Mistral (wind) - The Cause of The Mistral

The Cause of The Mistral

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 northeast.

The long and enclosed shape of the Rhone Valley, and the Venturi effect of funnelling the air through a narrowing space, is frequently cited as the reason 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 Rhone 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, three kilometers above the earth. The mistral is very strong at the summit of Mont Ventoux, 1900 meters 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.

In France, the mistral particularly affects Provence, Languedoc east of Montpellier, as well as all of the Rhone Valley from Lyon to Marseille, and as far southeast as Corsica and Sardinia. The mistral usually blows from the north or northwest, but in certain pre-alpine valleys and along the Côte d'Azur, the wind is channelled by the mountains so that it blows from east to west. Sometimes it also blows from the north-north-east toward the east of Languedoc as far as Cap Béar. Frequently, the mistral will affect only one part of the region.

In the Languedoc area, where the tramontane is the strongest wind, the mistral and the tramontane blow together onto the Gulf of Lion and the northwest of the western Mediterranean, and can be felt to the east of the Balearic Islands, in Sardinia, and sometimes as far as the coast of Africa.

When the mistral blows from the west, the mass of air is not so cold and the wind only affects the plain of the Rhone delta and the Côte d'Azur. The good weather is confined to the coast of the Mediterranean, while it can rain in the interior. The Côte d'Azur generally has a clear sky and warmer temperatures. This type of mistral usually blows for no more than one to three days.

The mistral originating from the northeast has a very different character; it is felt only in the west of Provence and as far as Montpellier, with the wind coming from either a northerly or north-northeasterly direction. In the winter this is by far the coldest form of the mistral. The wind can blow for more than a week. This kind of mistral is often connected with a low pressure area in the Gulf of Genoa, and it can bring unstable weather to the Côte d'Azur and the east of Provence, sometimes bringing heavy snow to low altitudes in winter.

When the flow of air comes from the northeast due to a widespread low pressure area over the Atlantic and atmospheric disturbances over France, the air is even colder at both high altitudes and ground level, and the mistral is even stronger, and the weather worse, with the creation of cumulus clouds bringing weak storms. This kind of mistral is weaker in the east of Provence and the Côte d'Azur.

The mistral is not always synonymous with clear skies. When a low pressure front over the Mediterranean approaches the coast from the southeast, the weather can change quickly for the worse, and the mistral and its clear sky changes rapidly to an east wind bringing humid air and threatening clouds. The position of the low-pressure front creates a flow of air from the northwest or the northeast, channeled through the Rhone Valley. If this low-pressure area moves back toward the southeast, the mistral will quickly clear the air and the good weather will return; but if the cold-weather front continues to approach the land, bad weather will continue for several days in the entire Mediterranean basin, sometimes transforming into what French meteorologists call an épisode cévenol, a succession of torrential rains and floods, particularly in the areas west of the Rhone Valley: the Ardèche, the Gard, Hérault and Lozère.

The summer mistral, unlike the others, is created by purely local conditions. It usually happens in July, and only in the valley of the Rhone and on the coast of Provence. It is caused by a thermal depression over the interior of Provence (The Var and Alpes de Haute-Provence), created when the land is overheated. This creates a flow of air from the north toward the east of Provence. This wind is frequently cancelled out close to the coast by the breezes from the sea. It does not blow for more than a single day, but it is feared in Provence, because it dries the vegetation and it can spread forest fires.