A "Mountain Wave" is a type of atmospheric phenomenon that occurs when strong winds flow over mountain ranges. As the air moves upward and then descends on the leeward side, it creates a series of waves in the atmosphere. These waves can lead to turbulence and are often associated with clear skies, making them difficult to detect visually. Mountain waves can also generate lenticular clouds, which are lens-shaped clouds that form at the crest of the wave. Pilots need to be aware of mountain waves, as they can affect flight stability and safety, especially in areas near mountains or hills.