Lenticular Clouds – defined by NOAA as “A very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.”
1. The most common source of UFO sightings until the advent of Chinese Lanterns
2. Their Latin name Altocumulus lenticularis mean “like a lens” because they look like a lens, obviously.
3. Lenticular clouds produce no rain
4. They are very stable clouds and hang essentially motionless in the sky.
The met office explain how they form much better than I can
“When air blows across a mountain range, in certain circumstances, it can set up a train of large standing waves in the air downstream, rather like ripples forming in a river when water flows over an obstruction. If there is enough moisture in the air, the rising motion of the wave will cause water vapour to condense, forming the unique appearance of lenticular clouds.”
5. Lenticular clouds are a sign of a stable, humid layer of air over hills and mountains. As such glider pilots tend to seek them out. The world record, according to Wiki, for a glider is 1,864 miles in distance covered and 50,721 ft in altitude by gliders in these clouds.
6. Pilots of aircraft that are powered will try to avoid lenticular as they harbour potentially dangerous turbulence