Plain English Meaning
Clouds that look like little castle towers sticking up from a flat base. They show that the air at that height is unstable and could grow into thunderstorms.
Full Definition
A cloud feature, typically attached to mid-level cloud layers, characterized by turret-like vertical projections rising from a common horizontal base. The presence of castellanus indicates instability and rising air at that altitude, often signaling the potential for thunderstorm development later in the day.
Context Anchor
Seen in cloud descriptions and aviation weather discussions, especially when identifying clouds that may grow into more active weather.
Derivation
From the Latin 'castellum,' meaning 'little castle' or 'fortress.' The name reflects the cloud's appearance: turrets rising from a shared base, like the battlements on a castle wall. Knowing this makes the cloud easy to recognize visually.