When the shallow marine boundary layer (MBL) sitting off the west coast of Namibia becomes saturated, thin stratus clouds form. Waves propagating in the lower troposphere and on the sharp inversion atop the layer can perturb the height of the top of this layer (like waves upon a the surface of water), moving it up and down, and allowing the top of the MBL to become either saturated (where the MBL is deepest) or unsaturated (MBL is shallow). Where unsaturated, clear regions form. Often these waves take the form of trains (with spacings a few km apart). They propagate from the land and from extratropical disturbances, sometimes leading to fantastic interference patterns.
These waves, on August 26th 2016 are particularly spectacular as seen in this MeteoSat SEVIRI visable imagery.