Lenticular cloud
Lenticular clouds are stationary clouds that form mostly in the troposphere, typically in parallel alignment to the wind direction. They are often comparable in appearance to a lens or saucer. Nacreous clouds that form in the lower stratosphere sometimes have lenticular shapes.
There are three main types of lenticular clouds: altocumulus standing lenticular, stratocumulus standing lenticular, and cirrocumulus standing lenticular, varying in altitude above the ground.
Formation and appearance
As air travels along the surface of the Earth, obstructions are often encountered, including natural features, such as mountains or hills, and artificial structures, such as buildings and other constructions, which disrupt the flow of air into "eddies", or areas of turbulence.When moist, stable air flows over a larger eddy, such as those caused by mountains, a series of large-scale standing waves form on the leeward side of the mountain. If the temperature at the crest of the wave drops below the dew point, moisture in the air may condense to form lenticular clouds. Under certain conditions, long strings of lenticular clouds may form near the crest of each successive wave, creating a formation known as a "wave cloud". Those wave systems can produce large updrafts, occasionally enough for water vapour to condense and produce precipitation.
Lenticular clouds have been said to be mistaken for UFOs, because many of them have the shape of a "flying saucer", with a characteristic "lens" or smooth, saucer-like shape. Lenticular clouds generally do not form over low-lying or flat terrain, so many people may have never seen one before and don't know that they can exist. Bright colours are sometimes seen along the edge of lenticular clouds.