Altocumulus lenticularis. These formations are caused by wave motions in the atmosphere and are frequently seen in mountainous or hilly areas. They are often called wave clouds. They may be triggered off by hills only a few hundred metres high and may extend downwind for over 100 km. The cloud elements form at the windward edge of the cloud and are carried to the downwind edge where they evaporate, as can be seen here. The cloud as a whole is usually stationary or slow moving.
Anyone who has been following my work over the last 4 years is well aware of my fascination for these unusual cloud formations, also called Lennies (by ME!); Mountain Wave Clouds, Cap Clouds, Magic Carpets, and … yes, Spaceship Clouds! The area I live and work here in the southern Washington Cascades breeds these ever changing and often brief formations prolifically at times. I should also mention that these clouds, particularly over Mount Adams and Mount Hood, have been in a variety of magazines, calendars, and on local news stations; they have been sold as large framed prints to a local North Shore Medical Clinic in White Salmon and also to private individuals, and have sold by the hundreds as greeting cards. Hope you enjoy my growing collection!