© Frank Le Blancq
Near Guernsey
Latitude: 49° 23' 43'' N
Longitude: 2° 28' 13'' W
02 August 2008 2012 (Local Time)
Camera direction: towards NW
Image P/S code: P.4.8
Image I.D.: 5168
CL = 8, CM = 3, CH = /
The relatively small white elements in the upper two thirds of this image are identified as Altocumulus. These elements make up an extensive sheet (species stratiformis). To this identification are added the varieties translucidus (the elements are sufficiently translucent to reveal the position of the Sun), perlucidus (small gaps between the elements reveal blue sky) and undulatus (separate elements are arranged in broadly parallel lines).
The Altocumulus and some higher cirriform cloud were advancing from the west (left in the photo) ahead of a weak warm front. Also visible are traces of Cumulus fractus (light grey in the top right) and a layer of dark grey Stratocumulus stretching towards the horizon (lower part of the image).
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A low of 999 hPa was centred north-west of Ireland, with high pressure of 1 027 hPa near the Azores, Portugal. Shallow wave depressions were running east from the Atlantic over southern England and along the English Channel. The photo location was in a weak ridge ahead of an approaching weak warm front. Wind at Altocumulus level was estimated to be blowing from 270° at 40 kt.
The sounding was taken upwind, 180 km to the south-west, and shows an unstable atmosphere from the surface to 947 hPa. The wet bulb effect occurs from 947 to 925 hPa, indicating the sonde ascended through and out of cloud. There is middle-level moisture from 575 to 490 hPa.
This layer of Altocumulus (species stratiformis) is clearly translucent (variety translucidus) and gaps between the elements permit the sky to be seen (variety perlucidus)(2). The elongated elements, almost parallel to one another indicate the variety undulatus(4). The layer was invading the sky.
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In this night-time image, the thin layer of medium-level cloud through which the moon is visible is Altocumulus stratiformis translucidus. On the left edge, there are gaps between the cloud elements, making this also the variety perlucidus. The optical phenomenon surrounding the moon is the aureole of a lunar corona.
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These regularly arranged cloud elements are Altocumulus as they have an apparent width of between 1 to 3 fingers at arm's length. They are of the species stratiformis (they are spread out in an extensive horizontal layer) and the varieties translucidus (they are sufficiently translucent to reveal the position of the Sun) and perlucidus (there are distinct spaces between the elements).
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This layer of medium-level cloud is Altocumulus. Its rounded masses and rolls have an apparent width of between 1° and 5° and there is some shading. The specific species of this Altocumulus is classified as stratiformis, due to the cloud’s extensive coverage of the sky. The sheet of cloud is sufficiently translucent to reveal the position of the Sun, and so it is also of the variety translucidus. In places there are spaces between the cloud elements, which also indicates the variety perlucidus, and the enlongated and broadly parallel rolls indicate that the variety undulatus also applies.
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The photo shows a layer of Altocumulus stratiformis undulatus perlucidus, viewed from an aircraft at an altitude of approximately 1 1000 m over the Italian Alps. At the lower right of the picture, some Cumulus clouds can be seen over the mountains.
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This thin and very high sheet is of the species stratiformis (the cloud is an extensive sheet) and the varieties translucidus (the greater part is sufficiently thin to reveal the position of the Sun), perlucidus (there are distinct, very small spaces between the elements) and undulatus (there are separate elements arranged in broadly parallel lines).
The regularly arranged elements (those more than 30° above the horizon) are about the width of a little finger at arm's length – borderline between Altocumulus and Cirrocumulus in size. The presence of shading suggests Altocumulus; however, an observer has to determine whether this shading is due to a low Sun angle (it was 6° above the horizon at the time of the photo). Continuous observation can assist with this, as the observer may have seen shading in the elements when the Sun was higher. This was the case in this instance, as observations from Hong Kong Airport, Hong Kong (China) reported Altocumulus translucidus throughout the day.
Cirrus fibratus is distinctly visible in a large gap in the Altocumulus layer. A leading edge of Cirrostratus is low on the horizon and a not-too-brightly coloured parhelion (sun dog) is visible to the north (right) of the setting Sun.
Completing the observation is Cumulus of the species mediocris, humilis and fractus.
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