These are stationary lens-shaped clouds that form in the troposphere, normally in perpendicular alignment 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.
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.
Reference: Wikipedia