An AIREP (air report) is a report from an aircraft in flight, provided in standard formats:

• Routine aircraft observations made during en-route and climb-out phases; these are generally requested only from aircraft with datalink capability.
• Special or non-routine observations made during any phase of flight.

Both types may be transmitted via datalink or voice to the appropriate controller, who then forwards them to the MWOs and WAFCs.

ICAO Doc 4444
4.12 REPORTING OF OPERATIONAL AND METEOROLOGICAL INFORMATION
4.12.3 Contents of special air-reports
4.12.3.1 Special air-reports shall be made by all aircraft whenever the following conditions are encountered or observed:

a) moderate or severe turbulence; or
b) moderate or severe icing; or
c) severe mountain wave; or
d) thunderstorms, without hail that are obscured, embedded, widespread or in squall lines; or
e) thunderstorms, with hail that are obscured, embedded, widespread or in squall lines; or
f) heavy duststorm or heavy sandstorm; or
g) volcanic ash cloud; or
h) pre-eruption volcanic activity or a volcanic eruption.

Note.— Pre-eruption volcanic activity in this context means unusual and/or increasing volcanic activity which could presage a volcanic eruption.

In addition, in the case of transonic and supersonic flight:

i) moderate turbulence; or
j) hail; or
k) cumulonimbus clouds.