What natural phenomenon provides visual proof that a mountain wave exists?

The presence of a rotor cloud is a strong indicator that a mountain wave is occurring. Rotor clouds form when winds flow over a mountain range, creating turbulent eddies and waves in the atmosphere. As the air flows, it can separate from the mountain, causing turbulent conditions and giving rise to these distinctive cloud formations. Rotor clouds typically appear as billowing, horizontal bands of clouds, often aligning with the ridgelines of the mountains beneath them.

In contrast, thunderstorms, stratus clouds, and cumulonimbus clouds do not specifically indicate the presence of mountain waves. Thunderstorms are characterized by intense upward motion of air but are not directly associated with mountain waves. Stratus clouds are layer-type clouds that signify stable atmospheric conditions rather than turbulence associated with mountain waves. Cumulonimbus clouds indicate vertical development and severe weather but do not provide visual evidence of the specific wave phenomenon caused by mountains. Thus, rotor clouds serve as the visual proof for mountain waves due to their association with turbulent airflow and mountain-induced disturbances.