What might indicate that aircrews are encountering mountain wave activity?

Clouds forming in wave patterns is a strong indicator that aircrews are encountering mountain wave activity. When strong winds blow over mountain ranges, the airflow is disrupted, causing oscillating waves of air to develop downwind. This results in the formation of lenticular clouds or other cloud types that exhibit a characteristic wave appearance. These clouds serve as visual cues for pilots, indicating that they may experience turbulent conditions associated with mountain wave activity.

In contrast, the other options do not directly signify mountain wave activity. Clear skies with no clouds indicate stable conditions without significant turbulence, while ice on the wings can be related to environmental moisture but does not specifically indicate mountain waves. Lastly, constant speed of the aircraft may suggest stable flight conditions but does not provide any information regarding the presence of mountain wave activity. Therefore, observing clouds in wave patterns is the most reliable indicator of this phenomenon.