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In connection with takeoff and landing, ATC will give the current wind direction and velocity to the pilot. In some cases, ATC will also report the variation in direction and/or current gust (max and min velocity). This is only done if the variation and/or gust is "significant".

How large does the variation in direction and/or gust have to be before ATC has to report it? Something tells me the mean wind velocity has to be at least 10 knots before wind variation must be reported, but I'm not actually sure.

Please include reference to the relevant regulations or recommended practices (ICAO, EASA or local).

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    $\begingroup$ In the USA, we must ensure the pilot has the representative wind (i.e. ATIS) and that's usually it. The only time we must issue the wind on frequency is "when authorizing the use of a runway and a tailwind component exists"—nothing about gusts. That said, if it's gusting above 15 knots or so I'll often toss that in even if it's a direct headwind. $\endgroup$
    – randomhead
    Feb 17, 2022 at 14:08
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    $\begingroup$ Actually, I think this question needs clarification. Are you asking about the definition of (or the criteria to describe the wind as) "variable" and "gusting"? In that case wbeard's answer would be partially relevant. Or are you asking if ATC has some requirement to specifically issue the wind if it is variable or gusting at such-and-such a level? $\endgroup$
    – randomhead
    Feb 17, 2022 at 16:12
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    $\begingroup$ @randomhead My local tower issues winds for nearly every takeoff and landing. Is that because it’s materially different from what’s on the ATIS? If so, is that required, or are they doing it on their own? $\endgroup$
    – StephenS
    Feb 17, 2022 at 18:32
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    $\begingroup$ @Stephen As long as you report having the current ATIS code they are not required to re-issue the wind unless 1) there is a tailwind component (3–5–3) or 2) it is a tower run by the Army, Navy, or Air Force (3–9–10i, 3–10–5e). It is possible that there are or were a large number of ex-military controllers at that tower and they mistakenly trained new arrivals that they always have to issue the wind. $\endgroup$
    – randomhead
    Feb 17, 2022 at 19:07
  • $\begingroup$ @randomhead The latter. I'm not asking about when variation or gust must be included in the met report, but about when ATC must give these details from the met report to the pilot. $\endgroup$
    – 60levelchange
    Feb 19, 2022 at 13:25

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TL;DR:

The weather ATC transmissions to aircraft are based on the local routine and special reports (not to be confused with METAR/SPECI), which are displayed in the ATS unit, and are ultimately covered by Annex 3, where no minimum mean is specified to start reporting a variation, and where "significant" variations are defined:

  • Wind speed: 10 knots above mean, or 5 knots above mean if noise abatement is in effect
  • Direction: when it's ≥ 60° (details below).

ICAO Doc 4444

Starting from Doc 4444, which is complementary to SARPs Annex 2 and directed mainly at ATC personnel (§ 2.1):

7.4.1.2.2 Prior to take-off aircraft shall be advised of:

[...] any significant changes in the surface wind direction and speed, the air temperature, and the visibility or RVR value(s) given in accordance with 7.4.1.2.1 [...]

Note.— The meteorological information listed above is to follow the criteria used for meteorological local routine and special reports, in accordance with Chapter 11, 11.4.3.2.2 to 11.4.3.2.3.

Likewise for landing (7.4.1.2.3), so let's check 11.4.3.2.2 and .3:

11.4.3.2.2 Meteorological information concerning the meteorological conditions at aerodromes, to be transmitted to aircraft by the ATS unit concerned, in accordance with Annex 11, Chapter 4 and this document, Chapter 6, Sections 6.4 and 6.6 and Chapter 7, Section 7.4.1, shall be extracted by the ATS unit concerned from the following meteorological messages, provided by the appropriate meteorological office, supplemented for arriving and departing aircraft, as appropriate, by information from displays relating to meteorological sensors (in particular, those related to the surface wind and runway visual range) located in the ATS units:

a) local meteorological routine and special reports [...]

11.4.3.2.3 The meteorological information referred to in 11.4.3.2.2 shall be extracted, as appropriate, from meteorological reports providing information on the following elements: [...]

Note. — Provisions relating to meteorological information to be provided in accordance with 11.4.3.2.3 are contained in Annex 3 — Meteorological Service for International Air Navigation, Chapter 4 and Appendix 3.

The local routine and special reports displayed to the controller are used, and are handled by Annex 3 Chapter 4, so let's go there:

Annex 3

4. OBSERVING AND REPORTING OF METEOROLOGICAL ELEMENTS [...]
4.1 Surface wind [...]
4.1.5 Reporting [...]
4.1.5.2 In local routine and special reports and in METAR and SPECI: [...]

c) variations from the mean wind speed (gusts) during the past 10 minutes shall be reported when the maximum wind speed exceeds the mean speed by:

  1. 2.5 m/s (5 kt) or more in local routine and special reports when noise abatement procedures are applied in accordance with paragraph 7.2.6 of the PANS-ATM (Doc 4444); or

  2. 5 m/s (10 kt) or more otherwise [...]

If you're still unsure about the term "significant", Annex 3 says:

Table A3-1. Template for the local routine (MET REPORT) and local special (SPECIAL) reports [...]

Significant speed variations (C)4

With the footnote saying:

4. To be included in accordance with 4.1.5.2 c).

4.1.5.2 c) is already quoted above.

Likewise for the direction:

b) variations from the mean wind direction during the past 10 minutes shall be reported as follows, if the total variation is 60° or more:

  1. when the total variation is 60° or more and less than 180° and the wind speed is 1.5 m/s (3 kt) or more, such directional variations shall be reported as the two extreme directions between which the surface wind has varied;

  2. when the total variation is 60° or more and less than 180° and the wind speed is less than 1.5 m/s (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;

(All bold emphasis added.)

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  • $\begingroup$ Hm. So you're taking the Note—"The meteorological information listed above is to follow the criteria used for METAR and SPECI"—to mean "when the wind meets the criteria to be described as gusty or variable, it must be issued to the pilot"? That wouldn't be my first interpretation; I would read it as "when issuing a 'significant change' [which is not defined here!], use the format prescribed for a METAR or SPECI." But I can see where you're coming from, for sure. $\endgroup$
    – randomhead
    Mar 19, 2022 at 15:36
  • $\begingroup$ @randomhead: 11.4.3.2.2 is literally about transmissions by ATS units; Doc 4444 does not handle METAR formatting. I improved the bold emphases now. $\endgroup$
    – user14897
    Mar 19, 2022 at 16:13

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