In the north it’s completely normal to have sub zero temperatures in the winter. My hometown doesn’t have a particularly high altitude (650m, i.e. not in the mountains) and the average low for January is below zero.

Spain has very different climates. I live only 30 kms away from the cost in the North of Spain and temperatures of -4 or -5 C are quite common in the winter. Some times we even have -10 C. So temperetures bellow zero are definitely common here.

Not true. In the north range mountains there are below zero temperatures every winter.