050 Meteorology topic guide
Jet Streams and Clear Air Turbulence
A jet stream is where the thermal wind effect concentrates. A strong horizontal temperature gradient stacked through a deep layer produces a correspondingly strong increase of wind with height, and that increase is largest exactly where the tropopause itself steps down, at the boundary between two air masses. The result is a narrow, fast-flowing ribbon of air rather than a smoothly graded wind field.
Two such ribbons matter for the syllabus: the polar front jet, tied to the boundary between polar and mid-latitude air, and the subtropical jet, tied to the poleward edge of the tropical Hadley circulation. Both sit at different typical heights, both strengthen in winter, and both carry the clear air turbulence that catches a cruising aircraft without any cloud to give it away.
Two jets, two origins
The polar front jet forms along the same polar front that produces mid-latitude depressions, so its position is variable, tracking the front itself. It is generally stronger and lower in winter, when the temperature contrast between polar and tropical air is greatest, and it is typically found roughly between FL300 and FL400, with winter cores commonly quoted as exceeding 200 kt.
The subtropical jet forms at the poleward edge of the Hadley cell, where air that has risen near the equator and travelled poleward aloft is deflected strongly eastward. It sits at a generally higher and steadier altitude than the polar front jet, is less tied to any single surface feature, and also strengthens in winter as the temperature gradient at its latitude sharpens.
- Polar front jet: tied to the polar front, more mobile, typically FL300 to FL400, strongest in winter
- Subtropical jet: tied to the edge of the Hadley cell, generally higher and steadier, also winter-favoured
Locating clear air turbulence around the core
Clear air turbulence is produced by wind shear, not by wind speed alone, so it concentrates on the cold, poleward side of the jet core, where the horizontal temperature and wind gradient is steepest, rather than directly in the core itself where the flow is comparatively smooth. Vertical shear is also strong immediately above and below the core, so changing level without changing side is not a reliable fix on its own.
Jet curvature and the entrance and exit regions of the jet add further along-stream acceleration or deceleration, which increases shear rather than reducing it. Significant weather charts commonly mark turbulence in exactly these locations: the cold side of a jet axis, and around ridges, troughs, and jet entrances and exits, rather than along the smooth core itself.
Seasonal shift and chart recognition
Both jets migrate with the sun's seasonal heating, moving poleward and weakening in summer and shifting equatorward and strengthening in winter, tracking the same temperature gradients that create them. A stem describing an unusually strong, low-latitude jet is therefore almost always describing winter conditions, not a random encounter.
On an upper wind or significant weather chart, a jet shows up as closely packed contour lines, or isohypses: the tighter the packing, the stronger the implied wind, which is exactly what the thermal wind relationship predicts from a sharp temperature gradient.
Worked example
Worked example: avoiding clear air turbulence near a jet
An aircraft is planning a crossing near a strong winter jet stream. To reduce the risk of clear air turbulence without a major route change, on which side of the jet core, and away from which features, should the crew expect the least disturbed air?
- AThe cold, poleward side of the core
- BThe warm, equatorward side of the core, away from the tightly packed contours
- CDirectly beneath the core at a slightly lower level
- DIn the jet exit region, where the flow is diverging
Show the answer and walkthrough
Correct answer: B
- A. This is exactly where clear air turbulence concentrates, since the horizontal shear is steepest there, the opposite of what reduces risk.
- B. Correct. The gradient, and the shear it drives, is gentler on the warm side of the core, and staying clear of the tightly packed contours avoids the strongest part of the jet.
- C. Vertical shear is also strong immediately above and below the jet core, so simply descending a little without changing side does not reliably avoid the turbulence.
- D. Entrance and exit regions add extra along-stream acceleration or deceleration, which increases shear rather than reducing it, so this is a favoured location for turbulence, not a calmer one.
Step by step
- Clear air turbulence is produced by wind shear, and the shear is greatest on the cold, poleward side of the jet core, where the temperature and wind gradient is steepest.
- The warm, equatorward side has a gentler gradient, so the shear, and the turbulence it produces, is weaker there.
- Vertical shear is also strong immediately above and below the core itself, so changing level alone does not reliably avoid the turbulence.
- Jet entrance and exit regions add extra acceleration or deceleration along the flow, which increases shear rather than reducing it.
- The combination that most reliably reduces risk is moving to the equatorward side of the jet and staying clear of the tightly packed contours that mark the core.
Common mistakes
Assuming the jet core itself is the roughest air
The core is where the wind is fastest, not necessarily where the shear, and therefore the turbulence, is greatest. Exam stems test whether a candidate separates wind speed from wind shear.
Placing both jets at the same altitude
The polar front jet is generally lower and more variable, while the subtropical jet sits higher and steadier. A stem naming a specific altitude is testing which jet is being described.
Forgetting the seasonal cycle
Both jets strengthen and shift equatorward in winter as the temperature gradient sharpens, so a question describing a stronger, lower-latitude jet is pointing to winter conditions, not a random encounter.
Related topic guides
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Last reviewed July 2026