all of the above mountain effects rolled into one. It is caused by low pressure passing to the north and a cold front passing across France. During the summer, the rain and showers associated with the cold front may not reach all the way south to the Mediterranean coast, but there will be a change to a northwesterly air flow. Initially, the wind is blocked by the mountains until it finally bursts through the gap and out into the Gulf of Lyon with often dramatic effects. In Marseilles a mistral can be blowing over 100 days a year – not always strong, but still a Force 6 or higher. The charts of average wind speed clearly show the path of the Mistral.
While it is blowing in the Gulf of Lyon, a strong Mistral will extend southwards to Minorca in the west and as far south as Sardinia or even Tunisia. How far east it reaches is a bit hit and miss. I have been becalmed near Cannes yet there has been 30kn 20 miles away in the bay of St Tropez. This is linked to how developed the lee depression becomes in the Gulf of Genoa. In the winter the Mistral is stronger and more widespread, as the diagrams of monthly averages show. Usually the wind blows out of a clear blue sky with perhaps a few lenticular clouds; these are lens shaped clouds formed by the wind coming over the mountains. From clear skies, the Mistral is known as a white Mistral and is the more common type. When a series of fronts crosses France there are bands of rain within the Mistral and it is known as a black Mistral. They have a reputation for arriving with little warning.
The Bora in the Adriatic is similar to a mistral coming off mountains often with a clear sky. By late summer the Bora may be preceded by a line of thunderstorms giving strong squally wind before it veers to the north and blows even stronger. It is a winter Bora that was reported to have de-railed a train inland. But the winds are not confined to mountains; the Sirocco is a southerly wind known by various names. Being a desert wind it can coat the boat with a fine dust. Although sailing in the Med may give ideal conditions some of the time, it also gives very strong local winds and weather forecasts should not be taken lightly. Along most coasts there are forecasts transmitted via VHF, often in English.