Altocumulus castellanus clouds take their name from their resemblance to the turrets of castles and are often a warning of thunderstorms.
Their towering appearance is a sign of medium-level instability in the air and they often serve as a precursor to thunderstorms or heavy precipitation. For this reason, they are sometimes also referred to as 'thunder altocumulus'.
As a mid-level cloud, they form at heights of around 2,000 - 5,500 m (7,000 - 18,000 ft).
The clouds usually appear as a collection of small individual closed circulations, known as cells, with the towers proceeding from the top of the cloud. If conditions are cool and moist enough, they can develop into large cumulonimbus clouds.
Last updated: 5 August 2014