A special air-report is a weather report made by an aircraft in flight when it encounters significant or hazardous meteorological conditions that are important for other aircraft and air traffic services. A special air-report should be given in case of any of the conditions mentioned below is encountered during flight.

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.