Greyish or bluish sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.
MAIN DIFFERENCES BETWEEN ALTOSTRATUS AND SIMILAR CLOUDS OF OTHER GENERA
Sheets or layers of Altostratus may, on rare occasions, degenerate into patches which may be confused with dense Cirrus of comparable optical thickness. Altostratus patches however have a greater horizontal extent and are predominantly grey.
A high and thin layer of Altostratus may be mistaken for a veil of Cirrostratus. It is sometimes possible to identify the doubtful cloud by remembering that Altostratus prevents objects on
the ground from casting shadows and that it may show a ground glass effect. Furthermore, if halo phenomena are present, the doubtful cloud is Cirrostratus.
Altostratus sometimes has gaps, breaches or rifts; care must then be exercised not to confuse it with an Altocumulus or Stratocumulus sheet or layer showing the same features. Altostratus is distinguishable from Altocumulus and Stratocumulus by its more uniform appearance.
A low, thick layer of Altostratus may be distinguished from a similar layer of Nimbostratus by the presence in Altostratus of thinner parts through which the sun is, or could be, vaguely r.evealed. Altostratus’ is also of a lighter grey and its under surface is usually less uniform than that of Nimbostratus. When, on moonless nights, doubt exists regarding the choice of the designation Altostratus or Nimbostratus, the layer is by convention called Altostratus, if no rain or snow is falling.
Altostratus is distinguishable from Stratus, with which it may be confused, by its ground glass effect. Furthermore, Altostratus is never white, as thin Stratus may be when observed more sun or less towards the sun.
CLOUDS FROM WHICH ALTOSTRATUS MAY FORM
Altostratus may evolve from a thickening veil of Cirrostratus (As cirrostratomutatus); it is sometimes formed by the thinning of a layer of Nimbostratus (As nimbostratomutatus). Altostratus may also develop from an Altocumulus layer; this happens when widespread ice crystal trails (virga) fall from the latter (As altocumulogenitus). Sometimes, particularly in the tropics, Altostratus is produced by the spreading out of the middle or upper part of Cumulonimbus (As cumulonimbogenitus).
Altostratus nearly appears as a layer of great horizontal extent [several tens or hundreds of kilometres (several tens or hundreds of miles)] and fairly considerable vertical extent [several hundreds or thousands of metres (several hundreds or thousands of feet)]. It is composed of water droplets and ice crystals. In the most complete case, three superposed parts may be distinguished, namely:
(a) an upper part, composed wholly or mainly of ice crystals,
(b) a middle part, composed of a mixture of ice crystals, snow.crystals or snowflakes and supercooled water droplets,
(c) a lower part, composed wholly or mainly of ordinary or supercooled water droplets or drops. In some cases, the cloud may consist of only two parts, either:
-an upper part like (a) and a lower part like (c) or
-an upper part like (b) and a lower part like (c).
Less frequently, the entire cloud may also be like (a) or like (b) alone. The constituent particles in the lower part of Altostratus are so numerous that the outline of the sun or moon is always dimmed and the surface observer never sees halo phenomena. In the thickest parts, the position of the luminary may be completely concealed. Raindrops or snowflakes are often present in Altostratus and below its base. When precipitation reaches the ground, it is generally of the “continuous” type and in the form of rain, snow or ice pellets.
The under surface of Altostratus occasionally exhibits a mamillated or ragged appearance due to precipitation trails (virga of rain or snow). Isolated virga are clearly seen when rain, before evaporating, falls farther in some places than in others. The presence of precipitation sometimes makes it difficult to distinguish a cloud base; this is particularly the case when uniformly falling snow completely evaporates, before reaching the ground. If, however, snow melts rapidly into rain, an apparent base may be observed at the melting level, as the visibility through rain is greater than through snow. This “base” is very clearly visible when the rain layer is thin, which is the case for instance if the raindrops quickly evaporate; it may be completely obscured when the rain layer is thick. Pannus clouds may be present; they occur under the Altostratus in the lower turbulent layers when these are moistened by evaporation from precipitation. Pannus clouds also show a tendency to form near the 0˚ C (32˚ F) level where the cooling of the air by melting snow increases the instability of the layer underneath. In the initial stage of their formation, pannus clouds are small, sparse and well separated, and usually occur at a considerable distance below the under surface of the Altostratus. Later, with a thickening Altostratus and a lowering of its base, this distance is greatly reduced. At the same time, the pannus clouds increase in size and number and may merge into a quasi-continuous layer.
Altostratus is not subdivided into species owing to the uniformity characterising its appearance and general structure.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
Virga and praecipitatio may be clearly visible.
Pannus clouds may be observed under Altostratus.
Altostratus may show mamma.