Flying doughnuts
Flying over the German Bight at about 13,000 metres I saw a completely annular cloud. It appeared unrelated to the other scattered clouds that were present and, for the two minutes that I was able to watch it, it didn鈥檛 change its appearance. It must have been 3 or 4 kilometres across and 300 or 400 metres thick, though this varied around its circumference. Could it have been natural and, if not, what could have caused it?
鈥� Such toroidal (doughnut 鈥� shaped) clouds usually result from a pulse of rising gas caused by, say, an explosion or a brief, hot flame. Over the North Sea, however, the most likely source would have been a bubble of a few tonnes of natural gas suddenly released from below the sea floor. It is not especially rare for a mass of hydrocarbon clathrate (a gas compound held in a solid matrix) to escape from the sludge confining it.
Whether it was ignited by static electricity as it emerged into the air, or simply expanded abruptly at the sea surface, the light, water 鈥� saturated gas would have risen into still air, cooling as its pressure dropped, until its moisture content condensed into cloud. Where I live, such condensation seldom lasts long, but in Europe鈥檚 humid atmosphere contrails often form a nucleus for persistent cloud.
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So it would not be surprising if a large, visible vortex were to rise to several thousand metres, growing as it went. Such toroidal vortices can be surprisingly stable, but even so, for it to have remained so spectacularly intact suggests that there was little air turbulence.
Other possibilities include a gas pipeline break, although it鈥檚 unlikely that such a large vortex was caused directly by human activity.
Jon Richfield
Somerset West, South Africa
鈥� Your cloud sounds to me like a lenticular (lens 鈥� shaped) cloud. These are triggered by wind hitting a hill or mountain and producing a stream of rising air. The air cools as it rises and reaches a point where moisture condenses, producing an unmistakably smooth lens 鈥� shaped cloud (see photograph top right). A good analogy is the hump of water you see in a fast-flowing stream just downstream from a rock or obstruction.
I suspect that if your cloud had a hole in the centre, you were seeing the front edge of it, where air was rising through the height at which the moisture was condensing, and the back edge, where higher air was condensing or freezing on its way back down.
If I am right, the cloud would probably be elliptical when viewed from above. There were almost certainly strong winds and a hill or mountain upwind of the cloud.
The smooth lift producing these clouds can extend to great heights, enabling gliders to climb well above the cruising height of an airliner. Visit any gliding club and you will find many people who are very keen to discuss such clouds.
Clive Stewart
Evesham, Worcestershire
鈥� The doughnut 鈥� shaped cloud was most probably a manifestation of a large 鈥� scale vortex in the atmosphere, caused by a stream of high 鈥� altitude wind. A few years ago, when sailing near the island of Krk in Croatia, I took this photograph (bottom right) of a 鈥済alaxy 鈥� shaped鈥� cloud about 1 kilometre in diameter floating at an altitude of about 4 kilometres.
The cloud was some 15 kilometres downwind of a mountain ridge which has a saddle where wind, known locally as the Bura, attains great force when pushed into a stream by the saddle.
Radko Istenic
Ljubljana, Slovenia
This week鈥檚 question
Down the plughole
Do giant whirlpools ever form in hydroelectric reservoirs where the water runs through the base of the dam? If they do, do they have adverse effects? And if they don鈥檛 form, why not?
Ben Cadoret
Sheffield, South Yorkshire