Mountain wave turbulence could be anticipated under which combination of conditions?

• A. Winds of 40 knots or greater blow across a mountain ridge and the air is stable
• B. Winds calm over flat terrain
• C. Moist air with no wind
• D. Strong winds with strongly unstable air

Answer: (A)

Mountain wave turbulence occurs when stable air flows over a mountain, producing vertical waves that can extend downstream and cause turbulence in the lee. The most reliable situation for anticipating these waves is strong winds—roughly 40 knots or more—blowing across the ridge while the air remains stable enough to support wave formation. In this setup, the airflow is forced upward and downward in a regular pattern, which creates the potential for significant turbulence, rotor activity, and sometimes cloud formations like lenticular clouds downwind. If the wind is calm, there’s no wave generation; if the air is moist with no wind, there’s still no forcing to create the mountain waves; if the air is strongly unstable, convection disrupts the orderly wave pattern, reducing the predictability of mountain wave turbulence.