The sinking air motion is very evident from this cloud layer. What is even strange in this cloud cover (writing as a cloud observer), is that it seemed to be arranged in a wave-like formation resembling the undulatus sub-species of altocumulus. Upon observing the weather sounding, the very strong winds present in the middle levels of the atmosphere was immediately striking (for example at an altitude of 5km, the wind was blowing at 120kmh hence explaining the pouch-like sacs which seemed to being blown away). The wave-like formation may have been due to wind shear which was clearly shown in the weather sounding and the mammatus features could have been initiated by falling precipitation from this cloud layer which evaporated and condensed into such cloud features (the first pic on the thumbnail shows virga from the cloud cover taken several minutes later which prove that the cloud was conductive to produce precipitation). However, the latter is only my hypothesis upon observing this weather as mammatus formations are still not very well understood by meteorologists.