Rotor and lee-side turbulence: the hazard you can't see
When wind meets a mountain or ridge, it is pushed up on the windward side, producing smooth lift. On the lee side, the air tumbles down and can break into rotor: a rolling, turbulent flow. It is invisible, which makes it dangerous.
Where rotor appears
- Behind ridges, cliffs and hills.
- Behind trees, buildings and hangars near a landing field.
- In the lee of a launch when wind is strong.
Warning signs
- Wind that is stronger than the forecast.
- Lenticular clouds (smooth lens-shaped clouds) on a mountain crest indicate strong waves.
- Ragged, shredded clouds and dust or leaves swirling.
- Other pilots struggling or deciding not to launch.
Rules of thumb
- Stay on the windward side when the wind is strong.
- Keep height above the ridge when crossing and give the lee a wide margin.
- Don’t launch in the lee, or in wind blowing off the back of the hill.
- Avoid landing behind obstacles with strong wind: pick a field with clear approach.
- If the forecast is for strong wind, stay on the ground.
If you get caught
Fly away from the terrain, keep the wing loaded with some speed, don’t over-react with the brakes, and be ready to use your active flying skills. Prepare early to leave the area.
Why this matters
Turbulent air close to the ground leaves little height to recover. Many serious accidents begin with a pilot entering turbulence at low altitude.