FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A dispatcher is planning a flight that will cross a mountain range where surface winds are forecast at 35 knots nearly perpendicular to the ridge line, with a stable air mass aloft. What condition should the dispatcher expect on the lee side of the range?
- ASmooth, laminar airflow with no turbulence
- BWidespread advection fog over the ridge
- CRadiation fog forming in the mountain valleys
- DMountain wave turbulence with possible rotor clouds
Show answer & explanationAnswer & explanation
Correct answer: D. Mountain wave turbulence with possible rotor clouds
Strong winds (generally 25+ knots) blowing perpendicular to a mountain ridge combined with stable air aloft produce mountain wave activity, often visible as lenticular clouds aloft and turbulent rotor clouds near the surface on the lee side. This can produce severe to extreme turbulence for aircraft.
Why the other options are wrong
- A. Strong perpendicular flow with stability produces waves, not smooth laminar flow.
- B. Advection fog requires warm moist air over a cold surface, unrelated to this wind/stability scenario.
- C. Radiation fog requires calm, clear conditions, not strong winds over terrain.
Mountain Wave Turbulence
Turbulence caused by strong wind flow perpendicular to a mountain ridge in a stable atmosphere, creating oscillating wave patterns downwind.
- Requires wind ≥25 kt roughly perpendicular to ridge and stable air aloft
- Lenticular clouds mark wave crests; rotor clouds mark turbulent rotors near the surface
- Can produce severe to extreme turbulence and strong up/downdrafts
Memory trick: Stable air + strong wind over a ridge = waves like water over rocks.