Lenticular clouds are stationary clouds that form mostly in the troposphere. They are often comparable in appearance to a lens or saucer. Nacreous clouds that form in the lower stratosphere sometimes have lenticular shapes.
Altocumulus standing lenticular (ACSL), stratocumulus standing lenticular (SCSL), and cirrocumulus standing lenticular (CCSL), are three main types of lenticular clouds. Because of their unique appearance, they have been suggested as an explanation for some unidentified flying object (UFO) sightings.