The mistral (French: [mistʁal]; Catalan: mestral; Corsican: maestrale; Croatian: maestral; Greek: μαΐστρος; Italian: maestrale; Maltese: majjistral) is a strong, cold, northwesterly wind that blows from southern France into the Gulf of Lion in the northern Mediterranean. It produces sustained winds averaging 50 km/h (30 mph), sometimes reaching 100 km/h (60 mph). It can last for several days. Periods of the wind exceeding 30 km/h (19 mph; 8.3 m/s; 16 kn) for more than sixty-five hours have been reported. It is most common in the winter and spring, and strongest in the transition between the two seasons. It affects the northeast of the plain of Languedoc and Provence to the east of Toulon, where it is felt as a strong west wind. It has a major influence all along the Mediterranean coast of France, and often causes sudden storms in the Mediterranean between Corsica and the Balearic Islands. The name mistral comes from the Languedoc dialect of the Occitan and means 'masterly'. The same wind is called mistrau in the Provençal variant of Occitan, mestral in Catalan, maestrale in Italian and Corsican, maistràle or bentu maestru in Sardinian, and majjistral in Maltese. The mistral is usually accompanied by clear, fresh weather, and it plays an important role in creating the climate of Provence. It can reach speeds of more than 90 km/h (56 mph; 25 m/s; 49 kn), particularly in the Rhône Valley. Its average speed during the day can reach about 50 km/h (31 mph; 14 m/s; 27 kn), calming noticeably at night. The mistral usually blows in winter or spring, though it occurs in all seasons. It sometimes lasts only one or two days, frequently lasts several days, and sometimes lasts more than a week.
Cause
The mistral takes place each time there is an anticyclone, or area of high pressure, in the Bay of Biscay, and an area of low pressure around the Gulf of Genoa. When this happens, the flow of air between the high and low pressure areas draws in a current of cold air from the north which accelerates through the lower elevations between the foothills of the Alps and the Cevennes. The conditions for a mistral are even more favorable when a cold rainy front has crossed France from the northwest to the southeast as far as the Mediterranean. This cold, dry wind usually causes a period of cloudless skies and luminous sunshine, which gives the mistral its reputation for making the sky especially clear. There is also, however, the mistral noir, which brings clouds and rain. The mistral noir occurs when the Azores High is extended and draws in unusually moist air from the northwest. The long and enclosed shape of the Rhône Valley, and the Venturi effect of funnelling the air through a narrowing space, are frequently cited as the reasons for the speed and force of the mistral, but the reasons are apparently more complex. The mistral reaches its maximum speed not at the narrowest part of the Rhône Valley, south of Valence, but much farther south, where the valley has widened. Also, the wind occurs not just in the valley, but high above in the atmosphere, up to the troposphere, 3 km (1.9 mi) above the earth. The mistral is very strong at the summit of Mont Ventoux, 1900 meters (6300') in elevation, though the plain below is very wide. Other contributing factors to the strength of the mistral are the accumulation of masses of cold air, whose volume is greater, pouring down the mountains and valleys to the lower elevations. This is similar to a foehn wind, but unlike a foehn wind the descent in altitude does not significantly warm the mistral. The causes and characteristics of the mistral are very similar to those of the Tramontane, another wind of the French Mediterranean region.
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