White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of layers laying on top of each other, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged.
Most of the regularly arranged small elements usually have an apparent width of between one and five degrees (when observed at an angle greater than 30 degrees above the horizon). This is between the approximate width of one to three fingers at arm's length.
A corona (ring around the sun or moon with red on the outside) may sometimes (albeit rarely) be seen. Irisation may appear along the thinner edges of the elements.
The most common species is Altocumulus stratiformis, occurring as an extensive sheet or layer of separate or merged elements.
The species Altocumulus castellanus is in the form of sproutings or small towers having a common base, or appears as small cumuliform tufts. When Altocumulus castellanus acquires a considerable vertical extent, it becomes a high based Towering Cumulus, and can even transition to Cumulonimbus.
Altocumulus lenticularis is a lens or almond shaped, often elongated cloud, commonly associated with mountain wave activity. Mountain waves can present a significant hazard to aviation.
Distinguishing Ac from other genera
Altocumulus differs from Cirrocumulus in that some of the Altocumulus clouds have shading. However, if the clouds are without shading but most of the elements have an apparent width of between one and five degrees, the cloud is to be called Altocumulus.
Altocumulus is distinguished from Stratocumulus by its smaller elements.
WMO technical notes do not associate any precipitation with Altocumulus, however an Australian convention associates showery precipitation with the castellanus species.