Description Altocumulus and differences with similar clouds

White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not he merged; most of the regularly arranged small elements usually have an apparent width between one and five degrees.

Altocumulus clouds sometimes produce descending trails of fibrous appearance (Virga). When this is the case, the clouds are regarded as Altocumulus and not as Cirrus, so long as they have a part without a fibrous appearance or a silky sheen.

Altocumulus may sometimes be confused with Cirrocumulus. In case of doubt, if the clouds have shading, they are by definition Altocumulus, even if their elements have an apparent width of less than one degree. Clouds without shading are by definition Altocumulus if most of the regularly arranged elements, when observed at an angle of more than 30 degrees above the horizon, have an apparent width between one and five degrees. A Corona or irisation is often observed on thin parts of Altocumulus but only infrequently on Cirrocumulus.

An Altocumulus layer may sometimes be confused with Altostratus; in case of doubt, clouds are called Altocumulus if there is any evidence of the presence of laminae, rounded masses, rolls, etc.

Altocumulus, with dark portions, may sometimes be confused with Stratocumulus. If most of the regularly arranged elements have, when observed at an angle of more than 30 degrees above the horizon, an apparent width between one and five degrees, the cloud is Altocumulus.

Altocumulus in scattered tufts may be confused with small Cumulus clouds; the Altocumulus tufts, however, often show fibrous trails (Virga) and moreover are, in their majority, smaller than the Cumulus clouds.