The PIREP, the Turbulence Code That Only Pilots Can Write, and the Real-Time Weather Picture Your EFB Cannot Build Without You

The TAF, the winds aloft, the METAR from forty minutes ago - these products tell you a lot about expected conditions. But the most accurate weather data in any preflight brief comes from pilots who flew that exact airspace within the last hour. That data is a Pilot Weather Report (PIREP), and it can validate your forecast products, contradict them, or reveal a gap in the picture that no model or satellite can fill.

What Is a PIREP and Why Does It Matter?

A PIREP is a real-time report filed by a pilot describing actual conditions encountered in flight - not a forecast, not a model output, not a satellite extrapolation. The FAA and the National Weather Service use PIREPs as one of the primary tools for validating forecast accuracy. When no PIREPs exist for a region, forecasters work without ground truth on what the atmosphere actually contains between the surface and cruise altitude.

For most general aviation flights under FAR Part 91, filing a PIREP is voluntary, not legally required. ATC may request one, and you should always provide it when asked. But “voluntary” doesn’t mean inconsequential - every preflight brief you’ve relied on was made more accurate by pilots who filed before you.

How Are PIREPs Coded and Formatted?

PIREPs come in two types. A routine report carries the designation UA. An urgent report - triggered by severe turbulence, severe icing, or a tornado - carries the designation UUA and receives priority dissemination.

After the type designation, each PIREP uses standardized coded fields:

• OV (Location) - Referenced to a nearby VOR or airport identifier with direction and distance. OV ARB030040 means 40 nautical miles on the 030° radial from the Ann Arbor VOR.
• TM (Time) - Zulu time
• FL (Altitude) - An aircraft at 9,500 feet reports FL095
• TP (Aircraft Type) - Critical context. What is light chop to a Boeing 737 can be moderate turbulence in a Piper Cherokee 140. Factor aircraft type into every turbulence report you read.
• SK - Sky coverage
• WX - Weather and visibility
• TA - Temperature in Celsius
• WV - Wind direction and speed
• TB - Turbulence
• IC - Icing

How Do You Read Turbulence in a PIREP?

Turbulence intensity uses an FAA-defined scale. Knowing the codes lets you evaluate how a report applies to your aircraft and altitude:

• NEG - No turbulence
• SMTH-LGT - Smooth to light
• LGT - Slight, erratic changes in attitude and altitude; no strain on aircraft or occupants
• LGT-MOD - Light to moderate
• MOD - Definite strain against seat belts; aircraft remains in control
• MOD-SEV - Moderate to severe
• SEV - Large, abrupt changes; aircraft may be momentarily out of control
• EXTRM - Structural damage possible

Type codes add important context. CAT (clear air turbulence) occurs in clear sky with no visual warning. CHOP describes rapid, continuous bumpiness without large altitude changes.

Frequency is reported as occasional (less than one-third of the time), intermittent (one-third to two-thirds), or continuous (more than two-thirds).

Altitude range is just as important as intensity. A report showing MOD CHOP from 8,000 down to 4,500 feet means moderate chop through that entire layer. Cruising at 5,500 feet puts you at the top of it. Don’t just read the intensity label - read the altitude range.

How Do You Read Icing in a PIREP?

Icing intensity codes parallel the turbulence scale:

• NEG - No icing
• TRC - Trace, barely perceptible accumulation
• LGT - Light
• MOD - Moderate
• SEV - Severe

Ice type matters. RIME ice is rough and opaque, forming in clouds with small supercooled water droplets. MIXED combines rime and clear ice. CLR (clear ice) forms from larger droplets, is denser and harder to shed, and can build fast enough to alter wing aerodynamics before you react.

The altitude range in an icing PIREP tells you whether your planned altitude puts you inside the layer, above it, or below the freezing level. A freezing level PIREP is its own report type and is especially critical in winter and spring. If the freezing level sits at 4,000 feet and you’re planning at 3,500 feet beneath a broken layer, you’re in exactly the environment where structural icing occurs - supercooled moisture in clouds below the freezing level.

Where Do PIREPs Show Up in Your Preflight Briefing?

Calling 1-800-WX-BRIEF for a standard briefing surfaces any relevant PIREPs along your route. Write them down and ask for the age of each report - the specialist can provide it.

