Map from Google - Cantabria

In the autonomous community of Cantabria, corresponding to the province of Santander, and located in northern Spain, the climate is cool and damp on the coast, due to the influence of the Atlantic Ocean, while in inland areas it becomes colder with increasing altitude, and more continental in the south. On the coast (called "la Marina") winters are mild and rainy, and summers are cool and fairly sunny. Atlantic frontal systems can pass all year round, although they grant a relative lull in summer, when, hoewever, thunderstorms can break out in the afternoon or in the evening.
The wind blows frequently, in fact there are several beaches frequented by surfers.
Also fog is common: it can present itself all year round, when there is no wind, and even on summer nights, especially along the coast and in the valleys.
In winter, cold air masses from the northeast can bring cold and windy periods, but on the coast very cold days are rare (the cold records are a few degrees below freezing). However, even on mild days the wind can increase the feeling of cold. Snow, very rare on the coast, becomes a bit more frequent in inland hilly areas, while in the mountainous peaks, heavy snowfalls may occur. Even frosts are rare on the coast, and in any case they are light, while they become more frequent, and at times can be intense, in the interior.
In summer, temperatures are cool along the coast and in inland mountainous areas; to the south, although there are no plains, hot days become more frequent even at 600/700 metres (2,000/2,300 feet), with highs around 30 °C (86 °F) or more, since this zone is close to the meseta, the spanish arid plateau. In summer, when high pressure systems are more common, there are many sunny days, during which, however, there may be morning mists, and thunderstorms can break out in the evening; there's still no shortage of gray days, when a kind of drizzle fall, called morrina, especially in late spring and early summer.
The rains are quite abundant in much of the territory, given that they hover around 1,000/1,200 millimetres (40 to 47 inches) per year on the coast and in the hills of the north-central, so that the landscape is green; in mountainous areas they can exceed 2,000 mm (79 in), while in the southern part (comarca of Campoo-Los Valles, see Reinosa, Valderredible) and in some sheltered valleys (Valley of Liébana, see Potes), precipitation drops to around 700 mm (27.5 in). Summer is everywhere the least rainy season.
In the interior we find hills, and in the southern part a mountain range, the Cantabrian Mountains, which separates this cool and moist part of Spain from the arid plateau; in particular, in the western part we find the "Peaks of Europe" (Picos de Europa), which Cantabria shares with the region of Asturias.
The wind that comes down from the mountains may cause rapid increases in temperature, due to the föhn effect, so that along the coast it may exceed 20 °C (68 °F) in winter, 30 °C (86 °F) in autumn and spring, and 35 °C (95 °F) in summer, when however this wind is rare.

Potes, and Picos de Europa in the background

On the coast there are bays and inlets, like the Bay of Santander, or the lagoons of Parque Natural de las Marismas de Santoña, Victoria y Joyel.
At about ten kilometres (6 miles) from the coast we find the Sequoyas del Monte Cabezón National Park, where a forest of Sequoia sempervirens grows, while a few kilometres (or miles) from the coast we find the Cave of Altamira, famous for its cave paintings of the Paleolithic, and UNESCO World Heritage Site.
In Cantabria the river Ebro has its source, in Pico Tres Mares, a 2,173 metre high mountain; in the outskirts of Reinosa the river widens into an artificial reservoir called embalse del Ebro, at 800 metres (2,600 feet) above sea level.

In the capital, Santander, located on the coast, the average temperature of January and February is 10.5 °C (51 °F), that of August is 20.5 °C (69 °F). The daily temperature variation is generally reduced, due to the influence of the ocean. In summer, a season that often has the taste of spring, the temperature very rarely reaches or exceeds 30 °C (86 °F) (it may not happen for a whole summer month). The lowest record temperatures are around -5 °C (23 °F), the highest around 37/38 °C (99/100 °F).
Here are the average temperatures.
Average temperatures - Santander
SantanderJanFebMarAprMayJunJulAugSepOctNovDec
SantanderJFMAMJJASOND
Min (°C)88910121517181614119
Max (°C)131315161820232322201614
Min (°F)464648505459636461575248
Max (°F)555559616468737372686157

Santander gets an average of 1.115 mm (44 in) of rain per year, with a maximum in autumn and winter, and a minimum in summer, when, however, it still rains 8/9 days per month.
Here is the average precipitation.
Average precipitation - Santander
SantanderJanFebMarAprMayJunJulAugSepOctNovDecYear
SantanderJFMAMJJASONDY
Prec. (mm)110858510075605070901151551251115
Prec. (in)4.33.33.33.932.422.83.54.56.14.943.9
Days131110121198910121013127

The amount of sunshine in Cantabria is never very good: in July and August there are 6 hours of sunshine a day along the coast and in Santander; in the other seasons, and especially in late autumn and winter, it is even worse, and the sky is often cloudy.

Castro Urdiales, beach

The sea along the coast of Cantabria is very cool even in summer, since it reaches 20 degrees (68 °F) in July and 21 °C (70 °F) in August, as you can see in the following table, on the temperature of the sea at Santander.
Sea temperature - Santander
SantanderJanFebMarAprMayJunJulAugSepOctNovDec
SantanderJFMAMJJASOND
Sea (°C)131213131517202120181514
Sea (°F)555455555963687068645957

When to go

The best time to visit Cantabria is the summer, from June to August or the first half of September, being the driest and sunniest. July and August are the warmest and least rainy months. Even in midsummer we must take into account some rain, and some nights a little cool on the coast, and a bit cold in inland areas.
In summer, in good weather you can take a break at the sea and sunbathe, with an air temperature often pleasant and a cool breeze from the ocean, although as I said the sea is a bit cold for swimming.