Turbulence

Turbulence is rough air. Name it by its cause:

• Mechanical: wind flowing over rough ground, hills, or buildings. Stronger with a stronger wind and rougher ground.
• Thermal (convective): rising thermals from daytime surface heating. Worst on a hot afternoon over dry ground; it fades after sunset.
• Frontal: the lifting of warm air plus the wind shear at the frontal surface. Worst at a fast cold front.
• Wind shear: a change in wind speed or direction over a short distance. Shear is the mechanism behind much of the rest.
• Mountain wave: standing waves downwind of a ridge. Needs stable air and a strong wind blowing across (perpendicular to) the ridge. Reaches far downwind and well above the ridge.
• Clear-air turbulence (CAT): shear near the jet stream at high altitude, with no cloud to warn you.

Mountain-wave cloud signs:

• Lenticular (lens-shaped) clouds sit on the wave crests.
• Rotor clouds mark the severe, tumbling turbulence beneath the wave. This is the dangerous part.

Low-level wind shear (LLWS) is shear near the ground, dangerous on approach and departure. Its sources are fronts (especially sharp, fast ones), low-level temperature inversions, and thunderstorms / microbursts.

Source note: the turbulence-cause physics and the mountain-wave cloud signs here are grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text; the physics is universal and matches the Canadian material (equivalent to TP 9352). The TC AIM MET stays cited for the Canadian products: CAT (2.2), wind shear (2.3), and the GFA turbulence categories (MECH, LEE WV, CAT, L LVL WS).

Trap: expecting a mountain wave in unstable air, or with the wind along the ridge. It needs stable air and a strong perpendicular wind. And do not read lenticular cloud as the hazard: the smooth lens marks the crest, but the rotor cloud below marks the violence.

Practice