rom a research on the Internet, I found that the photo was probably a `roll cloud` being defined as follows: `A roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be `rolling` about a horizontal axis. They are a solitary wave called a soliton, which is a wave that has a single crest and moves without changing speed or shape.` Humidity level was above 80% while temperature was probably around +25C at the time. The mid-level clouds helped to create contrast with this cloud hence proving two things: (1) The roll cloud was at a LOWER level than the Altocumulus classifying it as a low cloud AND (2) it was completely detached from the altocumulus itself or other present cloud feature. It might have been initiated by the onset of sea-breezes despite the mid-level cloud cover. An important indicator (at least to me) when I looked at the weather sounding was a low-level subsidence inversion at an altitude of around 600 metres just beneath a humid and possibly unstable air. This roll cloud formed very quickly, moved relatively swiftly westwards, darkened the sky over the area for a few minutes and perhaps also decayed rapidly after passing by (though I`m not sure as there were other clouds around). No rain fell during this event although I may have felt the wind increase just a little during this cloud`s passage overhead. In fact, I was very lucky to have the camera at hand to take this fabulous photo. A closeup shoto of this roll cloud resembled a cumulus clouds. Find the following on the right hand side of the screen: `A close-up shot of the roll cloud` in an attempt to reveal its characteristics and the weather sounding showing the air profile at the time. The second thumbnail is a photo of altocumulus perlucidus clouds indicating instability at the middle part of the atmosphere. Such cloud is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment. However, no such fronts were seem to be present at the time.