On holiday in Taormina, Sicily, about 30 kilometres north-east of Mount Etna, which we could see from our window, we awoke on Wednesday 10 May 2006 at 6.45 am to see an odd cloud drifting towards us (see image). What caused it? Was Etna responsible? My grandson suggests it鈥檚 a flying saucer, but I鈥檓 sure that鈥檚 not it.
The clouds shown are lenticulars, which are caused by waves in the air downwind of mountain ranges. Lenticulars do not drift, but form continuously as moist rising air from the upwind side of the mountain condenses. As the air descends on the downwind side it warms and the cloud evaporates. Any observed movement of the cloud is in fact caused by a change in wavelength.
Solitary mountains rarely produce waves strong enough to form clouds, but in this case airstreams deflected around each side of Mount Etna may meet on the downwind side and contribute to the updraft. Lenticulars are often best seen in the early morning before thermals that form during the day disrupt the wave system.
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Andrew Brown (glider pilot), London, UK
The formation is a lenticular cloud, or to give it its technical name, an altocumulus standing lenticularis, and is almost certainly connected with nearby Mount Etna. This is not because it is a volcano, but simply because it is a high mountain close to the sea. When stable moist air, such as wind blowing off the warm Mediterranean sea, flows over mountains, a series of large-scale standing waves can form on the leeward side, and lenticular clouds can form at their crests.
Not only are lenticular clouds striking to look at, they also provide useful signposts for aviators, albeit for quite contradictory reasons. Pilots of large aircraft attempt to avoid lenticular clouds because of the threat posed by the extremely powerful rotor forces that fashion their distinctive shape. Glider pilots, on the other hand, will actively seek out 鈥渓ennies鈥� to use those same vertical air movements to obtain lift. Indeed the current altitude and distance records for gliders were set employing this so-called 鈥渨ave lift鈥�.
鈥淣ot only are lenticular clouds striking to look at, they also provide signposts for aviators鈥�
It has been claimed that the phenomenon played a decisive role in the 1942 battle of the Coral Sea, which lies between Australia, Papua New Guinea and the Solomon Islands. A force of elderly US navy Devastator torpedo bombers, hunting the Japanese fleet and low on fuel, found their path blocked by Papua New Guinea鈥檚 Owen Stanley mountains. At almost the point of no return Commander W. K. Ault, the formation鈥檚 leader and an experienced glider pilot, identified the distinctive cloud patterns associated with wave lift and used them to enable his squadron to soar over the range, where they found and sank the carrier Shoho, the first major Japanese warship sunk during the second world war, in what proved a turning point in the Pacific war.
Your reader鈥檚 grandson is not the first to propose 鈥渇lying saucers鈥�. Veteran UFO debunker Donald Menzel, and the US government鈥檚 Condon report have pointed out how frequently this mistake is made.
Hadrian Jeffs, Norwich, Norfolk, UK