`The apparent permanency and stationary aspect of a cloud," wrote the 19th-century meteorologist J.F. Daniels, "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."
He was referring, if you get his drift at all, to Altocumulus lenticularis, a particularly attractive feature of the sky, sometimes seen in the vicinity of mountains or high hills. Unlike most other clouds that are carried along by the wind, lenticular cloud, as it is sometimes called, appears fixed in space, never moving from the spot where it has formed.
Imagine a moist air-mass briskly approaching a ridge of mountains in its path. Although the airflow may be quite smooth to start with, the distortion caused by the mountain barrier sometimes results in a wave-like undulation downwind.
On the lee, or sheltered, side of the obstruction, the air moves up and down in a way which resembles the ripples which form on the surface of a pond into which a stone is thrown. These ripples in the airflow can survive for a surprisingly long distance downwind of a mountain obstacle.
Now the air ascending towards the crest of each wave suffers a drop in temperature, as air nearly always does if forced to rise. If conditions are right, the temperature drop may be sufficient to require condensation to take place, allowing cloud to form.
But as the air descends into the next trough, it becomes warmer again and capable of reabsorbing the droplets of water moving along with it, and the cloud disappears. The result is a series of long shallow cloudbands at right angles to the flow of wind.
The clouds are called "lenticular" because they have thin and sometimes pointed ends with a thicker and broader centre, resulting in a cross-section similar to that of an optical lens - rather like an almond, in fact.
The waves which cause these lenticular clouds are called "standing" waves because they remain the same distance from the mountain, even though the air itself is moving along at considerable speed. And since the waves remain fixed in position, so, too, do the clouds which form at their crests.
In fact, as you look at a lenticular cloud, it is not the same cloud you see all the time; there is, as Mr Daniels had observed, an optical deception. The cloud is constantly being formed on the windward side as moisture condenses from the rising air, and disappearing on the lee side, as the air descends and reabsorbs the water droplets travelling with it.