The castellanus variety is recognised by vertical towers sprouting from a shared base, resembling the battlements of a medieval castle. It can appear in altocumulus, cirrocumulus, and stratocumulus, with altocumulus castellanus (Ac cas) being the most significant for forecasting.
Storm signal
Morning altocumulus castellanus is one of the most reliable indicators that thunderstorms will develop by afternoon. The towers reveal convective instability at mid-levels (3,000–6,000 m), and when this connects with afternoon surface heating, deep convection is triggered.