Altostratus
Altostratus (from Latin altum "height" and stratus or sternere "expand, spread out, cover with a layer"; abbreviation : As ) is a bluish to gray medium-high layer cloud without contours, which consists of (supercooled) water droplets and ice crystals . It is relatively clear gray, the sun is sometimes almost invisible behind the dark clouds. If the altostratus after cirrostratus (veil clouds ) pulls up over a large area, a warm front or occlusion is usually on the rise.
Number of hours until it starts to rain:
| approx. 9 h | The sun is a little clouded, but visible through the cloud layer with a sharp edge |
| approx. 6 h | The sun can only be seen as a bright spot in the cloud cover |
| up to 3 h | The sun has completely disappeared and a few ac pan are floating under the layer of clouds |
Altostratus has no special species, but has the subspecies translucidus , opacus , duplicatus , undulatus and radiatus . For special forms, accompanying and mother clouds, see the article cloud .
history
The cloud genus Altostratus was first named by Émilien Renou in 1855 and finally included in the internationally valid cloud classification.
Physical structure and appearance
The altostratus consists of water droplets and ice crystals. There are also raindrops and snowflakes in the cloud. It almost always has a large horizontal extent (up to hundreds of kilometers) and a fairly considerable vertical extent (up to thousands of meters). It can consist of two or more layers lying one on top of the other and sometimes intergrown with only a slight difference in height. Occasionally, waves or broad parallel bands can be seen.
Altostratus is generally so dense that even through the thinner parts, the sun can only be partially seen like through a frosted glass. The thicker parts can be so dense that they completely hide the sun.
Precipitation from the Altostratus can occur in the form of falling strips that hang down from the cloud base ( virga ). This sometimes gives the cloud base a warty or torn appearance. But when the precipitation falling from Altostratus reaches the ground, it is mostly permanent precipitation in the form of rain, snow, grains of ice or barrels of frost.
Clouds in the form of smaller shreds (pannus) can form under Altostratus within the lower lying turbulent layers if these are enriched with moisture as a result of evaporation from precipitation. In the early stages of their development, pannus clouds are small, not very numerous, and clearly isolated. They usually occur at a considerable distance below the underside of the Altostratus. Later, with increasing thickness of the Altostratus ceiling and sinking of its lower limit, this distance becomes significantly smaller. At the same time, the pannus clouds increase in size and number and can merge into an almost continuous layer.
Emergence
An altostratus layer usually forms when extensive layers of air are slowly lifted to sufficiently high heights. It can often be observed as high fog in autumn and winter, in calm weather conditions, as in winter 2005/06, when the maximum values for fine dust were exceeded.
Altostratus can also develop from a cirrostratus veil that increases in thickness; sometimes altostratus also arises as a result of a decrease in the thickness of a nimbostratus layer. Sometimes it also develops from an altocumulus layer, when ice crystal trails (virga) fall from the altocumulus to a large extent. Occasionally, especially in the tropics, altostratus occurs as a result of the expansion of the middle or upper parts of a cumulonimbus cloud.
Differences too
Cirrus
In rare cases, altostratus fields or layers can regress into spots, which can then be mistaken for spots of dense cirrus. Altostratus spots, however, have a larger horizontal extent and look predominantly gray.
Cirrostratus
The Cirrostratus has a bright white color and the sun is usually only slightly covered, even on the ground sharp shadows are cast.
A high, thin Altostratus layer can be mistaken for a Cirrostratus veil. In cases of doubt, it is sometimes possible to classify the cloud correctly, taking into account that with Altostratus objects do not cast shadows on the ground and that it has the same effect as frosted glass. If halo phenomena can be seen, the cloud is called cirrostratus in case of doubt.
Altocumulus and Stratocumulus
Altostratus sometimes has crevices, gullies, or cracks. Particular care should be taken to avoid confusion with altocumulus or stratocumulus fields or layers, as they have the same characteristics. Altostratus can be distinguished from Altocumulus and Stratocumulus by its uniform appearance.
Nimbostratus
A deep, thick Altostratus layer can be distinguished from a similar Nimbostratus layer in that Altostratus has thinner areas through which the sun may be weakly visible. Altostratus also shows a lighter gray, and its underside is usually less uniform than that of Nimbostratus. If, on moonless nights, doubts arise as to whether a cloud should be designated as Altostratus or Nimbostratus, the layer is designated Altostratus by convention, provided that neither rain nor snow falls.
stratus
In contrast to the Stratus, the sun, viewed through an Altostratus layer, is more of a bright spot - similar to when viewed through frosted glass - and has no sharp edges. Furthermore, in the case of an altostratus cover on the ground, often no shadows are visible, since the sun's rays are widely scattered when penetrating the cloud layer. The cloud layer may look white when viewed against the sun, but is gray in color.
It should be noted that one cannot always clearly distinguish between Altostratus and Stratus; z. B. in a stratus merging into Altostratus, the features can mix.
