© Gary McArthur
Shorewell Park TAS 7320, Australia
Latitude: 41° 3' 60'' S
Longitude: 145° 52' 43'' E
20 February 2004 0748 (Local Time)
Camera direction: towards W
Image P/S code: P.4.18
Image I.D.: 5858
CL = 0, CM = 7, CH = /
This photograph shows a layer of Altocumulus stratiformis cloud, typically displaying varying shades of grey. It is sufficiently thick and opaque to mask the Sun, which identifies it as belonging to the variety opacus. Of most interest are the well-defined wave-like features at 1 and 2 that resemble exaggerated undulations in the base of the cloud. This is the supplementary feature asperitas. Asperitas is more chaotic and has less horizontal organization than undulatus, and is characterized by waves in the cloud base that resemble a roughened sea surface when viewed from below. Varying levels of illumination and variations in cloud thickness can lead to dramatic visual effects.
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Strengthening north-west winds ahead of a fast-moving, intense polar low
An extensive and thick layer of Altocumulus (species stratiformis and variety opacus) is the only cloud visible in this photo. The chaotic wave-like structures on and descending from the under surface of the cloud are of the new variety, asperitas. It could be a mild form of asperitas or developing asperitas.
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This is a striking example of the new variety asperitas. It appears that the asperitas is a further development of a double system of undulations that is present. The first system of undualtions are aligned north-east to south-west, transverse to the upper-level winds, while the second system of undulations is aligned north-west to south-east. These two systems of waves are starting to interfere with each other, giving a slightly chaotic appearance in places.
The predominant cloud is of the genus Altocumulus. This can be determined by the size of the regularly arranged elements to the west (left) of the Sun, experience in judging height and an upper-air sounding from 190 km to the north-north-west.
The Altocumulus is in an extensive sheet (species stratiformis) and is sufficiently translucent in most parts to reveal the position of the Sun (variety translucidus). In addition, in places there are spaces between the elements that would reveal the position of the Sun (variety perlucidus).
There are also patches of Altocumulus lenticularis and Stratocumulus undulatus to the east and west; the latter may be asperitas.
There is very distant cirriform cloud on the horizon.
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The image shows Altocumulus stratiformis translucidus undulatus where the undulatus is developing into the new supplementary feature asperitas.
Asperitas boasts well-defined, wave-like structures in the underside of the cloud that are more chaotic and have less horizontal organization than the variety undulatus. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, that sometimes descend into sharp points; it is as if one were viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects.
Links in the image description will highlight features on the image. Mouse over the features for more detail.