Turbulence: Types, Intensity Levels, and Reporting
Learn where turbulence comes from, what the AIM's light-to-extreme intensity scale means, and how to report a rough ride other pilots can use.
Turbulence intensity is graded on a four-step scale defined by what the air does to the airplane, not by how it feels: light, moderate, severe, and extreme. The FAA sets each level in AIM 7-1-21 by the aircraft’s reaction and the reaction inside the cabin, so that one pilot’s report means something specific to the next. “Moderate” is not “kind of bumpy.” It is a defined term with a defined airframe response and a defined effect on the people strapped into it.
That precision matters because turbulence is the one hazard almost every flight meets, and the only common language we have for it is the intensity word in a pilot report. Get the word right and the report is usable. Get it wrong, or skip the report entirely, and the pilot behind you is flying on a forecast that can only say turbulence is possible. This post covers where turbulence comes from, what each intensity level actually means, why the same air is “moderate” in one airplane and “light” in another, and how to report it so it earns its place on the next briefing. It sits under our guide to aviation weather hazards, which maps how the phenomena fit together.
Where turbulence comes from
Rough air is rough air to the airframe, but the source tells you where to expect it, how to plan around it, and what to call it in a report.
Mechanical turbulence is air disturbed by terrain or obstructions. Wind flowing over hills, ridgelines, buildings, or a tree line breaks into eddies downwind. It is strongest on the lee side, worse with stronger surface wind, and it is why the last few hundred feet into a gusty, obstructed runway can be the roughest part of the flight.
Convective or thermal turbulence is the bumpy ride of a sunny afternoon. Uneven surface heating sends columns of rising air up through the cooler air around them, and every thermal you punch through is a small jolt. It builds through the day, peaks in the afternoon, and fades after sunset. Pushed far enough, the same convection grows into thunderstorms, where the turbulence turns violent. Our post on thunderstorms and general aviation covers that end of the scale; the roughest air near a storm is not always inside the cloud.
Wind shear turbulence comes from a sharp change in wind speed or direction over a short distance, vertically or horizontally. A temperature inversion with a strong wind above it, a frontal boundary, or the edge of a low-level jet can all stack fast-moving air against slow-moving air, and the boundary between them is turbulent. This is where the winds aloft forecast earns its keep: a large change in wind between two adjacent forecast levels, or a big directional swing over a few thousand feet, is a shear cue you can read on the ground before you ever feel it.
Mountain wave is shear and terrain working together. Stable air blowing across a mountain range at speed sets up a standing wave downwind, sometimes for a hundred miles, with smooth lift and sink in the wave itself and violent rotor turbulence in the churning air beneath the crests. Lenticular clouds mark the wave; ragged rotor clouds mark the worst of it.
Clear air turbulence (CAT) is shear turbulence at altitude, usually near the jet stream, with no cloud to warn you. It matters most to high-flying aircraft, and the AIM asks that high-level turbulence above roughly 15,000 ft ASL that is not tied to cumuliform cloud be reported as CAT so it reads as what it is.
Wake turbulence belongs in a different bucket. It is not weather. It is the pair of counter-rotating vortices trailing off the wingtips of the airplane ahead of you, strongest behind a heavy, slow, clean jet on approach or departure. It is a separation and sequencing problem, not a briefing item. Named here only so you do not mistake a wake encounter for atmospheric turbulence and report it as weather.
What the intensity levels actually mean
The scale lives in AIM 7-1-21, TBL 7-1-11. Each level is written twice: what the aircraft does, and what happens to the occupants. Here is the working version.
| Intensity | What the aircraft does | Inside the cabin |
|---|---|---|
| Light | Momentarily causes slight, erratic changes in altitude and/or attitude | Slight strain against seat belts; unsecured objects may shift slightly; walking is barely affected |
| Moderate | Similar to light but greater; definite changes in altitude and/or attitude, but the aircraft stays in positive control throughout, usually with variations in indicated airspeed | Definite strain against belts; unsecured objects dislodge; food service and walking are difficult |
| Severe | Large, abrupt changes in altitude and/or attitude, usually with large airspeed variations; the aircraft may be momentarily out of control | Occupants forced violently against belts; unsecured objects tossed about; walking is impossible |
| Extreme | The aircraft is violently tossed about, practically impossible to control, and structural damage is possible | — |
Two things anchor the scale. First, the dividing line between moderate and severe is control: in moderate turbulence you keep positive control the whole time, in severe you may lose it momentarily. Second, “structural damage is possible” is the phrase that makes extreme a genuinely different category, not just a bigger number.
The AIM pairs intensity with a duration word, because a single hard jolt and a continuous pounding are different reports. Occasional is less than a third of the time, intermittent is one-third to two-thirds, and continuous is more than two-thirds. “Occasional moderate” and “continuous moderate” describe very different legs.
