Under which conditions might one expect a major turbulence event in mountainous flying?

A major turbulence event in mountainous flying is most likely to occur when there is a stable air layer above the mountains. This situation can lead to a phenomenon known as "mountain wave turbulence." When strong winds flow over mountains, they can generate waves of air that propagate downstream. If the air is stable, or if there is a temperature inversion, these waves can become pronounced, potentially leading to severe turbulence as the aircraft encounters the oscillating air currents.

Stable air layers act to trap the turbulent air below them, and when an aircraft moves through these conditions, it can experience significant updrafts and downdrafts associated with the waves. This turbulence can be exacerbated if the winds are strong, leading to even more challenging flying conditions.

In contrast, calm weather or light winds might not produce the same level of turbulence, while descending through low clouds could suggest other weather-related factors at play, which may not be directly related to the mountain wave phenomena.