Cruise can be glass-smooth for an hour and then, with a clear sky ahead and nothing on the radar, the aircraft starts to buck. More often than not the cause is the jet stream, and understanding it turns a nasty surprise into something you can plan around.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What a jet stream is
A jet stream is a narrow, fast-flowing river of air high in the atmosphere, near the tropopause. It forms where there is a strong horizontal temperature contrast, such as the boundary between cold polar air and warmer mid-latitude air, and it can wind for thousands of miles around the globe while being only a few hundred miles wide and a few thousand feet deep.
As the FAA Aviation Weather Handbook describes, the cores are commonly 100 knots or more. The two that matter most at mid-latitudes are the polar-front jet, which is lower and stronger and shifts with the seasons, and the subtropical jet, which is higher and further toward the equator.
Two thresholds get muddled here, and they are worth separating, because one defines the thing and the other only decides whether it is drawn.
- What makes it a jet stream at all is a speed floor, though the figure depends on who is defining it. The WMO and ICAO convention puts the minimum core speed at 60 knots (30 m/s), as SKYbrary sets out, while the FAA's own definition in AC 00-30C uses 50 knots. Below the relevant floor you have a strong upper wind rather than a jet.
- What gets it onto the chart is a higher bar. On the high-level significant weather charts produced under the World Area Forecast System, a jet is drawn as a bold arrow only when its core reaches 80 knots or more, with the core height and speed labelled. That figure is a charting convention, set out in the WAFC and Met Office guide to interpreting WAFC SIGWX forecasts at section 3.5.1, and not a wind-speed threshold in ICAO Annex 3 itself, which sets none.
The gap between the two is a real trap. A 65-knot core is a jet stream by definition and will not be drawn on the chart, so a chart with no jet arrow across your route is not a promise of no jet and no shear. Read the forecast winds aloft as well.
Why the shear turns into turbulence
The danger is not the strong wind itself, which an aircraft flies through happily, but the way the wind speed changes around the core. The fast core sits next to much slower air above, below and to either side, so over a short distance the wind speed and direction change sharply. That change is wind shear, and where it is strong enough it breaks down into clear-air turbulence (CAT).
The word "clear-air" is the whole problem. As the FAA AIM and SKYbrary note, CAT forms in cloud-free air, so there is no cloud to mark it and weather radar, which sees precipitation, does not detect it. You feel it before you can see anything.
Where to expect it
CAT is not scattered at random. It clusters in predictable places relative to the jet:
- on the cold (poleward) side of the core and just below it, where the shear is sharpest;
- near sharp upper troughs and where the jet curves tightly or two jets cross;
- in the shear zone just under the tropopause.
A worked example
Suppose your route crosses a polar-front jet and the forecast winds aloft show the core at FL340 running 130 knots, while the level you are planning, FL300, shows 70 knots from a slightly different direction. That is a change of roughly 60 knots and a shift in direction across only 4000 feet of height: a classic strong-shear signature on the cold side and below the core.
Nothing on the satellite picture or the radar marks it, but the significant weather chart draws the jet arrow there and the shear is exactly where CAT lives. The practical move is the same one professional crews make: note it at the planning stage, keep the seatbelt sign on through the band, and if the ride deteriorates, a change of a few thousand feet up or down often climbs out of the worst shear layer. A pilot report passed on helps the aircraft behind you.
Common pitfalls
- Radar will not save you. Weather radar paints precipitation, not the dry shear of CAT, so a clear screen is no promise of a smooth ride near a jet.
- The strong wind is not the hazard; the change in it is. A steady 120-knot wind is smooth; it is the gradient at the edges of the core that bites.
- It is a cruise-level phenomenon. Because jets sit near the tropopause, CAT is mostly a concern at the higher levels, not in the circuit.
- No arrow on the chart is not no jet. The 80-knot mark is the WAFC charting threshold, not the definition; a jet core of 60 to 79 knots is still a jet and still shears, and it will not be drawn.
In Pilot EFB
Pilot EFB helps you read the winds and temperatures aloft and the forecasts that flag where shear is likely, so the jet is on your mind before the flight rather than a mid-cruise surprise, and it keeps the pilot reports and significant weather products you have pulled together in one place. A briefing you have already pulled stays readable with no signal; fetching fresh forecasts needs a connection. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.