FAA Sport Pilot Airplane (SPA)WeatherHard
A pilot planning a flight over mountainous terrain is briefed that stable air is flowing at 35 knots perpendicular to a ridge line. What hazard should the pilot expect on the leeward (downwind) side of the ridge?
- ARadiation fog forming along the ridge crest
- BStrong up- and downdrafts, severe turbulence, and possible rotor clouds
- CSmooth, laminar airflow with reduced headwinds
- DReduced density altitude that improves climb performance
Show answer & explanationAnswer & explanation
Correct answer: B. Strong up- and downdrafts, severe turbulence, and possible rotor clouds
Strong, stable winds blowing perpendicular to a mountain ridge create mountain waves on the lee side, producing severe turbulence, strong up/downdrafts, and rotor clouds that can be extremely hazardous to light aircraft.
Why the other options are wrong
- A. Radiation fog requires calm, clear conditions, not strong winds over terrain.
- C. This is the opposite of the turbulent conditions actually produced.
- D. Wind and terrain interaction do not directly change density altitude.
Mountain Wave Turbulence
Occurs when stable air is forced over a mountain ridge by strong perpendicular winds, creating oscillating waves and severe turbulence on the leeward side.
- Requires strong wind (typically 25+ kt) roughly perpendicular to ridge
- Produces rotor clouds and lenticular (cap) clouds
- Can cause severe turbulence and updrafts/downdrafts far above ridge height
Memory trick: Mountain wave = wind 'surfing' over the ridge, crashing hard on the lee side.