Isolated lenticular cloud or altocumulus lenticularis cloud at sunrise and moonset. Lenticular clouds get their name from their smooth, lens-like shape. They are usually caused when wind forms standing waves of air on the lee side of mountains. Lenticular clouds form when moisture in the air condenses at the top of these waves and dissipate when the air descends again. The cloud pattern depends upon the wind speed & the shape of the mountains. A constant wind may produce clouds which are stable and remain virtually stationary in the sky for long periods. Lenticular clouds are sometimes mistaken for alien 'flying saucer' spacecraft. Photographed taken on November 2, 2001.