What is clear air turbulence?
It is turbulence with nothing to see: no cloud, no storm, nothing on the weather radar in the nose. That is exactly why it has to be forecast rather than avoided by eye — and why it is the kind that arrives without warning.
Clear air turbulence sits where the wind changes speed fastest — usually the edge of the jet stream.
Where clear air turbulence comes from
Air is a fluid, and fluids tear when two parts of them slide past each other at different speeds. High in the atmosphere there is a river of wind doing exactly that — the jet stream, a ribbon of air moving at two hundred knots or more with much slower air on either side of it. The boundary between the two is not a clean line. It shreds into eddies, and an aeroplane flying through those eddies gets bumped.
Meteorologists call the underlying quantity wind shear: how sharply the wind changes speed or direction across a short distance. Where shear is strong and the air is not stable enough to resist being stirred, smooth flow breaks down into turbulence. There is no cloud involved because there is no moisture involved. It is dry, clear, and completely invisible.
This is the kind that makes the news, because it is the kind that arrives with the seatbelt sign off. The bumps you can see coming — the grey tower of a thunderstorm, the ragged air over a mountain range — are the ones a crew can steer around. Clear air turbulence has to be known about in advance or not at all.
The three places bumpy air is usually made
Only the first of these is clear air turbulence in the strict sense, but all three are what a forecast has to account for along a route.
The edges of the jet stream
The classic source, and the one that puts bumps in the middle of long-haul flights over oceans and continents alike. Strongest in winter, when the jet stream is faster and further south, and worst where it bends sharply around a trough.
Mountain waves
Wind hitting a mountain range gets pushed upward, then oscillates downwind of it like water over a rock in a stream. Those waves reach cruising altitude and can extend for hundreds of kilometres past the last peak, which is why bumps over the Rockies, the Alps and the Andes have a reputation.
Convection near storms
A thunderstorm punches warm wet air up through the cruise levels and the air around it has to go somewhere. The rough air spreads well beyond the visible cloud, which is why crews stay at least twenty nautical miles clear of a cell rather than skirting the edge of it.
Why weather radar cannot see it
The radar in the nose of an airliner works by bouncing energy off water. Raindrops, hail and wet snow all send a return, which is why a storm cell shows up as a solid blob the crew can steer around and why thunderstorms are, for all their drama, the manageable kind of weather.
Clear air turbulence contains no water. There is nothing for the beam to reflect off, so the radar shows an empty screen through air that will shake the cabin. Nothing carried on the aircraft can detect it ahead, which leaves two practical defences: a forecast computed on the ground before departure, and reports radioed back by the crews who flew through the same air twenty minutes earlier.
FlyBud uses both. The forecast comes from global weather models sampled along your route at your altitude, and it is raised where pilot reports say the real air was rougher than the model expected.
What clear air turbulence does and does not do
- ✓It is uncomfortable, not structurally dangerous. No large modern airliner has been lost to turbulence of any kind, and certification loads sit far above what the atmosphere produces.
- ✓It injures people who are not belted, and almost nobody who is. That is the entire risk, and it is the reason the crew ask you to keep the belt loosely fastened when you are seated.
- ✓It is seasonal. Winter jet streams are faster, so the North Atlantic and North Pacific crossings are reliably bumpier between November and March.
- ✓It appears to be becoming more common. Studies of jet stream shear over the North Atlantic report a measurable increase across recent decades, consistent with a warming atmosphere sharpening the temperature gradients that drive the jet.
- ✓It is forecastable hours ahead, which is why airline dispatch offices plan routes and altitudes around it and why this page can hand you a number of minutes before you board.
Clear air turbulence questions
Can pilots see clear air turbulence coming?
Not visually and not on radar. They know about it from the forecast they were given before departure, from reports passed on by air traffic control, and from other crews on the same route ahead of them. That is why an unexpected patch is announced after it starts rather than before.
How far ahead can it be forecast?
Usefully, about three days. The global weather models that drive it are published several times a day and their skill at turbulence falls off fast past 72 hours, which is why the forecaster on this site stops there rather than pretending to know about next week.
What altitude does it happen at?
Mostly between 25,000 and 40,000 feet — the same band airliners cruise in, because that is where the jet stream lives. A forecast that ignores altitude is not much use: the air at 33,000 feet can be rough while 37,000 is smooth.
Is clear air turbulence what causes the plane to drop?
The drop is smaller than it feels. The aircraft is moving through air that is sinking, and the height actually lost in a bump is usually a few feet to a few tens of feet. Your stomach reports it as much more because it responds to the change in acceleration, not to the distance.
Can the crew fly around it?
Sometimes. If the forecast puts rough air at one altitude, the usual answer is to request a different one, which is why a smooth flight can involve a step climb you never noticed. Going around it horizontally costs fuel and is only worth it for a large area.
See where it is on your route.
Two airports and a date. The forecaster on this site reads the same models along your flight path and tells you where the clear air gets rough.