Fantastic lenticular cloud at Sheregesh (Kemerovo region, 11.27.2016)
Lenticular clouds, technically known as altocumulus standing lenticularis, are stationary lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction.
As air flows along the surface of the Earth, it encounters obstructions. These are human-made objects, such as buildings and bridges, and natural features, like hills, valleys, and mountains. All of them disrupt the flow of air into eddies. The strength of the eddies depends on the size of the object and the speed of the wind. It results in turbulence classified as ‘mechanical’ because it is formed through the “mechanical disruption of the ambient wind flow”.
Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves. Under certain conditions, long strings of lenticular clouds can form, creating a formation known as a wave cloud.
Power pilots tend to avoid flying near lenticular clouds because of the turbulence of the rotor systems that accompany them, but sailplane pilots actively seek them out. This is because the systems of atmospheric standing waves that cause “lennies” (as they are sometimes familiarly called) also involve large vertical air movements, and the precise location of the rising air mass is fairly easy to predict from the orientation of the clouds.
As of 2016 the gliding world records for both distance (over 3,000 km; 1,864 mi) and absolute altitude (15,460 m; 50,721 ft) were set using such lift.
Lenticular clouds have been mistaken for UFOs (or “visual cover” for UFOs) because these clouds have a characteristic lens appearance and smooth saucer-like shape.