From SKYbrary Wiki
A layer of patches of cloud composed of small lumps, heaps, or rolls of cloud, often arranged in waves or lines which may be joined to form a continuous layer.
A characteristic of this cloud is Coronae - one or more coloured rings around the sun or moon when viewed from below the cloud. Ac comprising small, but distinctive, cumuliform clouds indicates rapidly forming mid level cloud in unstable conditions (Ac Castellanus). In these circumstances, the vertical development of the cloud may be sufficient to produce light precipitation or virga (light rain which doe not reach the surface).