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Turbulence, particularly clear air turbulence, is a significant and often unexpected hazard to aircraft during flight.  Turbulent eddies with wavelengths of 10 m to 1 km are felt as bumpiness, say by aircraft flying at ~250 m s-1. Aircraft are more sensitive to vertical gusts than to lateral gusts.

Clear air turbulence is often caused by:

  • shear instabilities (Kelvin–Helmholtz instabilities).
  • upper-level fronto-genesis.
  • large-amplitude mountain waves.
  • breaking convectively-generated gravity waves.

Eddy dissipation rate (EDR) is the International Civil Aviation Organization (ICAO) standard for aircraft reporting of turbulence and hence is also the standard measure for clear-air turbulence (CAT).  The units of EDR are m2/3 s-1.


In the IFS the energy dissipation rate is derived from:

  • vertical wind shear and the total deformation (Ellrod1 index).
  • subgrid contribution to gravity wave drag by breaking convectively generated gravity waves.
  • tendencies for horizontal momentum.This has contributions from
    • vertical diffusion scheme. This includes:
      • dissipation due to turbulent mixing.
      • orographic wave drag.
      • orographic blocking.
    • dissipation due to convective momentum transport.


Clear air turbulence correspond approximately to the eddy diffusion rate

CATEDRecChart colours (Fig1)
Weak<0.05 m2/3 s-1

Light blue


Moderate0.05 – 0.3 m2/3 s-1

Blue, dark blue,

green, yellow 

Strong to severe>0.3 m2/3 s-1

Orange, dark orange,

red, dark brown, purple

Severe (observed example)>0.5 m2/3 s-1

Light purple and clear


Table1: Relationship between CAT descriptions and eddy dissipation rate.


Further reading

ECMWF Newsletter 168

ECMWF Technical Memorandum No874


Fig1: ecChart of energy dissipation rate. DT 12UTC 2 March 2023, VT 03UTC 3 March 2023.  Colours given in table 1.


Fig2: ecChart of 300 hPa contours (dam).  300hPa winds (kt) shown as arrows and isotachs (coloured). DT 12UTC 2 March 2023, VT 03UTC 3 March 2023. Severe CAT values of EDR to the east of the strongest jet and extend into the unshaded area enclosing winds below 50kt.  Pin shows Dallas, Texas.  Severe turbulence was encountered by an aeroplane in this system causing emergency diversion to an available airfield.