FAA Sport Pilot Airplane (SPA)WeatherMedium
While flying near a mountain range, a pilot notices smooth, stationary, lens-shaped clouds hovering downwind of the ridge line, unmoving despite strong winds aloft. What condition do these clouds indicate?
- ADeveloping radiation fog along the ridge
- BAn approaching warm front with widespread stratus
- CFormation of a temperature inversion trapping surface pollutants
- DMountain wave activity with possible severe turbulence downwind of the ridge
Show answer & explanationAnswer & explanation
Correct answer: D. Mountain wave activity with possible severe turbulence downwind of the ridge
Standing lenticular altocumulus clouds form in the crests of mountain waves and remain stationary even in strong wind, signaling potential severe turbulence and strong up/downdrafts in the wave pattern downwind of the ridge.
Why the other options are wrong
- A. Radiation fog requires clear, calm nighttime conditions near the surface, not lens-shaped clouds at altitude.
- B. Warm fronts typically produce widespread layered stratus, not stationary lenticular clouds.
- C. Inversions trap pollutants near the surface and are unrelated to this cloud shape at altitude.
Standing Lenticular Clouds
Lens-shaped altocumulus standing lenticularis (ACSL) clouds form at the crest of mountain waves and remain stationary relative to the terrain despite strong winds, indicating turbulence.
- Smooth, lens or almond shaped, appear stationary
- Form on the leeward side of mountain ridges
- Warn of possible severe turbulence and strong vertical currents
Memory trick: Lenticular clouds look like flying saucers parked over the danger zone.