Lenticular, or lee wave, clouds form downwind of an obstacle in the path of a strong air current. In the Boulder, Colorado, area, the obstacle is the Front Range of the Rocky Mountains, seen through clouds at the bottom of the picture.
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Courtesy of UCAR Digital Image Library
Lenticular clouds form on the downwind side of air flowing over a large mountain or mountain range. Unlike most clouds that move across the sky, lenticular clouds stay in one place as air appears to blow through them. What is really happening is that the cloud forms on the side closest to the mountains where air is uplifting over the top of the mountain. Then the cloud moves with the wind and evaporates on the downwind side, so it appears stationary even though air is moving through the cloud.
As this photo on this page shows, lenticular clouds are lens-shaped and resemble flying saucers.
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