lenticular cloud
A lenticular cloud is a lens- or disc-shaped cloud that forms on the lee side of mountains as a result of wavelike airflow. It is often mistaken for a flying saucer.
A lenticular cloud is a smooth-edged, lens- or disc-shaped cloud type, often stacked in layers one above another. Its most common form is lenticular cloud, but it can also form at lower and higher levels. It typically appears on the lee side of mountains, behind and above the ridge, and often stands almost motionless in the sky for hours while the wind blows through it.
How it forms
When a flow of moist air meets a mountain range, the air rises, then sinks and rises in a wavelike motion behind the mountain (wave flow or mountain wave). Where the upward-curving branch of the wave cools the air below the dew point, the water vapor condenses and a cloud appears. Where the air moves downward, the cloud dissipates. This way the cloud stays in place even though the air flows through it. Its typical altitude is a few thousand meters.
Connection to UFO phenomena
Because of its smooth, sharp-edged, often evenly lit and motionless shape, many people take a lenticular cloud for a flying saucer or another unknown object. According to skeptics and meteorologists, numerous UFO reports can be explained by lenticular clouds, especially at sunset, when the setting sun can paint the cloud in vivid colors.
Official and skeptical view
Meteorology describes it as a well-known, natural cloud formation. According to skeptics, such sightings are typical examples of how an everyday atmospheric phenomenon can be misinterpreted. Some ufologists say unknown objects may hide behind lenticular clouds, but this has not been proven.