Gusts

[BOAC]

Yes, yes, yes, but did you read it?

Supplementary - so how do you get a tailwind out of it?

[hetfield]

BOAC

If no direction is given, one has to assume a tailwind out of it, don't you?

[BOAC]

I take it that is your way of saying you did not read ALL of Seite 20?

[hetfield]

BOAC

If you are referring to the passage

"Given a generally smooth land surface area, when the wind
blows strong and the weather is stormy, the most powerful gusts blow in the same direction as the wind
above the boundary layer. In the northern hemisphere the wind veers clockwise about 10° to 20° with
respect to the mean wind direction. These assumptions do not hold good in hilly terrain, thunderstorms,
thermal weather conditions, during the passage of fronts and so forth."

Yes, I've read it

And that's exactly what my question is about!

Cheers

[FlightDetent]

Ok, the behaviour of gust (whatever we think it is) is established. The actual conditions will get measured and reported. Here's what the coding should look like, Annex 3 [my bolding]:

4.1.3 Averaging
4.1.3.1 The averaging period for surface wind observations shall be:
a) 2 minutes for local routine and special reports and for wind displays in air traffic services units; and
b) 10 minutes for METAR and SPECI, except that when the 10-minute period includes a marked discontinuity in the wind direction and/or speed, only data occurring after the discontinuity shall be used for obtaining mean values;
hence, the time interval in these circumstances should be correspondingly reduced.

Note.— A marked discontinuity occurs when there is an abrupt and sustained change in wind direction of 30° or more, with a wind speed of 20 km/h (10 kt) before or after the change, or a change in wind speed of 20 km/h (10 kt) or more, lasting at least 2 minutes.

4.1.3.2 Recommendation.— The averaging period for measuring variations from the mean wind speed (gusts) reported in accordance with 4.1.5.2 c) should be 3 seconds for local routine and special reports and for METAR and SPECI and for wind displays used for depicting variations from the mean wind speed (gusts) in air traffic services units.

4.1.5 Reporting
4.1.5.2 In local routine and special reports and in METAR and SPECI:
....
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 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;
_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;
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) 10 km/h (5 kt) or more in local routine and special reports when noise abatement procedures are applied in accordance with paragraph 7.2.3 of the PANS-ATM (Doc 4444); or
_2) 20 km/h (10 kt) or more otherwise;

4.1.5.3 In local routine and special reports:
...
c) when variations from the mean wind direction are reported in accordance with 4.1.5.2 b) 2), the two extreme directions between which the surface wind has varied shall be reported; and
d) when variations from the mean wind speed (gusts) are reported in accordance with 4.1.5.2 c), they shall be reported as the maximum and minimum values of the wind speed attained.
4.1.5.4 In METAR and SPECI, when variations from the mean wind speed (gusts) are reported in accordance with 4.1.5.2 c), the maximum value of the wind speed attained shall be reported.

hetfield: Based on above I think the BFU report is not 100% sincere with regards to wind shear reporting and coding. What we had been discussing under gust is in fact wind shear i.e. change in speed and direction. However, for Annex 3, the term gust is used for windspeed change only and varying direction, if any, is dealt with separately.

Therefore 24015G27KT
- is a METAR/SPECI code based on 10 minute average (no min speed provided);
- indicates windspeed change with direction less than 60 deg from average; the direction change (if any) associated with gust is already calculated into the average direction reported as 240;
- the observed conditions may by extreme (my approximation) be: steady 220/10 with wind shears 260/27;

It's all academic and complicated but it serves one purpose, to provide pilots with reliable numbers. If I would like to depart from 27L/R EGLL and the wind is 27010G20kt I would assume true conditions 250/6 gusting 280/20 based on the fact that rules to include varying code into the wind (60 deg offset from average) report had not been met. To summarize, if no varying component is reported, the measured gust is "reasonably" close to the wind direction provided.

Sincerely,
FD (the un-real)

[hetfield]

Thx FD for clarification

That's exactly what I'm looking for.

Fly safe