December is one of the wettest months in Spain, so come prepared.
Remember that we're talking averages here. Weather throughout Europe is unpredictable, so don't take what you read on this page as gospel.
Weather in Madrid in December
Madrid can get bitterly cold in December, especially at night where it could be below zero. In my first winter in December we had just one heater to share between three bedrooms and it was cold, believe me! There is also a fair chance of rain in Madrid in December.
The average maximum temperature in Madrid in December is 52°F/11°C and the average minimum temperature is 36°F/2°C.
See also: 100 Things to Do in Madrid
Weather in Barcelona in DecemberThe sea has a warming effect in winter, so Barcelona isn't as cold in December as in Madrid, but it can still get pretty cold. Rain and overcast days are common, though there should be some sunny (cold) days.
The average maximum temperature in Barcelona in December is 57°F/14°C and the average minimum temperature is 43°F/6°C.
Read more about Barcelona
Weather in Andalusia in December
If you're one of those people that seizes up in the cold, Andalusia is your only (near) guarantee of good weather. It can get chilly at night (less so in coastal areas). However, on average it rains on one-in-three days in Andalusia, but nothing is perfect!
The average maximum temperature in Malaga in December is 63°F/17°C and the average minimum temperature is 48°F/9°C.
Read more about Andalusia
Weather in Northern Spain in DecemberI spent New Year's Eve in San Sebastian in 2006 and I was able to go out in a t-shirt (see this picture of San Sebastian's beach that I took that day) but the locals were extremely surprised by this virtual heatwave. On average it rains on 50% of days in December in Bilbao, so dress appropriately.
The average maximum temperature in Bilbao in December is 57°F/14°C and the average minimum temperature is 45°F/7°C. Note that it can get quite a bit colder further inland.
Weather in North-West Spain in DecemberThe wettest region in Spain has its wettest month in December (it rains on 21 days out of 30 in December in Santiago). It doesn't get as cold as it does inland (especially at night, when it's as warm as the Costa del Sol) but you won't notice this as you'll be soaked to the bone.
The average maximum temperature in Santiago de Compostela in December is 55°F/13°C and the average minimum temperature is 9°C.
Read more about North-West Spain