Altocumulus castellanus are midlevel cloud patches with little turrets rising from a shared base. Their castle-like shapes can indicate instability at middle altitudes, although they do not guarantee thunderstorms.
The ‘castle tower’ clue
Instead of a smooth or uniformly lumpy cloud, look for small vertical protrusions rising from an altocumulus layer. The tops can resemble distant battlements.
What creates the towers
Air pockets that are buoyant relative to their surroundings can rise within an elevated moist layer. The resulting shallow convection produces upright cloud elements.
What it means for the forecast
The pattern can suggest instability aloft, which may be relevant to later convective development when combined with surface heating, moisture and lifting. Many sightings do not lead to severe weather.
How to distinguish it
Regular altocumulus may form rounded patches without pronounced upward turrets. Towering cumulus has a lower base and typically much greater vertical extent.
Frequently asked question
Do these clouds always form in the morning?
No. They can appear at different times depending on atmospheric conditions.
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