Most clouds are carried along briskly by the wind, to the extent, indeed, that the speed of movement of a cloud is a good approximation of the wind speed at that level.
But not so with altocumulus lenticularis; as the 19th-century meteorologist J.F. Daniels put it, succinctly but obscurely: "The apparent permanency and stationary aspect of a cloud is often an optical deception, arising from the solution of vapour on the one side of a given point, while it is precipitated on the other." Let Weather Eye explain.
Lenticular cloud, as altocu- mulus lenticularis is more often called, has sharply defined edges and thin, somewhat pointed, ends. The total effect resembles that of an elongated almond, or as its name implies, the cloud has a cross-section rather similar to that of an optical lens.
It is often to be seen in the vicinity of mountains or high hills, arranged in well-separated bands stretching across the sky, and as we have said, it remains stationary, often for hours on end, locked in position relative to the ground below.
It is, of course, the mountains themselves that are responsible for the cloud's formation. As the large obstacle disturbs the smooth flow of air, wave-like undulations in the airflow form downstream. The resulting waves are called standing waves, because even though the air in which they form may be moving very rapidly, the position of each crest and each trough remains fixed in position relative to the mountain.
Lenticular cloud forms in these conditions if the air is moist. Air cools as it rises, and if it contains sufficient moisture even a slight fall in temperature may be enough to cause its relative humidity to reach 100 per cent, in which case condensation occurs and water droplets form.
It sometimes happens that condensation takes place in this way as the air rises to the crest of each standing wave, but as it descends again into a trough, the air becomes warmer and capable of reabsorbing the little drops of water moving with it.
The result is a series of long shallow cloud-bands arranged at right angles to the flow of wind, each one marking the position of the crest of one of a succession of standing waves.
In fact, when you look at a lenticular cloud, it is not, in a sense, the same cloud you see all the time, which is the point Mr Daniels tried to make. The cloud is constantly being formed on the windward side of each wave, and disappearing on the lee side, as the air descends and the water drops evaporate again.