Air that flows over a mountain forms air waves on the leeward side of the mountain. In the back of the wave where the air rises, the air cools and forms a cloud.
The air flows on through the cloud and down into the wave "valley" where the temperature rises enough to dry out the cloud droplets and clear the air.
Typical for such clouds is the elongated, smooth form. The Orographic clouds are often called almond clouds or lenticular clouds, and they move very little, in spite of a often very strong wind.
In the upper picture we can see clouds formed on top of a air wave just over the top of Beerenberg. Over the mountain clouds can be very stable and remain relatively
unchanged for many hours.
In the lower picture I have photographed wave clouds located about 30 km south of Beerenberg.
