WINDS AND WEATHER
Biscay is frequently under the influence of either the Azores high or lows passing along the English Channel. This causes the prevailing winds in summer to be west in the northern part and northwest in the southern part. However, other pressure systems are common and changeable Atlantic weather is the norm.
In addition to winds driven by large-scale weather systems, local sea breeze effects are very important. Atlantic France is well supplied with all the features necessary to generate a big sea breeze during the day. As a rule of thumb, a clear sky and a line of fluffy clouds along the line of the coast indicates that a sea breeze is developing and the afternoon wind will blow freshly onto the shore from the southwest gradually veering to the west. Quiberon Bay and Pertuis d'Antioch (between Île de Ré and Île d'Oleron) are both famous for the strength and complexity of their sea breezes.
There is a reverse phenomenon that is particularly important when anchoring for the night. A land breeze, known by the French as a brise de terre, can develop at about midnight starting gently and increasing to a fresh breeze from the northeast. This breeze can be particularly strong if it blows down cliffs or along rivers. Île de Groix is a good example. A night breeze blowing down the valleys and rivers in Lorient will blow directly from the northeast into the harbour at Port Tudy with very uncomfortable results.
Sunshine and rain
The weather in the southern part of the area is quite a bit better than the north. On average, there is less rain, about two hours a day more sunshine, temperatures are about 5°C higher and humidity is 10% lower. However, when fronts cross southern Brittany or further south they can result in a few days of unsettled weather. One of the attractions of North Biscay as a cruising area is that there is always a protected inland waterway close at hand. The Rade de Brest, Lorient, the Morbihan, the Vilaine, the Charente and the Gironde are all protected and each has interesting towns so it is possible to get away from any unpleasant sea conditions. Further South however between the Gironde and the Basque ports there is no port of refuge and a reliable long-range forecast is needed before embarking on the passage.
Fog, mist or haze can be frequent in the summer. On average, visibility is less than five miles on one day in five. Real fog, with visibility of less than 0.5M, averages one day in twenty. The coast is so well marked by beacons and towers that navigation in poor visibility is possible, particularly with the help of GPS and radar. However, fog can be particularly unpleasant in narrow tidal waters and rivers.
Swell is generated by storms and winds of Force 6 and above. With persistent winds of Force 8 or more, large waves are created that can take a few days to die down and will radiate out into areas that were never affected by the strong winds.
A large swell will break heavily on bars and in shallow water and can make some entrances, such as Belon, Etel and the Vilaine, dangerous even in fine weather. If swell enters a narrowing inlet, it tends to increase in height and steepness and funnel up the entrance to make anchorages uncomfortable or even untenable. In open water it can break intermittently and dangerously on rocks that rise from deep water, even if the depth over them is apparently safe.
In Biscay swell occurs mainly on the northwest and west coasts of Brittany, particularly in the vicinity of Ouessant and northeast of Le Four. It is less frequent in the Bay of Biscay. However, any anchorages that are open to the Atlantic, such as those on the west and south side of Île de Groix, Belle-Île and Île d'Yeu should only be used in settled weather. Before using such an anchorage, it is worth checking that there have been no recent disturbances in the Atlantic that could generate a sudden swell. French weather forecasts include predictions of the height of the swell (la houle).
Biscay is very well served for weather forecasts. The many available sources are summarised in an excellent, 30 page booklet called Le Guide Marine from Météo France. This is available free in every port office. It also contains an invaluable lexicon of meteorological terms in French and English.
Navtex forecasts are available from CROSS Corsen on 518kHz (A) and 490kHZ (E). The latter provides more local detail but in French. In the northern part of the area, less detailed forecasts can also be received from Niton on 518kHz (E) and in the southern part from La Coruña on 518KHz (D).
CROSS Corsen and CROSS Etel transmit area forecasts on VHF channel 63 several times a day. These are detailed and generally accurate. They are in French but, with the aid of the Météo France lexicon mentioned above, even non-French speakers should be able to understand them.