Mountains are a barrier to wind flow. Air can flow around the mountain, over it or a combination of both. When it comes to a mountain range, there is not an option for the wind to go around the mountain. Thus, the air that does get to the other side must go over the mountain range. The displacement of air over a mountain range can initiate a wave-like motion to the air, especially if the air becomes trapped in a stable layer. This is analogous to two people holding a rope and one person suddenly lifting up on one end to initiate a wave pattern. The air overshoots and undershoots where it is most stable and this creates a wave-like pattern after the air passes over the mountain range and continues the journey downstream. The diagram below shows the wave-like pattern of the wind flow moving downstream from the mountain range.

When this air is close to saturation, it will be able to develop clouds where the air is rising, cooling and condensing out moisture. Where the air sinks the clouds will clear up since sinking air warms and sinking air decreases in relative humidity. This can create an alternating pattern of cloudy and clear areas downstream from the mountain range. The birdís eye view below shows the elongated sheets of clouds that can be produced. The thickness and size of the sheets will depend on how much moisture is in the air and how the stability profile changes as the air moves downstream. If there is abundant moisture present then it is possible to have a thick sheet of clouds everywhere downstream from the mountain range. Even the normally clear areas will still have clouds since the sinking motion in those regions is not enough to dissipate the clouds.