Under what conditions is mountain wave turbulence likely to occur?

Mountain wave turbulence is most likely to occur when strong winds of 40 knots or greater blow across a mountain ridge, especially when the air is stable. This phenomenon happens due to the interaction of fast-moving air with the terrain, leading to the formation of powerful wave patterns in the atmosphere.

In stable air conditions, the air cannot rise freely, causing it to flow around and over the mountain ridges instead. As the wind encounters the mountains, it generates waves that propagate downwind, creating turbulence both in the upwards and downwards motion of the air. This can result in significant atmospheric disturbances, which are particularly hazardous for aircraft flying near mountainous terrain.

The other choices reflect conditions that do not typically lead to mountain wave turbulence. Low wind speeds may not generate the necessary energy for wave formation, and the dynamics involved with thunderstorms create a different type of turbulence not associated with mountain waves. Additionally, a lack of wind does not contribute to the development of the waves required for this turbulence type.