Regarding variable wind direction in METAR reports, what are the criteria for VARIABLE when the authority uses "VRB"?
An Advisory Circular mentions the authority uses "V" when the wind direction varies by 60 degrees. But how about "VRB"?
Unless you are asking for a specific country, ICAO annex 3 to the Chicago Convention is the reference for meteorological information:
If a member state departs from a given ICAO recommendations, then the applicable elements and the reference to the ICAO paragraph will appear in the AIP (AIP, Section GEN 1.7 Differences from ICAO Standards, Recommended Practices and Procedures).
Annex 3 was strongly amended with amdt 73. Be sure to use documentation including the amendment (starting with 15th edition).
Variable wind is indicated by "VRB" or "V"
1. V: More than 6 km/h and variation less than 180°. Mean wind direction and variation. Annex 3, § 184.108.40.206:
1) when the total variation is 60° or more and less than 180° and the wind speed is 6 km/h (3 kt) or more, such directional variations shall be reported as the two extreme directions between which the surface wind has varied
E.g. 31015G27KT 280V350.
2. VRB: Less than 6 km/h or variation more than 180°. No mean wind direction. Annex 3, § 220.127.116.11:
2) when the total variation is 60° or more and less than 180° and the wind speed is less than 6 km/h (3 kt), the wind direction shall be reported as variable with no mean wind direction; or
3) when the total variation is 180° or more, the wind direction shall be reported as variable with no mean wind direction"
Thanks to mins, I could find out the answer.
US AIP Section GEN 1.7 mentions below;
In the U.S., the criterion for a variable wind is: wind speed greater than 6 kt and direction varies by 60 degrees or more. If the wind is >1 kt but <6 kt, the wind direction may be replaced by ”VRB” followed by the speed or reported as observed. “VRB” would be spoken as “wind variable at .”
The conclusion is VRB is used when variable less than 60° and less than 6 kt.