Map from Google - Galicia

In the autonomous community of Galicia, in north-western Spain, the climate is different from what is normally expected in Spain, and is mild oceanic, with relatively mild and rainy winters, and cool and quite sunny summers. In summer, short heat waves are possible, with highs around or above 30 degrees. In July 2013, the temperature reached 36 degrees in La Coruña, 38 °C in Pontevedra and 39.7 °C in Vigo.
Wind blows frequently, and can be stormy in the cold season, so there are several beaches frequented by surfers.
Even fog is common: it can present itsfelf all year round, when there is no wind, even during summer nights.
Snowfalls are very rare on the coast (where for example it snowed on the 10th of January 2010), they are very rare (once or twice a year) in low-lying inland areas, while they become abundant in the mountains during winter.
The rains are usually abundant, since they exceed almost everywhere 1,000 millimetres per year, and are close to 2,000 mm along the west coast and on west facing slopes. However, summer is the driest season, and the rains, though possible also in this season, become quite rare.
In the interior there are mountain ranges, where the climate gets colder with increasing altitude, and in winter you can experience heavy snowfalls. There are also skilifts. The highest peak is Peña Trevinca (2,124 metres), in Serra do Eixe, while the altitude exceeds 1,700 metres in Serra de Queixa and Sierra de Ancares.
The coast is indented, and there are coves (called rías) as well as islands. The most important islands are Illa de Arousa (inside the rías of the same name and in the province of Pontevedra), the Cies Islands (off the coast of Vigo, where the famous Praia das Rodas is located), and the Ons Island (just to the north, and west of Pontevedra).
The most western point of Galicia and Spain is Cape Touriñán, just north of Cape Finisterre, which is traditionally (and wrongly) considered as the most westerly. This westernmost part of the coast is called Costa da Morte (or Costa de la Muerte in Castilian), because of the frequent shipwrecks that occurred in the course of history, of the dangerousness of the coast as well as the frequent storms.
About fifty kilometres south-west of Santiago de Compostela we find a large sandy beach, called Dunas de Corrubedo, protected in a natural park.
In the northeast there's a beach, called Praia das Catedrais or Praia de Augas Santas, where you will find characteristic rocky arches.

Praia das Catedrais

On the northwest coast, La Coruña is mild in winter, while in summer it remains cool: highs in July and August are just around 22/23 °C.
Here are the average temperatures.
Average temperatures - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDec
La CorunaJFMAMJJASOND
Min (°C)88910121416161513119
Max (°C)141416161821222322191614

In La Coruna, approximately 1,050 mm of rain per year fall; the wettest seasons are autumn and winter, the least rainy is the summer: from June to August precipitation drops below 50 mm per month, though on average it still rains 6/7 days per month.
Here is the average precipitation.
Average precipitation - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDecYear
La CorunaJFMAMJJASONDY
Prec. (mm)11590809080453535651351401351045
Days15121214117668131515135

Further south, and in the interior, we find Santiago de Compostela, a bit colder because of the distance from the sea and of altitude, and rainier, as it receives nearly 1,900 mm of rainfall per year.
Lugo, located in the interior and below 450 metres of altitude, gets pretty cold in winter, with night frosts that become quite frequent, while in summer, when the days are pleasant, the nights are still cool.
In Sarria, at no great distance from Lugo and practically at the same altitude, and which is often chosen as the starting point for the Camino de Santiago (or Way of St. James), the temperatures are similar.
Here are the average temperatures of Lugo.
Average temperatures - Lugo
LugoJanFebMarAprMayJunJulAugSepOctNovDec
LugoJFMAMJJASOND
Min (°C)2235710121210853
Max (°C)111215161922242523181411

In Lugo, the rain pattern is very similar to that of La Coruna: 1.115 mm of rain fall in a year, with a peak between October and January (when it exceeds 110 mm per month), and a minimum in summer.
Here is the average precipitation.
Average precipitation - Lugo
LugoJanFebMarAprMayJunJulAugSepOctNovDecYear
LugoJFMAMJJASONDY
Prec. (mm)115908510590553540701401451501115
Days14121214137568141515135

In the southwestern part of the coast, in cities such as Vigo and Pontevedra, the climate remains cool enough; inside the rías, winter temperatures are a bit lower than on the Atlantic coast, while in summer they become a little higher.
Here are the average temperatures of Vigo.
Average temperatures - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDec
VigoJFMAMJJASOND
Min (°C)567810131515141186
Max (°C)121316171923242523191512

The rains in the coast exposed to westerly winds are particularly abundant: in Vigo they reach 1,800 mm per year, including more than 200 mm per month from October to January; sometimes they reach spectacular values. But even here, in summer fortunately the rains diminish in intensity and frequency.
Here is the average rainfall in Vigo.
Average precipitation - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDecYear
VigoJFMAMJJASONDY
Prec. (mm)2101651401551256045451052352502651805
Days14121214127558131415131

In the southeastern interior area and at low altitude, the climate becomes almost Mediterranean, and the summer it gets quite hot: in Orense (or Ourense), 250 metres above sea level, highs in July and August touch 30/31 degrees, although nights remain cool.
Here are the average temperatures.
Average temperatures - Orense
OrenseJanFebMarAprMayJunJulAugSepOctNovDec
OrenseJFMAMJJASOND
Min (°C)33579131515121064
Max (°C)131519202328303128221613

In Orense, precipitation fall below 1,000 mm per year, and summer rains are quite rare.
Here is the average precipitation.
Average precipitation - Orense
OrenseJanFebMarAprMayJunJulAugSepOctNovDecYear
OrenseJFMAMJJASONDY
Prec. (mm)957060757035202060115110115850
Days119912105346111111102

To the north-east of Orense, we find the Sil River, which gives birth to the canyon of the same name.

The amount of sunshine in Galicia is good in the summer, when sunny days prevail, especially in the southern part (in July and August there are approximately 8 hours of sunshine per day in La Coruña, 8/9 in Santiago de Compostela, and even 9/10 in Vigo and Pontevedra), while in the other seasons, and especially in late autumn and winter, the sky is often cloudy.

Cies Islands, beach

The sea along the northern coast of Galicia is a bit cold even in summer, as you can see in the following table, regarding the temperature of the sea at La Coruna.
Sea temperature - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDec
La CorunaJFMAMJJASOND
Sea (°C)131313131416181918171514

On the west coast, the ocean temperature is practically the same, as can be seen from the data of Vigo.
Sea temperature - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDec
VigoJFMAMJJASOND
Sea (°C)141313141516181918171615

When to go

The best time to visit Galicia is the summer, from June to August or the first half of September, being the most dry and sunny. July and August are the warmest and less rainy months. Even in midsummer you must take into account some rain, and some nights a bit cool on the coast, and a bit cold in inland areas. Those who fear the heat, to avoid the albeit rare hot days, can choose May (especially the second half), June and September (especially the first half), knowing though that it might rain more frequently.
In the summer, on sunny days (which are quite frequent, especially in the south-west), you can take a break at the sea and sunbathe, with an air temperature often pleasant and a cool breeze from the ocean, but as mentioned the sea is cold for swimming.