Lenticular clouds are stationary lens-shaped clouds
that form at high altitudes, normally aligned
perpendicular to the wind direction.
Due to their shape, they are often mistaken as UFOs.
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. If the temperature at the
crest of the wave drops to the dew point, moisture in the
air may condense to form lenticular clouds. As the moist
air moves back down into the trough of the wave, the
cloud may evaporate back into vapor. Under certain
conditions, long strings of lenticular clouds can form near
the crest of each successive wave, creating a formation
known as a 'wave cloud'. The wave systems cause large
vertical air movements and so enough water vapor may
condense to produce precipitation. Bright colors
are sometimes seen along the edge of lenticular clouds.