Relief rainfall, also known as orographic rainfall, is a type of rain caused by warm, moist air that is forced to rise as it passes over higher elevations. As the moist air rises, it also cools and condenses, forming clouds and eventually, rain.
After the air flows over the top of the mountain and has dropped down to the other side, it warms once again. Because of this, the air now has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area, opposite of the relief rainfall, is called the rain shadow.
Relief rainfall is very common in mountainous areas, especially on the west coast of the United Kingdom, since the prevailing weather also comes from that direction.