If a thick, multilayered cloud system approaches and crosses a high and extended mountain range, the various cloud layers will be influenced differently. Cloud layers up to the height of the mountain range will be decelerated and even intensified by Stau processes; the cloud layers above the mountain ranges will not be particularly influenced and will therefore cross the mountains without any retardation. As a consequence lower and higher cloud become decoupled.

The consequence of such a process is that a double structure in the frontal cloud band develops: the cloudiness associated with the surface frontal position stays rather longer along the mountain range while the high frontal cloud band passes quickly over the mountains.

These processes appear frequently in the region of the Alps, and the following schematics and examples relate to this mountain range.

The loop below shows an example in which the Decoupling process can be observed twice: at about 19.00 UTC a smaller, fibrous cloud band moves into N. Italy and the remaining cloud band in the W. Alps is very dominant; at about 21.30 UTC a thicker cloud band moves over N. Italy and further eastward. The remaining low and middle level cloud in the mountain range gradually decreases. The process of different displacement velocities is reflected very well by Cloud Motion Vectors (CMVs) which are computed from the cloud displacement between two successive IR images. In the example below the cloud band over N. Italy is accompanied by much longer displacment vectors than the cloud band over the W. Alps.