species of Altocumulus
Altocumulus which presents, in at least a fraction of its upper part, cumuliform protuberances in the form of turrets which give this cloud a crenulated appearance. The cumuliform cloud elements have a common horizontal base and appear to be arranged in lines. The castellanus character is especially evident when the cloud is seen from the side. The presence of this cloud is a sign of instability at its level; when it acquires a considerable vertical extent, Altocumulus castellanus becomes Cumulus congestus, and sometimes Cumulonimbis.