ICAO Standard:
Annex 3 — Meteorological Service for International Air Navigation
4.3 Routine observations and reports
Note.— Meteorological information used in ATIS (voice-ATIS and D-ATIS) is to be extracted from the local routine report, in accordance with Annex 11, 4.3.6.1 g).
4.6 Observing and reporting meteorological elements
4.6.1 Surface wind
4.6.1.1 The mean direction and the mean speed of the surface wind shall be measured, as well as significant variations of the wind direction and speed, and reported in degrees true and kilometres per hour (or knots), respectively.
If you call tower and get winds, they will be in magnetic.
Specific Pre-Flight Information Briefing enroute and terminal winds will be in true.
Believe me, this drove me crazy for a while. I have the measured wind data, which is always measured true, and the data from the ac from AMDAR for enroute, is true. The ac itself reports the winds in true.
Then, for the ATC screen you see above, this is what the controller sees in relation to each runway, which of course, is in mag. Mag is used to compute the crosswind/head-tailwind/etc component winds for each runway. As the system can be installed worldwide, built in to the interface in the settings, the magvar is input. This is then applied as applicable, but not globally to the wind output! Only the screens where ATC will provide the wind call. Other outputs, such as the METAR and ATIS do not have the magvar couple.
Thus, the surface winds (at 10m) that ATC reports shown are in mag, but virtually everything else is in true, (as are any forecasts)
Last edited by underfire; 7th Jun 2017 at 08:51.