The wind. We can’t see, but do feel and experience it. Among the atmospheric phenomena on our planet, the wind is perhaps the most dynamic and changeable. Still, some large scale patterns are observed regularly, such as trade winds and westerlies. But also on a smaller and more local scale, there are some recurring wind phenomena worthy of their own specific name, due to their distinct characteristics and effects. Today, as the first in a longer series, we take a look at one of the most (in)famous winds in the Mediterranean: the Mistral.
The Mistral is a north to north-westerly, generally cold and dry wind taking place in France, blowing from the Rhône Valley to the Mediterranean Sea, predominantly taking place during the winter and spring seasons. The wind, with windspeeds typically averaging between 30 and 40 kts (7-8 Bft) and up to >50kt (10-11 Bft), with gusts over 80kts and up to 100kts, has gained a strong reputation. That is also caused by its persistence: it is not uncommon that Mistral episodes take place for days continuously, driving the local people to madness and despair. The effect has been noticed already in Roman times: its name originates from the Latin adjective Magistralis, meaning ‘masterly’ or ‘belonging to the master’, being indicative for the force of this wind phenomenon. Due to local dialects, its name has been adopted to Mistral, which is currently the name for this wind phenomenon.
The Mistral develops typically in a specific synoptic situation. For the development of Mistral winds, high pressure is required over the Bay of Biscay or Atlantic, and low pressure over the Mediterranean, typically over the Gulf of Genoa (Figure 1). This results in a strong pressure gradient over eastern France, and consequently in a strong northerly flow over central and southern France. Cold and dry polar air originating from northerly regions is then transported southward over France and into the Mediterranean in general — and the Gulf of Lion specifically. The flow is enhanced by means of a funneling effect due to the topography of the area: the wind is confined by the Alps and the Central Massif, and pressed through the N-S oriented Rhône Valley, where the wind gets accelerated and its top speeds are reached eventually.
Next to the general Mistral, caused by the pressure gradient and this funneling effect, more local effects may take place, further enhancing the wind speed. As the Rhône Valley is confined by the Central massif to the west and the Alps to the East, many smaller scale valleys in these mountain ranges are nearby. Local downbursts might occur due to the topography in the area, posing a further, more local threat to the area. This latter phenomenon is generally called Mistral Local, to distinguish it from the larger scale Mistral, the Mistral Général.
Figure 1: Animation of the synoptic setting required for Mistral winds to develop. Figure made by Weerplaza, one of Infoplaza’s brands.
Although fierce, the Mistral is generally associated with fairly sunny and dry weather, as cold and dry air is being transported to the area. The Mistral winds, in combination with the topography of the area, are able to form characteristic lenticular clouds.
The population in southern France, especially in the Provence region, is well accustomed to the wind effects, which is proved by the typically southward facing orientation of trees and also buildings such as farmhouses, and the open frameworks of church bell towers, to avoid wind damage to these constructions. They even make profit from the wind effect, as for the growth of grapes for wine production, the wind is even beneficial. With its drying nature, the Mistral is able to prevent diseases and hence promote growth.
But despite being known for its effects onshore in southeastern France, the Mistral leaves its mark at the Mediterranean Sea. The coast in the Gulf of Lion is particularly vulnerable, as wave heights may rise rapidly under Mistral conditions, being unsuitable for many smaller and sometimes larger ships and ferries. Wave heights corresponding to storm conditions are not uncommon, reaching several meters high. During some episodes, the Mistral can also affect larger parts of the western Mediterranean. The Balearic Islands and the west and south coast of Corse and Sardegna are not unfamiliar with the Mistral. In the Strait of Bonifacio, located between these islands, a renewed funneling effect may take place, intensifying the Mistral once more. In some occasions, Sicily, Malta and even northern Africa experience the effect as well. The usually relatively calm Mediterranean Sea can turn into a rough and stormy sea in such circumstances, being a threat for sailors and offshore workers in large parts of the Mediterranean.
Overall, the Mistral is an infamous wind in southern France and the western Mediterranean, generally taking place in a distinct synoptic setting. It's usually present in the spring and summer seasons, reaching values of over 50 kts. This has gained the wind its reputation, making it one of the better known winds in Europe. In addition to its effects on land, sailors in the Gulf of Lion and the western Mediterranean should take the Mistral into account, so they aren’t caught off guard by its strength and force.
In a few weeks time, this series will be continued with another famous wind phenomenon in the Mediterranean; the Sirocco. For more information on current weather and in depth-articles, for instance over the mentioned funneling effect, take a look at our blogs on this website.