Chop is not turbulence
The AIM keeps a separate term for the rapid, rhythmic bumpiness that rattles the airframe without moving it around. Light chop is slight, rapid, somewhat rhythmic bumpiness with no appreciable change in altitude or attitude. Moderate chop is the same character with more intensity: rapid bumps and jolts, still without appreciable changes in the airplane’s flight path.
The distinction is real and worth reporting accurately. Chop is a texture; turbulence is displacement. A ride that buzzes the seat but holds altitude is chop, and calling it “moderate turbulence” oversells it to the next pilot. If the airplane is being shoved off altitude or attitude, it is turbulence, and the intensity word applies.
Why “moderate” isn’t the same in every airplane
Read the criteria again and notice what they are built on: the airplane’s reaction and the occupants’ reaction. Both depend on the airframe. The same parcel of rough air that shoves a Skyhawk off altitude and strains everyone against their belts may register as light chop on a 737 that is an order of magnitude heavier with far more wing loading. A lighter airplane is accelerated more by the same gust, so it reacts more.
The AIM does not spell this out for turbulence the way it does for icing, but it builds the dependence into the report itself: type of aircraft is one of the required elements of a turbulence PIREP (AIM 7-1-21). That field is there precisely so the next pilot can weigh the report against their own airframe. A “moderate” call from a regional jet at your altitude may be a rough ride in a 172. Always read the type before you trust the intensity.
Maneuvering speed, and why it drops as you burn fuel
There is a speed that turns a turbulence encounter from a structural question into a stall question: maneuvering speed, Va. At or below Va, a sudden full deflection of a single flight control, or a sharp gust, stalls the wing before the airframe reaches its limit load factor. Above Va, the same input can overstress the structure. This is why the airplane flight manual gives a turbulent-air penetration speed at or near Va: slow down, and the wing becomes the pressure-relief valve.
Here is the part that catches pilots out: Va is lower when the airplane is lighter. Manufacturers publish Va at maximum gross weight, and it decreases as weight comes off, whether that is fuel burned over a long leg or an empty back seat. The reason is angle of attack. A lighter airplane needs less lift to hold altitude, so at any given speed it flies at a lower angle of attack, further from the critical angle. That leaves more room to load the wing before it stalls, which means a lighter airplane can reach its limit load factor before it stalls. Lowering the target speed restores the margin, so the wing again stalls first. Burn down to a light airframe, keep flying the gross-weight Va into turbulence, and the protection you think you have is thinner than the placard suggests.
One caveat the accident record wrote in hard: Va protects against a single full deflection of one control. It does not license large, rapid, or reversing control inputs stacked on top of each other. In turbulence the right technique is to slow to the turbulent-air speed, keep the wings roughly level, and accept altitude excursions rather than fighting them with aggressive control movement.
Reporting it so the next pilot can use it
A turbulence PIREP is short and its fields are fixed. AIM 7-1-21 asks for aircraft location, time in UTC, intensity, whether the turbulence was in or near clouds, altitude or flight level, aircraft type, and the duration. That is the whole report:
“Denver Center, Skyhawk Three Four Five Whiskey Bravo, pilot report. One five miles south of Falcon, one niner one five Zulu, occasional moderate turbulence, in clear air, eight thousand five hundred, Cessna one seventy-two.”
Notice how much that gives a pilot planning the same route: intensity, that it was not tied to cloud, the altitude, and a type they can calibrate against. Give the report to whoever you are already talking to, center, approach, or tower, or call Flight Service if you are not talking to anyone. Our full walkthrough on reading and filing PIREPs covers the radio call, the slash-delimited format the specialist builds from your words, and how the report feeds the turbulence graphics you briefed from.
A thirty-second call that the ride smoothed out on top of a layer, or that the bumps started at 6,000 and quit at 9,000, is real information. Say it on frequency, where the pilot behind you can use it.
Common questions
- Is moderate turbulence dangerous in a small airplane?
- It is uncomfortable and makes the airplane harder to fly precisely, but by definition the aircraft stays in positive control throughout moderate turbulence. Slow to maneuvering speed, keep the wings level, and secure loose items. Severe is the category where control can be lost momentarily, and where you want to already be slowed down and heading out of the area.
- Should I file a PIREP for a smooth ride?
- Yes. A report that the air smoothed out on top of a layer, or that the bumps quit above a certain altitude, tells the next pilot exactly what they want to know before they climb. Negative reports are real information, and the low and middle altitudes where general aviation flies are chronically under-reported.
- Why is maneuvering speed lower when the airplane is lighter?
- Because a lighter airplane flies at a lower angle of attack for the same speed, leaving more room to load the wing before it stalls, so it can reach its limit load factor before stalling. Lowering the target speed restores the margin so the wing stalls first. Manufacturers publish Va at maximum gross weight, and it drops as fuel burns off.