Lenticular Clouds

A lenticular cloud over a mountain



Lenticular clouds are very rare and usually form around hills or mountains. These clouds often look like giant flying sausers or upside down pancakes. They form when the air goes back and forth between higher and lower elevations. As the air gets higher in altitude it cools forming these clouds.

Lenticular clouds in Hawaii



Sources:
http://www.wisegeek.com/what-are-lenticular-clouds.htm

http://www.warren-wilson.edu/~physics/PhysPhotOfWeek/20050415LentCloud/index.shtml

Picture Sources:

www.starrynightphotos.com

www.vinaminh.com