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 be merged; most of the regularly arranged small elements usually have an apparent width between one and five degrees.
MAIN DIFFERENCES BETWEEN ALTOCUMULUS AND SIMILAR CLOUDS OF OTHER GENERA
Altocumulus sometimes produces descending trails of fibrous appearance (virga). When this is the case, the clouds are regarded as Altocumulus and not as Cirrus, as 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 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. 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, roils, 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.
CLOUDS FROM WHICH ALTOCUMULUS MAY FORM
Altocumulus may form by an increase in size or a thickening of at least some elements of a patch, sheet or layer of Cirrocumulus (Ac cirrocumulomutatus), by subdivision of a layer of Stratocumulus (Ac stratocumulomutatus), or by transformation of Altostratus or Nimbostratus (Ac altostratomutatus, Ac nimbostratomutatus). Altocumulus may also be produced by the spreading out of the summits of Cumulus clouds which reach a stable layer while in the process of vertical development (Ac cumulogenitus). Occasionally, the stable layer cannot stop the vertical development completely; in this case, after a temporary spreading out, the Cumulus clouds resume their growth above the stable layer, at least in places. Thus, the Altocumulus may appear on the lateral portion of Cumulus. Altocumulus may also be observed on or near the lateral portion of Cumulonimbus. This Altocumulus often forms while the mother-cloud is still in the Cumulus stage. The clouds are nevertheless, by convention, called Altocumulus cumulonimbogenitus.
Altocumulus is, al leasl in Ihe main, almosl invariably composed of waler droplels. This is evident from Ihe fairly low transparency of Ihe macroscopic elemenls and from the fact that the latter show sharp outlines when separate. Nevertheless, when Ihe temperature is very low, ice crystals may form. If the droplets then evaporate, the cloud becomes entirely an ice cloud and its macroscopic elements cease to present sharp outlines. The formation of ice crystals may take place in all species of Altocumulus; it occurs most frequently in Altocumulus castellanus and fioccus. A corona or irisation is often observed in thin parts of Altocumulus. Parhelia or luminous pillars are sometimes seen in Altocumulus, indicating the presence of tabular·shaped ice crystals.
During the initial stages of its formation, Altocumulus is frequently a fairly smooth cloud of moderate horizontal extent. This cloud then subdivides into more or less regularly arranged small elements, in the form of laminae or tessellations.
Altocurnulus in the shape of lenses or almonds often forms in clear air as a result of local orographic lifting of a layer of moist air.
Altocumulus frequently occurs at different levels in the same sky and is in many instances associated with Altostratus. In this case, the air is often hazy immediately below the sheets or layers of Altocumulus or between the elements constituting them.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
Virga may appear with most of the species of Altocumulus. Altocumulus floccus frequently dissipates, leaving very white trails of ice crystals, which are then identified as Cirrus. Mamma are sometimes visible in Altocumulus.