Turbulence and Jet Streams

The module ends with the wind at its roughest and its fastest. Turbulence is what disturbed air does to an aircraft, and it comes in a handful of nameable types with nameable homes; the jet streams are the fastest winds in the atmosphere, ribbons of 60 kt (knots) or more threading the tropopause breaks. The two subjects belong together, because the nastiest turbulence of all, clear-air turbulence, lives beside the jets.

Turbulence and its types

Turbulence is the irregular, chaotic motion of the air: the succession of bumps, jolts and attitude disturbances an aircraft feels when the flow it rides is no longer smooth. Gustiness is the same irregularity read by an anemometer: rapid fluctuations of wind speed and direction around the mean. The exam's list of common types sorts turbulence by what stirs the air:

Convective turbulence — thermals rising from heated ground in unstable air.
Mechanical turbulence — the friction layer's eddies, wind tumbling over rough ground and obstacles.
Orographic turbulence — disturbed flow over hills and mountains, mountain waves and their rotors included.
Frontal turbulence — the lifting and wind change along a front.
Clear-air turbulence (CAT) — high-level turbulence away from cloud, the subject of its own section below.

Where turbulence forms

Each type forms where its stirring mechanism lives. Convective turbulence needs sunshine and instability: thermals leave the heated surface, and every climb or descent through the convective layer of a summer afternoon crosses them. Mechanical turbulence needs wind and roughness: the stronger the wind and the rougher the surface, the deeper and coarser the eddies — the friction layer of the module's first lesson at its most physical. Orographic turbulence is the mountain's version, from simple tumbling in the lee of a hill to the organised violence of the previous lesson's rotors. Frontal turbulence rides the boundary where one air mass forces another aloft.

Put on a chart, the homes of turbulence are predictable: rough ground and strong relief, inversion layers, cumulonimbus (CB) and thunderstorm (TS) zones, and unstable layers generally. The inversion deserves its place on that list: the smooth, stable lid itself is calm, but the shear across an inversion's top (fast air sliding over the stagnant layer beneath) makes the boundary bumpy to cross.

Clear-air turbulence

CAT is significant turbulence at high level away from convective cloud: no cloud to mark it, no radar echo to paint it, no warning before the first jolt. It forms where the wind changes speed or direction sharply over a short distance: strong wind shear rolls the smooth flow up into invisible eddies. That places CAT where the shear is: beside and below jet streams, in sharp high-level troughs, and in other disturbed high-level flows — the last section of this lesson maps the jet's worst corner, and the flight-hazards module returns to it.

Two operational facts close the subject. CAT cannot be remotely sensed from satellites, and its forecasting is limited: the charts outline probability areas, not certainties. Which is why pilot reports of turbulence are a very valuable source of information — the aircraft ahead is the only instrument that has actually measured the air, and its report is the freshest truth the system has.

Jet streams

Jet streams are narrow, flattened ribbons of very strong wind near the tropopause, with marked shear on every side of the core. The definition is numeric: a jet stream's wind is 60 kt or more; anything slower is just strong wind. The typical dimensions explain the ribbon image: thousands of kilometres long, a few hundred kilometres wide, and only a few kilometres deep — a shape like a seam of fast air pressed flat between the layers.

The formation mechanism is the warm and cold columns of the pressure lesson, stood side by side at a front. A tall warm column beside a short cold one means a pressure difference between them that grows with height, so the wind that difference drives grows with height too — strongest where the contrast has had the whole troposphere to accumulate, just below the tropopause. A jet, in one sentence, is the wind of a strong horizontal temperature contrast, concentrated at the top of the troposphere.

Four jets carry names, and their properties are exam recall. All the westerly jets share one seasonal rule: they are strongest in winter, when the temperature contrast that drives them is greatest, and they follow their parent fronts and cells equatorward in winter, poleward in summer.

Polar front jet — westerly, above the polar front near FL 300 (flight level 300), mid-latitudes (roughly 40-60°), all year but strongest and furthest equatorward in winter; it meanders with the front, and winter cores of 150-200 kt occur, the strongest over east Asia and Japan.
Subtropical jet — westerly, higher, near FL 390-400 over the sinking branch of the Hadley cell (roughly 25-35° in winter, drifting poleward in summer); blowing all year, with winter cores commonly around 100 kt, steadier than the polar front jet's extremes.
Arctic front jet — westerly, at the arctic front in high latitudes, lower (around FL 150-250) and generally weaker than the polar front jet, mainly a winter phenomenon.
Tropical easterly (equatorial) jet — the exception in every column: easterly, and a northern-summer phenomenon only, blowing high (around FL 450-500) over southern Asia and Africa with the summer monsoon, with cores around 100 kt.

Turbulence and its types

Where turbulence forms

Clear-air turbulence

Jet streams

Check you understood

Jets, the tropopause and CAT

What the exam asks