FAA Private Pilot Airplane (PAR)Weather Theory and ServicesHard
A pilot planning a flight over mountainous terrain reviews the weather briefing and notes strong winds forecast to blow perpendicular to a mountain ridge, combined with a stable atmosphere. Which hazard should the pilot be most prepared to encounter on the downwind (lee) side of the ridge?
- AWidespread radiation fog forming in the mountain valleys
- BMountain wave turbulence, possibly including rotor clouds and severe turbulence
- CIsolated afternoon thunderstorms due to strong surface heating
- DSmooth, laminar airflow with negligible turbulence
Show answer & explanationAnswer & explanation
Correct answer: B. Mountain wave turbulence, possibly including rotor clouds and severe turbulence
Strong winds blowing perpendicular across a mountain ridge in a stable atmosphere create mountain wave activity, producing turbulence, strong up- and downdrafts, and rotor clouds on the lee side that can extend well above the ridge height. This hazard is often noted in an AIRMET Tango for turbulence.
Why the other options are wrong
- A. Radiation fog requires calm, clear conditions at night, not strong winds over terrain.
- C. Thunderstorms require instability and strong surface heating, not the stable conditions described.
- D. Strong wind flow over ridges in stable air produces turbulence, not smooth laminar flow.
Mountain Wave Turbulence
Mountain wave turbulence forms when strong winds blow perpendicular to a mountain ridge in a stable atmosphere, creating oscillating airflow, severe turbulence, and rotor clouds on the lee side.
- Requires strong wind (typically 25+ kt) roughly perpendicular to the ridge
- Occurs in stable air, unlike convective turbulence
- Can produce severe turbulence, downdrafts, and rotor clouds far downwind
Memory trick: Stable air + strong wind over a ridge = wave and rotor rough ride