Altocumulus castellanus (Ac cas) - CCH 1953

Cumuliform turrets rising vertically from cloud elements connected by a common horizontal base. The turrets:

• Seem to be arranged in lines
• Give the cloud a crenelated (castle battlement) appearance
• Are sometimes taller than they are wide
• Are especially evident when the cloud is seen from the side

Altocumulus castellanus is an indicator of instability at that level; when it acquires considerable vertical extent, it is classified as:

• Cumulus congestus altocumulogenitus, if it is strongly sprouting or is of great vertical extent
• Cumulonimbus altocumulogenitus, if part of its upper portion is smooth, fibrous or striated, or if the cloud produces lightning, thunder or showers of hail