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.
Wind "CALM" means a wind speed less than 0.5m/s (1 kt) according to ICAO Annex 3, Appendix 3:
188.8.131.52 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.