© 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 I.D.: P.4.18
CL = 0, CM = 7, CH = /
Links in the image description will highlight features on the image. Mouse over the features for more detail.
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 resembling exaggerated undulations in the base of the cloud. This is the supplementary feature asperitas. Asperitas is more chaotic with less horizontal organisation than undulatus and characterised by waves in the cloud base which resemble a roughened sea surface viewed from below. Varying levels of illumination and variations in cloud thickness can lead to dramatic visual effects.
Strengthening NW winds ahead of a fast moving intense polar low.
This is a striking example of the new variety asperitas. It appears that the asperitas is a further development of the double system of undulations that is present. The first system of undualtions are aligned NE-SW, transverse to the upper level winds. The second system of undulations is aligned NW-SE. 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 NNW.
The Altocumulus is in an extensive sheet (species stratiformis), is sufficiently translucent in most parts to reveal the position of the sun (variety translucidus) and in places there are spaces between the elements that would reveal the position of the sun (variety perlucidus).
There is very distant cirriform cloud on the horizon.
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 the new variety, asperitas. It could be a mild form of, or developing, asperitas.
Altocumulus stratiformis translucidus undulatus where the undulatus is developing into the new supplementary feature asperitas.
Well-defined, wave-like structures in the underside of the cloud; more chaotic and with less horizontal organization than the variety undulatus. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects.