Lenticular

Lenticular clouds (altocumulus standing lenticularis) are stationary lens-shaped clouds formed at high altitudes. Their specific features cause them to often be mistaken as UFO's. 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 crest of these waves. Please read the complete excerpt at Wikipedia.com.