PIREP Turbulence Codes: LGT, MOD, SEV and CHOP Decoded

PIREP turbulence codes live in the /TB field, and they are the only place in aviation weather where a human tells you what the air actually felt like. A forecast predicts turbulence. A PIREP reports it.

The field packs three separate things into a few letters: how often, how hard, and what kind. Read them in that order and the codes stop looking like an alphabet soup.

The PIREP turbulence codes, decoded

The intensity words are the core of the field. These are the values the wxdecoder parser reads.

A frequency qualifier often comes first, describing how much of the time the turbulence was there.

And a type qualifier can follow, describing what kind of air it was.

What the intensity words actually mean

The words are defined by what happens in the cabin, not by a number on an instrument. That is deliberate, because it makes them reportable by anybody.

Light turbulence causes slight erratic changes in attitude or altitude. Occupants feel a slight strain against their belts. Objects stay put.

Moderate is the important one. The aircraft changes attitude or altitude, but it remains under control throughout. Occupants feel definite strain against belts, and unsecured objects move. This is where cabin service normally stops.

Severe means large abrupt changes, usually with large variations in indicated airspeed. The aircraft may be momentarily out of control.

Extreme means it is tossed about violently and control is impossible. Structural damage is possible. Reports of it are rare and always worth acting on.

Chop is not the same as turbulence

CHOP is the qualifier people skip over, and it changes the meaning quite a lot.

Chop is rapid rhythmic bumpiness without an appreciable change in attitude or altitude. The aircraft stays where it is put. It just vibrates getting there.

Ordinary turbulence moves the aircraft. Chop rattles it. So MOD CHOP is uncomfortable and steady, while plain MOD means the aeroplane is being shifted off its flight path.

That distinction matters when you are deciding whether to change level. Chop often persists across a wide band and moving may not help. Turbulence tied to a shear layer or a wave often clears with a few thousand feet.

Reading a full turbulence field

Put the parts together in the order they appear.

/TB CONS LGT CHOP 350 reads as continuous light chop at flight level 350. Steady, mild, rhythmic, and not going to move the aircraft around.

/TB OCNL MOD CAT 280-320 reads as occasional moderate clear-air turbulence between flight level 280 and 320. That is a band to avoid or to cross quickly, and the altitudes tell you which way to go.

Where two intensities are given, such as LGT-MOD, the pilot is reporting a range. Plan for the higher one.

If there is no altitude, the turbulence was at the level in the /FL field earlier in the report. Read the whole PIREP rather than the one field.

Why negative reports matter

NEG and SMOOTH look like empty reports. They are among the most useful ones in the system.

Turbulence forecasts cover large areas and long periods, and they are cautious by design. A forecast band of moderate turbulence may be smooth in most of it. Only a pilot flying through can tell you which part.

So a run of NEG reports through a level, from aircraft of a similar size, is real evidence that the level is usable. One SMOOTH from a heavy jet says less about what a light twin will feel, which brings us to the last point.

Aircraft type is in the /TP field for a reason. Moderate in a Cessna 172 and moderate in a Boeing 737 are not the same air. Always read the type alongside the intensity.

Where turbulence reports come from

Not every turbulence report is typed by a pilot, and the source changes how you weigh it.

A standard PIREP is voice, passed to a controller or a flight service station and typed up. It carries UA in the header. An urgent one carries UUA, and severe or extreme turbulence is one of the conditions that earns it.

An AIREP is different. It starts with ARP and is downlinked automatically by the aircraft. The turbulence group has no slash in front of it and the fields sit in fixed positions rather than being tagged.

Automated reports are frequent and consistent. Voice reports are rarer and often richer, because a pilot adds remarks a data link cannot. Both end up in the same feed, so check the header to know which you are reading.

Verdict on PIREP turbulence codes

Read the /TB field as three questions. How often, from the frequency qualifier. How hard, from the intensity word. What kind, from the type.

Then read the aircraft type and the altitude before you weigh any of it. A moderate report from a light aircraft two thousand feet below you is a different warning from a moderate report by a jet at your level.

Last updated August 29, 2026