Mountains generally act as the headwaters of rivers. Clouds moving along a weather frontal system are advected to a higher altitude when they encounter mountains, causing condensation and rainfall. Unfortunately, in most parts of the world, weather patterns follow a common direction most of the time. Therefore, when the clouds encounter the mountain range, it is usually from one direction. This results in the majority of rainfall falling only on one side of the mountain, and the mountain acting as a barrier preventing cloud systems from carrying moisture across the mountain.