The weather on one side of a mountain can be sunny and dry, while on the other side it can be cloudy and wet. This is because of the way the mountains force all of the moisture from the air on the windward side, leaving very little for the lee side. This is called a rainshadow.
It explains why the west of the UK is wetter than the east. With very high mountains the lee side may be so dry it's a desert.
