FAA Ground School Practice Test

When could possible mountain wave turbulence be expected?

1. When the winds blow gently across a valley

2. When winds of 40 knots or greater blow across a mountain ridge in unstable air

3. When winds of 40 knots or greater blow across a mountain ridge, and the air is stable

4. When winds are calm in mountainous areas

The correct answer is: When winds of 40 knots or greater blow across a mountain ridge, and the air is stable

Possible mountain wave turbulence is most likely to occur when strong winds, specifically 40 knots or greater, blow across a mountain ridge, and the air is stable. In stable air conditions, the atmosphere resists vertical motion, which can lead to the air being forced up and down as it flows over the ridges. This phenomenon creates a pattern of turbulence both above and downwind of the ridges, as the stable air tends to trap the energy of the wave created by the wind interacting with the mountain. In such scenarios, the wind's speed contributes significantly to the intensity of the mountain wave effect, and the stable air forms a crucial element in amplifying that turbulence. This creates conditions that can be hazardous for aircraft as they may experience sudden changes in altitude and severe downdrafts or updrafts. In contrast, when the winds are gently blowing across a valley or when they are calm in mountainous areas, there is insufficient wind energy and instability to create mountain wave turbulence, making these scenarios less conducive to such turbulence. Additionally, while winds of 40 knots or more indeed contribute to turbulence, the air must also be stable to create significant mountain wave activity, reinforcing the importance of both factors in this context.