Mountain Waves

"A lenticular cloud is a lens-shaped cloud that normally develops on the downwind side of a mountain or mountain range. This occurs when stable, moist air flows over a mountain, creating a series of oscillating waves. If the temperature at the crest of the wave equals the dew point temperature, condensation occurs in a lens formation. As the air falls down the trough of the wave, where the temperature and dew point temperature are not equal, evaporation occurs. Thus, a wave cloud, or a series of lenticular clouds, is capable of forming."  - Weather Underground