Lenticular Clouds (aka 'Lennies' and often bearing a marked resemblance to a UFO) are stationary
lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction.

Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing
waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves.
(from the Wikipedia)

Superstition Mountain in the distance.

Photographed in the Sonoran Desert from north of Mesa, Arizona USA.
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