Brittany, where it lies

The climate of Brittany, a region of north-western France, is oceanic, with relatively cold winters and cool summers, and with frequent cloud cover and wind throughout the year. However, the weather is more pleasant in summer.
Here are the average temperatures of Saint-Malo, located in the northern part of the coast.
Saint-Malo - Average temperatures
Min (°C)33469111313121064
Max (°C)891113172022222016129
Min (°F)373739434852555554504339
Max (°F)464852556368727268615448

Precipitation is abundant on the west coast (see Brest), where it exceeds 1,100 millimeters (43 inches) per year, while it drops to 950 mm on the south coast (see Lorient), below 800 mm (31.5 in) along the north coast (see Saint-Malo), and to about 700 mm (27.5 in) in the eastern inland area (see Rennes), although the rains are frequent throughout the year everywhere. The least rainy season is summer, when it rains for about 10/12 days per month.
Here is the average precipitation in Saint-Malo.
Saint-Malo - Average precipitation

Winter, from December to February, is quite cold: the temperatures are usually above freezing (0 °C or 32 °F), but wind and humidity increase the feeling of cold. In addition, very cold periods may occur, with temperatures below freezing, when France is affected by cold air masses from Russia. In these cases, there can be snowfalls, generally not abundant, and frosts; the coldest records are around -10 °C (14 °F). On the contrary, when the wind from the ocean prevails, on milder days, the temperature can reach 12/14 °C even in winter, although the wind can be strong.

Spring, from March to May, is very cool, if not cold, especially in the first part. In March, it can still snow; in April, and sometimes in the first half of May, there can still be cold days. The rains are frequent, but less than in winter. In addition, the days are longer, and you can even see the sun, maybe between one shower and another, or with clouds running across the sky.

Summer, from June to August, is generally cool or mild; some days can be pleasantly warm, but cooled by lively afternoon breezes. The coolest part is the west coast: in Brest, highs in July and August are around 20 °C (68 °F); on the north and south coasts, in cities like Saint-Malo and Lorient, they are around 21/22 °C (70/72 °F), while in Rennes, situated at a certain distance from the sea, they even reach 23/24 °C (73/75 °F). However, the temperature varies according to weather situation: when Atlantic disturbances reach the region (which may occur even in summer), the weather is cool and cloudy, with some chance of rain, but when the Azores anticyclone settles in the region, usually for a few days, there can be fine and sunny days. Sometimes, it can even be hot: occasionally, the temperature can reach 30 °C (86 °F) on the coast, and up to 35 °C (95 °F) in Rennes.

Autumn, from September to November, is still mild and pleasant in September, but later, it gradually becomes more cold, dull and rainy, as well as windy.

Saint Malo

The amount of sunshine in Brittany is low in late autumn and winter, when the sky is often cloudy, while it becomes quite acceptable from April to September. Of course, we are still in the least sunny region of France (along with Normandy and the north-east), but there can be some sunny days, or more easily, days with variable weather. Here are the average sunshine hours per day.
Saint-Malo - Sunshine

The temperature of the sea in Brittany is very cool even in summer, and may be too cold for swimming, at least for tourists coming from southern Europe or for those who suffer from the cold: it reaches 17 °C (63 °F) in August in Brest, and 18 °C (64 °F) in Saint Malo. Here are the average sea temperatures near Saint-Malo.
Saint-Malo - Sea temperature
Temp (°C)1099101214171817151312
Temp (°F)504848505457636463595554

Best Time

The best time to visit Brittany is summer, from June to August, since it is the warmest of the year, although even in this season it's better to bring a sweatshirt, a light jacket and an umbrella. June is a bit cooler, so you may prefer July and August. September is a good month as well: it's a bit rainier than the summer months, but the temperatures are still pleasant, especially in the first part.
In Saint-Malo, frequented mainly by tourists from central and northern Europe, as previously mentioned, the sea is very cool even in summer, however, on sunny days, when the breeze is not too cool, you can sunbathe.