Northern Switzerland has a fairly cold climate caused by sea winds from the Atlantic Ocean, but southern Switzerland tends to be much warmer and sunnier.

Mountains prevent cold air from blowing over into the south.

Explanation:

There are mountains between Northern and Southern Switzerland, and when air from Northern Switzerland strikes the mountain, it is reflected back in the same direction it came from, preventing air from crossing the mountain and reaching Southern Switzerland. And air is unable to reach the peak of the mountain. Northern Switzerland has a relatively cold climate due to Atlantic Ocean sea winds, but southern Switzerland is significantly warmer and sunnier.