Mountain waves & turbulence

C. Mountain waves

Lens-shaped clouds that stay still over or downwind of a ridge mark the crests of standing mountain waves. Ragged edges mean turbulence.

A. Along the rotor cloud, on the leeward side of the ridge

The rotor is a rolling eddy that forms downwind of the ridge, under the wave. The air in and along the rotor cloud is violently turbulent, like the inside of a thunderstorm. Treat the whole rotor zone on the lee side as a no-go area.

A. Stable

A stable layer near ridge level makes the air bounce after crossing the ridge, like a spring, setting up the waves; less stable air above lets them extend upwards. Unstable air would just give convection.

B. 7 m/s

Small hills need about 7 m/s (around 15 kt) at ridge level; large ranges about 15 m/s.

A. 15 m/s

At ridge level about 15 m/s (around 30 kt) is needed over large mountain ranges, and about 7 m/s over small ones.

B. Lift off before its rotation point and stay above its flight path

Vortices start when the heavy aircraft's nose wheel lifts off and then sink, levelling off 500-1000 ft below its flight path. Lift off before that point and climb above its path and you stay out of them.

B. Unstable (less stable)

The wave needs a stable layer at about ridge height, which acts like a spring. The air above it on the leeward side is less stable, and that lets the wave grow upward and the lenticular clouds form in its crests.

A. Stable air and strong winds blowing across the mountains

Needed: a wind of adequate strength blowing within about 30 degrees of the perpendicular to the ridge, increasing with height, and stable air near ridge level.

A. 30 degrees

The flow must hit the ridge nearly head-on - within about 30 degrees of the perpendicular. Wind blowing along the ridge does not set up waves.

B. In the early afternoon

Thermal turbulence follows the heating of the ground, which peaks in the early afternoon - the bumpiest time for low-level flying on a hot day.

A. On the leeward side of the ridge

Rotor (roll) clouds form downwind of the ridge on the leeward side, under the wave crests. The strongest rotor is usually in the first wave downwind. Cap cloud sits on the ridge itself, and lenticulars sit higher up in the wave crests.

B. Climb above its flight path

Wingtip vortices sink below the path of the aircraft that made them, typically 500-1000 ft, and then drift downwind. So the safe air is ABOVE the generating aircraft track. Descending through the vortex core is the one thing that puts you into it.

A. Rotor clouds

The rotor zone under the wave crests holds the most violent turbulence of a mountain wave - comparable to a violent thunderstorm, with up- and down-gusts.

B. Mountain waves

Rotor (roll) clouds form downwind of a ridge, under the crests of strong mountain waves, and mark the worst turbulence in the wave system.

C. Greater on the windward side

The air is forced to rise on the windward slope, cools at the adiabatic lapse rate, reaches saturation and forms orographic cloud and rain there. Having dropped its moisture it descends the far side and warms, which is why the leeward side is comparatively dry - the rain shadow.

A. Sink below its flight path and drift downwind

The pair of vortices descends at a few hundred feet a minute and levels off roughly 500-1000 ft below the path of the aircraft that made them. Near the ground they cannot keep sinking, so they move outwards along the surface and then drift with the wind. Both parts matter: stay above the path, and think about where the wind is taking them.

A. Heavy, clean and slow

The correct answer is: Heavy, clean and slow.

A. Beyond its touchdown point

The heavy aircraft stops making vortices at the point where its wheels touch and the wing stops lifting. The air beyond that point is therefore clear, and the vortices lie behind it. Touching down beyond its touchdown point keeps you out of them; landing short puts you straight into them.

A. Upwind vortex, which can be held over the runway

Once the vortices reach the ground they move outwards at a few knots. A light crosswind of about that speed cancels the outward movement of the UPWIND vortex, so instead of clearing the runway it sits on it. The downwind one is carried away quickly. A light crosswind is therefore worse than a strong one.