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The Mistral and Other Med Winds

26 May 2009By Chris Tibbs

With summer rapidly approaching, the Mediterranean season is getting under way. The Med is considered to have a very pleasant climate – so much so that the term “Mediterranean-type climate” is used around the world to describe areas with dry, hot summers and mild winters; Southern California is one such place.

But the Med is not perfect, and strong winds can develop here quickly. Part of the reason is the geography of the area. It is an inland sea with a narrow entrance, largely surrounded by mountains close to the coast, and these have a telling effect on the weather. They not only deflect weather systems, but the daily heating and cooling of their rocky slopes also generates thermal winds that can be surprisingly strong. These winds can be extremely localised; so much so, that there are numerous names to describe them throughout the Med.

Captains entering the Med through the Straits of Gibraltar after a transatlantic crossing say the weather often has a last throw of the dice here, with strong to gale-force winds blowing through the narrow entrance. Almost always, the wind is either westerly or easterly, accelerated by the mountains of Spain to the north and Africa to the south. This local wind blows strongest through the Straits, but it continues downwind for a considerable distance. Whilst local forecasts will largely pick this up, Grib files are generally less successful at predicting it and some modification is necessary to the forecast.

Of all the winds in the Med, the most famous (or infamous, depending on how you look at it) is the Mistral of Southern France. Although strongest in winter, the Mistral can occur in any month and even in summer, it can become gale-force.

This well-known wind is most likely to develop when there is a northerly blowing across Northern France, usually from a depression crossing Northern Europe, with a cold front approaching the Alps. The mountains hold the wind back and funnel it down the Rhone Valley where it bursts out over the Gulf of Lyon. The Mistral will start abruptly and at times a gale can be blowing in the Gulf of Lyon whilst a few miles down the coast, calm conditions prevail. The wind blowing across the mountains also generates a lee depression in the Gulf of Genoa, strengthening and prolonging the mistral. Although the wind is at its worst along the south coast of France, it may extend as far as Menorca and Corsica, where a surprisingly large swell can persist.

The Mistral is generally well forecast. Meteo France and Monaco Radio should be regularly monitored, which can be done on VHF or via the Meteo France website.

Perhaps the second strongest Med wind is the Bora, which like the Mistral can arrive quickly. Local legend has it that the Bora has been known to derail a train. It is confined to the eastern side of the Adriatic along the former Yugoslavian coast. Many years ago, I spent three seasons here with little in the way of weather forecasts available. The local fishermen would look at the water levels before making a prediction on the chances of a Bora; low water caused by high pressure over the Balkans was a sign that one was possible. Late in the season, thunderstorms moving down the Adriatic from the northwest often preceded a Bora. These were particularly troublesome as a strong wind from the northwest was quickly followed by a swing to the north or northeast.

Farther east and through the summer (May to September), the Meltemi (or Etesian) is a wind of note that blows with great regularity through the Aegean. Caused by the development of a ridge of low pressure extending from the heat low of Asia, combined with lee troughing from the mountains of Turkey and higher pressure to the north, this wind can occasionally reach gale force during the late afternoons. The Meltemi will ease at night close to the mainland coasts.

The Mediterranean covers a large area with numerous wind systems. It is difficult to give a seasonal prediction as to whether this will be a good or bad year, although general expectations are for a warmer-than-average summer. This is likely to increase the strength of thermally driven wind, and may well increase the number of thunderstorms.

Chris Tibbs is a meteorologist who previously worked as a skipper in the yachting industry. He is the author of the Onboard Weather Handbook (McGraw-Hill).