FAA Part 107 Remote Pilot (UAG)WeatherMedium
A remote pilot plans to operate a small UA in a valley located just downwind of a mountain ridge during moderately strong winds aloft. What hazard should the pilot most expect near the surface on the lee (downwind) side of the ridge?
- AA sea breeze circulation reversing the surface wind
- BA steady, laminar tailwind with no turbulence
- CA persistent radiation fog layer
- DStrong sinking air and rotor turbulence from orographic effects
Show answer & explanationAnswer & explanation
Correct answer: D. Strong sinking air and rotor turbulence from orographic effects
When wind flows over a mountain ridge, it is forced upward on the windward side and then descends rapidly on the lee side, often forming mountain waves and rotor turbulence near the surface. This orographic lifting effect can produce strong, unpredictable downdrafts and severe turbulence that are hazardous to small UA.
Why the other options are wrong
- A. A sea breeze requires a coastline and thermal contrast between land and water, not mountainous terrain.
- B. Airflow over ridges is rarely smooth; lee-side turbulence and rotors are common, not laminar flow.
- C. Radiation fog forms from surface cooling on clear calm nights, not from wind over terrain.
Orographic Lifting / Mountain Wave
Forced ascent of air over rising terrain, which can create mountain waves and severe rotor turbulence on the lee side.
- Windward side: forced ascent, cloud formation, precipitation
- Lee side: descending air, mountain waves, rotor turbulence
- Turbulence can extend far downwind and be severe
- Rotor clouds may form beneath the wave crest
Memory trick: 'Lee side leaps and rolls' — air rolls into rotors after crossing the ridge.