Acknowledgements: EUROCONTROL SKYbrary August 2018 (Copyright © 2018 EUROCONTROL, all rights reserved)

 (Ed.Note: In view of current weather conditions around Europe, the following is a timely             reminder from EUROCONTROL which they have asked that we pass on to our pilot colleagues)

 Cumulonimbus (Cb) cloud forms when three conditions are met:·      

 There must be a deep layer of unstable air

 The air must be warm and moist

 A trigger mechanism must cause the warm moist air to rise, such as heating of the layer of        air close to the surface, or rising ground forcing the air upwards (orographic uplift), or  a front forcing the air upwards

Flight into a Cb is highly dangerous – threats include:

        severe turbulence

        in-flight icing

        electrical disturbance


        severe downdrafts

The only sensible defence against the hazards associated with a Cb is to avoid flying into one in the first place.


Flight crews can:

         alter their routings to avoid forecast Cb activity

         carry extra contingency fuel in case they have to re-route in flight to avoid the storms

          burn additional fuel because of the potential use of aircraft de/anti icing systems.


The following will help controllers and flight crews to plan operations to avoid the associated hazards:

         Awareness of the conditions which lead to the formation of a Cb

         Recognition of a developing and mature Cb

         Awareness of the signs which indicate the proximity of a Cb

 Weather Radar

In addition to visual recognition, weather radar is a particularly valuable aid to avoiding Cb clouds.

 In-flight avoidance

In certain circumstances, navigating through a line of Cb cells may be the only option open to a pilot, either because his destination is beyond the line of cells, or because he is unable to climb over them. In such circumstances, the aircraft may have to diverge from track by many, perhaps hundreds of miles, in order to find a gap in the wall of Cb clouds. 

 The PiC will need to judge the least hazardous track to follow through line of cells, something which will require total attention on the part of all crew. The Weather Radar is invaluable in this situation. If the Cb cell is situated over the destination aerodrome, then the pilot would be well advised to hold off or divert rather than attempt a landing.


Tony Birth