A Lenticular cloud is a stationary lens-shaped cloud that forms at high altitudes, normally aligned perpendicular to the wind direction. 

These clouds have a characteristic lens appearance and smooth saucer -like shape, that's why they are often mistaken for  mysterious unidentified flying objects [UFOs].

Generally, lenticular clouds are formed as moist air speeds up while moving around a large land object such as a mountain or range of mountains, creating a series of large-scale standing waves on the downwind side.

If the temperature at the crest of the waves drops to dew point, moisture in the air may condense to form the lenticular clouds.

Lenticular clouds have also been known to form due to shear winds created by a front, even where there is no mountain.

Aircraft pilots avoid lenticular clouds due to its turbulent effect on the rotor systems. But gliders actively seek them out because the 'wave lift' resulting from the clouds is often very strong, enabling them to soar to remarkable altitudes and great distances.

 lenticular cloud #1