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What is the exact criteria for wind speed to be considered "CALM". Different countries might have different values and i do not mind getting answers from those. But an answer from an ICAO annex would be prefered.

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Wind "CALM" means a wind speed less than 0.5m/s (1 kt) according to ICAO Annex 3, Appendix 3:

4.1.5.2 In local routine and special reports and in METAR and SPECI:

...

d) when a wind speed of less than 0.5m/s (1 kt) is reported, it shall be indicated as calm;

Before Amendment 75 of ICAO Annex 3 this read:

d) when a wind speed of less than 2 km/h (1 kt) is reported, it shall be indicated as calm;

See also Annex 3:

  • Appendix 5, recommendation 1.2.1
  • Attachment C

The FAA, in their Order JO 7110.65X (Air Traffic Control), defines a limit of 3 knots.

2−6−3. REPORTING WEATHER CONDITIONS

....

b. Describe the wind as calm when the wind velocity is less than three knots.

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