Clear Air Turbulence

Meteorology · Chapter 16 · DGCA CPL and ATPL

Turbulence with nothing to see: where it hides around jet streams, mountains and storm tops, and how to get out of it.

Remember

CAT: bumpiness at high level in clear air (or cirrus), where air streams of different speeds meet. Needs strong wind shear + steep lapse rate.
Commonest 23 000-39 000 ft, just below the tropopause, on jet fringes (cold side, below the axis), above the core, near upper fronts; also lee waves and over CB tops.
India: Oct-May north/central/NE (STJ), Jul-Aug south (TEJ); severe Dec-Feb. Patches ~120 km N-S × 250 km E-W, ~2000 ft deep.
Avoid jets; climb, descend or turn N/S; don't cruise just under the tropopause (1 km above often clear).

Why can a cloudless sky shake an airliner?

Bumps in a clear sky

Five cards. Tap Next.

No cloud, no warning CAT is the bumpiness felt at high altitude in clear air (CI aside), with nothing to see on radar or out of the window. It forms where bodies of air moving at very different speeds rub past each other. The worst region is the upper troposphere, about 23,000-39,000 ft, near the tropopause.
Shear and a steep lapse rate Two conditions: strong horizontal or vertical wind shear, and a steep lapse rate. Both are found just below the tropopause, close to a jet core. Even in the stable lower stratosphere, a sharp drop of wind with height can make patches of CAT.
On the fringes Most CAT is on the fringes of a jet stream, not in its core, and near upper fronts where temperature contrasts are strong. Strongest just below the axis on the cold side.
Waves that break Strong winds across a mountain range can set up gravity waves reaching up into the stratosphere; when they break, CAT follows, worst on the lee side. Mountain waves plus a jet stream give severe CAT. Strong winds blowing over CB tops make waves far downstream, sometimes marked by lenticular cloud.
Avoiding CAT Keep away from jet streams. If you meet CAT, climb or descend, or turn north or south to get out of it. About 1 km above the tropopause is often clear of it; just below the tropopause is the worst place.

Memory hook: CAT needs strong shear + a steep lapse rate. Find it on the fringes of jets, above mountains and over CB tops.

Takeaway: Clear air turbulence is bumpiness at high level in cloud-free air (apart from CI), where layers of air moving at very different speeds meet. It needs strong wind shear and a steep lapse rate, so it lives just below the tropopause on the fringes of jet streams, near upper fronts, over and downwind of mountains and above big CB tops.

Where round a jet stream is the turbulence?

Where CAT hides

A slice through a westerly jet: the equator on the left, the pole on the right. Tap Next.

The jet A westerly jet sits just below the break in the tropopause. Its isotachs (lines of equal wind speed) crowd together round the core, so the wind changes fast with height and from side to side. That change is called shear. CAT needs strong shear and a steep lapse rate. Jets with more than about 110 kt at the core often have significant turbulence.
The CAT layer CAT is most common high in the troposphere, from about 23,000 to 39,000 ft, close to the tropopause. Most of it is near jet streams. Unstable air makes it worse.
On the fringes Most CAT is on the edges (fringes) of a jet, not in its core. It is worst where the shear is strongest: on the cold, polar side just below the axis. There is also CAT above the core, where the speed falls off with height. This happens even in the stable lower stratosphere.
Upper fronts High up, a front is a zone where the temperature changes sharply over a short distance. Near these upper fronts the wind shear is strong too, and so is the CAT.
Crossing a jet Seen from above: you fly from the equator towards the pole across a westerly jet in the northern hemisphere. As you cross, the outside air gets colder, and the strong wind comes from your left.

Memory hook: Shear + steep lapse rate. Fringes, not core. Cold side, below the axis.

Takeaway: Clear air turbulence (CAT) is bumpiness high up in clear air or thin cirrus, where streams of air of very different speeds meet. It needs strong wind shear and a steep lapse rate. It is most common from 23,000 to 39,000 ft, just below the tropopause: on the edges (fringes) of jets, worst on the cold side just below the axis, above the core, and near upper fronts.