In ForeFlight or a similar EFB, PIREPs appear as color-coded symbols on the map. Tap any symbol to decode the raw report into plain text. The visual layer is useful, but always check the timestamp before weighting a report. A PIREP from 20 minutes ago in stable conditions is fairly reliable. A PIREP from 90 minutes ago in a fast-moving weather system may describe an atmosphere that no longer exists along your route. PIREPs have a practical expiration even when the system is still displaying them.

What Does It Mean When There Are No PIREPs Along Your Route?

No PIREPs is itself information. It might mean the route has been flying smooth and nobody thought to file. But it might mean nobody has recently flown that corridor, and you have a gap in your weather picture.

In IMC, in areas of marginal visibility, or in icing-possible environments, the absence of PIREPs means your forecast products are unverified by real-world observation. The National Weather Service Area Forecast Discussion (AFD) will sometimes explicitly flag this - when a forecaster notes that observational data is sparse and model verification is limited, they’re telling you the forecast is carrying more uncertainty than usual. That is a signal worth reading before departure.

What Do PIREPs Actually Change in a Go/No-Go Decision?

Consider a cross-country in a Cessna 172 from Ann Arbor to Indianapolis - roughly 210 nautical miles - planning to cruise at 7,500 feet.

Indianapolis is reporting 4 miles in mist, overcast at 2,500 feet. The TAF shows improving conditions through the morning. AIRMET Sierra is valid along the route until noon Zulu.

Two PIREPs come back from your briefing:

1. 45 minutes ago, near Fort Wayne, 7,000 feet - Beechcraft Bonanza; light turbulence below 5,000 feet, smooth above; tops of overcast at 4,500 feet; negative icing.
2. 1 hour ago, near Indianapolis, 4,000 feet - Diamond DA40; broken ceiling at 2,500 feet, visibility about 3 miles in mist; light turbulence in descent.

This picture validates the forecast. Two aircraft flew the corridor in the last hour and found conditions broadly consistent with what the products predicted. That’s PIREPs working as intended.

Now run the same scenario with different reports: moderate turbulence continuous at 7,000 feet and again at 8,500 feet, plus an icing PIREP showing trace to light icing in clouds between 4,000 and 7,000 feet.

The turbulence now spans multiple altitude slices with no smooth layer within range. The icing report puts supercooled moisture in the layer you’d transit on climb and descent. In a non-deiced Cessna 172, that changes the conversation entirely: Can you climb above the tops before picking up significant ice? What is the icing layer thickness? Is there an alternate that avoids the worst of it?

A single PIREP doesn’t automatically flip a decision from go to no-go. But the right PIREPs define what questions you must answer before departure.

How Do You File a PIREP in Flight?

When you encounter conditions worth reporting - turbulence above light, any icing, ceiling significantly different from the TAF, or a smooth ride through an area forecast for turbulence - you have two options:

1. Request a relay through ATC on any frequency. Tell the controller you have a PIREP to file.
2. Contact Flight Service directly on 122.2.

You don’t need to file on every clear-sky training flight. But when conditions diverge from the forecast in either direction, that’s exactly what the system needs. File the negative turbulence PIREP. File the ceiling report when the bases differ from what the products said. File the smooth ride through forecast turbulence. That data improves the weather picture for every pilot briefing two hours behind you.

You get out of this system what other pilots put in. Filing PIREPs is one of the most direct ways to be a good citizen of the airspace.

What Does the Oral Exam Expect You to Know About PIREPs?

The Airman Certification Standards (ACS) weather evaluation task is not asking you to recite PIREP format from memory. It’s asking whether you can take a PIREP, place it in context with your other products, and explain what it means for the specific flight you’re planning on that specific day.

That is a synthesis skill, not a memorization skill. Be prepared to walk an examiner through how a PIREP interacts with an AIRMET, how aircraft type affects how you weight an intensity code, and how you’d use PIREPs to validate or challenge a TAF prediction.

Key Takeaways

• A PIREP is the only weather product reflecting what the atmosphere actually contained at a specific time, altitude, and location - not a model prediction.
• UUA designates an urgent PIREP (severe turbulence, severe icing, tornado); UA designates a routine report.
• The TP (aircraft type) field is critical context - light chop to a heavy aircraft may be moderate turbulence in a light GA plane.
• Always check the timestamp before weighting a PIREP; a 90-minute-old report in a fast-moving weather system may describe conditions that no longer exist.
• The absence of PIREPs along a route is information - forecasts are unverified and carry higher uncertainty when no real-world observational data supports them.
• Filing PIREPs when conditions differ from the forecast - including negative reports - feeds the system that the next pilot depends on.