Formation of a patch of dense Altocumulus (Ac) cloud due to the arrival of a cooler airmass. However, neither the surface pressure chart nor the weather sounding were showing the presence of a front but the air felt a bit cooler and drier afterwards. And normally such clouds form when warmer air slides over cooler air. The first thumbnail on the right shows the cloud approaching looking more like the perlucidus sub-species. But in this photo it seemed to have contained wave-like features hence going to undulatus more so that the model was showing a bit of speed shear at the cloud level. In any case, this cloud seemed to have contained some characteristics of the sub-species Undulatus, Perlucidus and Opacus, always in the genre or sub-species of Ac.