I have owned a second home in Hot Southern Spain near the coast for many years, I lived there full time for 10 ...my Daughter lives and works further south but inland up in the mountains..completely different area to mine around 7 hours away but still in the southern region .
Bilbao in Northern Spain has a climate much cooler than the south or even the rest of Spain. Summers are warm but not hot, and are often very wet. Winters are averagely mild, and rarely drop below about 10 degrees . However it's not somewhere I personally would take a holiday. It's quite a rough, industrial city, which is trying to improve it's image in the last few years which might be ok for the indigenous population, but for tourists, ?..no,.... there are far more interesting, cleaner, and less crime ridden cities in Spain to visit. IMO