Altocumulus
Altocumuli ( lat. Altum "height" and cumulus "accumulation"; abbreviation : Ac ) are white and / or gray spots, fields or layers of clouds , generally with their own shadow consisting of scale-like parts, balls, rollers, etc., which sometimes look partially fibrous or diffuse and may have grown together. Because of the stronger winds , they are usually somewhat longer, smaller and more frayed than lower clouds, but despite everything they are difficult to distinguish from the stratocumuli and cirrocumuli . There are still a few differences: First, the width - altocumuli are up to three fingers wide (with an outstretched arm), all the layer clouds that are larger are then just stratocumuli, and the cirrocumuli are less than a finger wide. And secondly, the shadow of its own which does not appear in the cirrocumulus. Clouds without a shadow of their own are called altocumulus if the greater part of the cloud parts is one to three fingers wide.
Special types of Altocumulus are stratiformis , lenticularis , castellanus and floccus , as well as the subspecies translucidus , perlucidus , opacus , duplicatus , undulatus , radiatus and lacunosus . For special forms, accompanying and mother clouds, see the article cloud .
history
The cloud genus Altocumulus was first named by Émilien Renou in 1855 and was finally included in the internationally valid cloud classification.
Physical structure and appearance
- Altocumulus consists predominantly and almost always of water droplets. However, ice crystals can also form at very low temperatures .
- Altocumulus occurs mostly in the form of an extensive field, which consists of individual, fairly regularly arranged cloud parts ( small clouds ) ( stratiformis ). Sometimes the clouds also take the form of elongated, parallel rollers, which can be separated from each other by sharply defined, cloudless paths. In very rare cases, altocumulus fields also have small, more or less regularly distributed, round, often fringed gaps, which give the appearance of a net or a honeycomb ( lacunosus ). Altocumulus fields often occur simultaneously at two or more heights ( duplicatus ).
- Altocumulus also occurs in lenticular or almond-shaped banks, which are often very elongated and have clearly pronounced outlines ( lenticularis ). These banks consist either of small, closely spaced pieces of cloud or of a single, more or less smooth-looking cloud. In the latter case there are clear shadows of their own.
- In rarer cases, Altocumulus occurs in the form of small, isolated tufts, the lower parts of which are somewhat frayed ( floccus ). Fibrous trails are common in these clouds. Another, equally rare form of altocumulus looks like a series of turrets growing out of a common, horizontal base ( castellanus ).
- The transmittance of altocumulus clouds varies considerably. In some cases the position of the sun can be determined through the larger part of the cloud ( translucidus , perlucidus ). In other cases the cloud is so dense that the sun is completely covered ( opacus ). Dense altocumulus layers can have an uneven underside, against which individual cloud parts stand out in a clearly relief-like manner. Almost always some shadow of its own can be seen in Altocumulus. Also corona formation or iridescence is frequently observed in altocumulus.
- Crystals falling from Altocumulus can cause halo appearances in the form of sub-suns or pillars of light .
Emergence
Altocumulus
- often arises at the edge of an extensive layer of air during uplift or also during turbulence or convection in the middle cloud storeys.
- can also arise from the expansion or growth of at least some cloud parts of a cirrocumulus field, from the breaking up of a stratocumulus layer or from transformation processes in the Altostratus or Nimbostratus.
- also forms when cumulus or cumulonimbus clouds spread
- in the shape of a lentil or almond usually arises when a layer of moist air is locally orographically conditioned.
Differences too
Cirrus cloud
In Altocumulus there are sometimes hanging, fibrous-looking trails (Virga). As long as the entire appearance is not fibrous or silky, the clouds are called altocumulus and not cirrus .
Cirrocumulus
Altocumulus can sometimes be confused with Cirrocumulus . In cases of doubt, the cloud, if its own shadows occur, is defined as an altocumulus, even if the cloud parts are less than 1 degree wide. By definition, clouds without their own shadow are still considered to be altocumulus if most of the regularly arranged cloud parts have a width of 1–5 degrees at an observation angle of more than 30 degrees above the horizon. Corona formation and iridescence is often observed on thin parts of altocumulus, but only rarely in cirrocumulus.
Altostratus
An altocumulus layer can sometimes be confused with altostratus ; In cases of doubt, the clouds are referred to as altocumulus if there are any signs of scale-like parts, balls, rollers, etc.
Cumulus
Altocumulus in the form of scattered tufts can be confused with small cumulus clouds; however, the altocumulus tufts often have fibrous trails (virga) and, moreover, are for the most part smaller than the cumulus clouds.
