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 mountains. Wind blows most types of clouds across the sky, but lenticular clouds seem to stay in one place. Air moves up and over a mountain, and at the point where the air goes past the mountaintop the lenticular cloud forms, and then the air evaporates on the side farther away from the mountains.
As this photo on this page shows, lenticular clouds are lens-shaped and look like flying saucers.
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