The Mistral is a strong and cold North Westerly wind that blows into the Mediterranean from France. In certain French valleys and along the Côte d'Azur, the wind is channelled by the mountains so that it blows from East to West.
The mistral is caused by the conjunction of an area of high pressure in the Bay of Biscay and an area of low pressure around the Gulf of Genoa. The flow of air generated between the two systems draws in a current of cold air from the north which then accelerates through the foothills of the Alps and the Cevennes.
The conditions for a mistral are particularly favorable when a wet cold front has crossed France from the northwest to the southeast and reached the Mediterranean. This creates a cold, dry wind and a period of cloudless skies and bright sunshine, which gives the mistral its reputation for making the sky especially clear. However, there is also 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.
A Mistral usually forms overnight. Forecasts are notoriously short notice but well broadcast on local radio. Look out for telltale signs in the northwest as wispy high-altitude cirrus clouds may appear. Typically, lens-shaped alto-cumulus clouds appear the evening before it arrives and golden clouds turning to pink and then grey with a red sunset are likely. It is said that the faster the clouds turn grey, the harder the wind will blow.
After sunset, short gusts of wind appear with long intervals in between. These intervals reduce and as the wind sets in the surface current starts to flow towards the east. Sea state increases into an uncomfortable, steep, short sea which can be dangerous, especially to small vessels. Full development usually takes effect between midnight and sunrise.
The Mistral is most prevalent in Winter or Spring and may blow for just a day or two or for as long as a week. In winter it can average anything up to a Force 6 or 7 during the day, easing overnight although it is known to cause storms, especially in the Mediterranean between Corsica and the Balearic Islands. In Summer Mistrals are less common but more violent due to the increased sea temperature, making passage planning and suitable ports of refuge an important consideration for the cruising sailor.