The mistral is a strong and cold wind from the northwest that blows from southern France into the Gulf of Lion, particularly in winter and spring – and especially in the transfusion period from winter to sprint. It accelerates at it passes trough the valleys of the rivers Rhône and Durance, and reaches the Mediterranean coast at the Camargue area.
The mistral will usually last for a few days, but sometimes it is over within a day or two, and there are also examples of mistrals that lasted for over a week. The mistral is typically accompanied by clear, fresh weather and it plays an important role in the climate of the plain of Languedoc and Provence. It also influences the weather along the French Mediterranean coast, and is known to cause sudden storms between Corsica and the Balearic Islands (Mallorca, Menorca, Ibiza, etc).
The mistral is one of the reasons why Provence has such as sunny climate, with around 2700 to 2900 hours of sunshine per year. The powerful mistral can very quickly clear the sky from clouds and it not uncommon for a completely overcast sky to be completely clear in less han two hours. The mistral doesn’t only remove clouds; it also clears the air of suspended dust and pollution, creating conditions where the visibility often exceeds 150 km. It is not difficult to understand why so many painters, including impressionist and post-impressionist ones, have been attracted to these parts of France.
Sustained winds exceeding 65 km/h are common, with wind speed occasionally reaching up towards 185 km/h. Wind speeds over 90 km/h are especially common in the Rhône Valley.
The mistral will typically have higher windspeeds during the day and slow down considerable during the night.
The mistral has always been a part of life in this region and archeological findings at the prehistoric site Terra Amata in Nice have shown that people living here around 40,000 B.C. built a low wall of rocks andbeach stones on the northwestern side of their fireplaces – most likely to protect them from the mistral. And even before humans arrived, the wilderness bowed to the mistral. If you visit places such as the plain of La Crau, you can see that trees here have a tendency to lean south – away from the mistral.
Traditional Provencal farmhouses are built facing south, with their back to the mistral, and if you look at the church bell towers in Provencal villages you can see that they tend to consist of very open iron frameworks that will allow the wind to simply pass through instead of wrecking havoc with the tower. Even the traditional Provencal Natitivy scene pays homage to the mistral by including the figure of a shepherd holding his hat and with his cloak blowing in the mistral.
The mistral is associated with an elevated risk of wildfires. When the tree and plant life is already dry from a lack of water, the drying effect of the mistral can be the straw that breaks the camels back. Also, the strong mistral will help spread a wildfire once it has started.
To help protect their crops from the drying effect of the mistral, farmers in this region have traditionally planted rows of cypress trees next to their fields.
How the mistral is formed
The mistral blows when there is an area of high pressure in the Bay of Biscay and an area of low pressure around the Gulf of Genoa. The air is sucked from the high pressure area to the low pressure area, causing cold air from the north to be blown down to the Mediterranean. As the wind moves through the lower elevations between the foothills of the Alps and the Cevennes mountains in south-central France, it picks up speed.
The likelihood of a significant mistral forming is especially high when a cold rainy front has crossed France from the northwest and reached the Mediterranean in the southeast.
As mentioned above, the mistral tends to increase in wind speed as it blows through low-laying areas. For a long time, it was believed that the long and enclosed shape of the Rhine Valley was the reason for the mistrals high speed and forcefulness, but new research indicate that the answer is more complex than that. The mistral reaches its highest speed south of Valence, where the Rhône Valley has already begun to widen again. Also, the mistral isn’t limited to the valley; it blows high above the Earth’s surface in the troposphere too. At Mont Ventoux, very strong winds have been measured, even though the plain below isn’t narrow at all.
The mistral usually blows from the north or northwest, but during parts of its journey mountains can channel it, causing it to locally change direction. There are for instance pre-alpine valleys and some places along the Côte d’Azur where it blows from east to west.
Mistral from the west
The mistral blowing from the west will usually bring sunny weather and warm temperatures to the Mediterranean coast and a high risk of rain for the interior. The mass of air is not very cold, and the wind chiefly affects the plains of the Rhône delta and the Côte d’Azur. It is unusual for this mistral to blow for more than three days.
Mistral from the northeast
The mistral blowing from the northeast is especially strongly felt in western Provence. In the winter, this mistral is known to bring cold temperatures. It is a long-lived mistral that sometimes blows for over a week.
Along the Côte d’Azur and eastern Provence, the northeastern mistral can bring rain or heavy snow to low-altitude areas, particularly if there is a low pressure area in the Gulf of Genoa.
The mistral is typically associated with clear skies and bright sunny days, but if the Azores High is extended and draws in particularly moist air from the northwest we can get a mistral noir instead. A mistral noir (a dark mistral) brings with it clouds and rain.
An épisode cévenol is a succession of torrential rains and floods named after the Cévennes mountain range in south-central France. These episodes typically affect the departments Ardéche, Gard, Hérault, and Lozére. They occur when a low pressure front over the Mediterranean approaches the coast from the southeast, making the mistral and its clear sky change rapidly to an east wind and a sky filled with rain clouds. If the low pressure front keeps moving towards the land, the Mediterranean basin can experience several days of rainy weather.
The summer mistral in the valley of the Rhône and coast of Provence
During the summer, the valley of the Rhône and coast of Provence is sometimes affected by what is locally known as the summer mistral, although this mistral has little to do with the actual mistral described above. The summer mistral doesn’t come blowing in from far away; it is created by purely local conditions. The summer mistral is the result of the land becoming extra heated due to a thermal depression over the Var and Alpes de Haute-Provence in the interior of Provence. The heat creates a flow of air from the north in direction of eastern Provence. As it reaches the coast, it is rarely strong enough to persist when faced with the sea breeze.
The summer mistral, which is especially likely to occur in the month of July, will normally not blow for more than a single day. Still, it is feared by the locals, since it can help spread forest fires and thereby cause a lot of damage in a short time-span.