Definition
A PIREP is a report of actual weather conditions encountered in flight, made by a pilot and relayed to a ground facility (typically Flight Service or ATC) for distribution to other pilots, forecasters, and dispatchers. PIREPs describe what the pilot is actually experiencing — such as cloud bases and tops, in-flight visibility, turbulence, icing, wind, and ride quality — at a specific location, altitude, and time. They are issued in two forms: routine (UA) and urgent (UUA), the urgent form being used for hazardous conditions such as severe turbulence, severe icing, hail, or low-level wind shear.
Plain English
A short report from a pilot in the air, telling people on the ground what the weather is actually like up there right now. Other pilots, controllers, and forecasters use it to know what to expect.
Context Anchor
Pilots see PIREPs during preflight weather planning and may give one to air traffic control or flight service when they encounter important weather in flight.
Derivation
Pilot Report — literally, a report from a pilot. The value of the term lies in who is making the observation: a person actually flying through the conditions, not a sensor on the ground or a forecast model.
Why Pilots Care
They supply real-time observations that forecasts cannot provide, helping pilots anticipate and avoid dangerous conditions such as icing or severe turbulence.
Intuition Check
Do not think of a PIREP as a forecast. A forecast predicts what may happen; a PIREP reports what a pilot actually encountered.
Example Sentence 1
Before descending into the area, the pilot asked Flight Service for any recent PIREPs on cloud tops and icing.
Example Sentence 2
Before takeoff the instructor checked recent PIREPs and decided to delay the flight due to reported moderate turbulence.