In meteorology, Altocumulus castellanus or Altocumulus castellatus[1] (ACCAS) is a cloud type named for its tower-like projections that billow upwards from the base of the cloud.

Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude lapse rate.

[2] They may be a harbinger of heavy showers and thunderstorms and, if surface-based convection can connect to the mid-tropospheric unstable layer, continued development of Castellanus clouds can produce cumulonimbus clouds.

Altocumulus castellanus clouds are typically accompanied by moderate turbulence as well as potential icing conditions.

For these reasons, flight through these clouds is often best avoided by aircraft.