Metar

[Thomas Huben]

For some reason my Metar WIND appears 20* different than most of the other controllers. Any clue why that would happen?

It never happened at KMSP but it always happens when I am on SFO_G_GND.

Thanks,

Tom Huben
HT

[Michael Mund-Hoym]

Thomas, I believe what you have seen is that controllers issue the wind in "magnetic" value, rather than the "true" value. The written METAR gives the wind direction in reference to true north while anything said over the radio is given in reference to magnetic north. In ZOAs skies the difference about 15° (magn. north is 15° East of true north). We round up to 20° and subract that from the true wind direction.

2 idioms that might help you remember this:

1. typed (as on a piece of paper or screen) is true, mentioned is magnetic.
2. east is least, west is best (subtract eastern variation, add western variation).

I hope that answers your question.

[Tucker Emerson]

Michael is correct. I believe we still don't mention this in the SOP's which is why some controllers don't know about it.

[Charan Kumar]

It's not an SOP to give wind variation of 20 kts. It is the field which determines that. During training we are required to explain to students why we do that.

If you look at the ground charts, you will know how much variation to add/subtract, and since we are always rounding it off to the 10s, we use 14.2, 14.8, 15.3, etc as 20 :)

[Thomas Huben]

Thanks!!!

Tom Huben
HT

[Mike Fries]

Shouldn't 14.2 and 14.8 round to 10 then ?

[Charan Kumar]

Shouldn't 14.2 and 14.8 round to 10 then ?:D, I thought twice before writing that, I myself wondered why we round off to 20 and not 10.

So Mike, why do we round off to 20?

[Mike Fries]

I don't know. At RHV we round to 10 :)

[Charan Kumar]

Ok, I remember hearing 20 deg difference on liveatc, but I will check again.

[Mike Fries]

haha maybe we are the only ones doing it like that. I thought SQL and SFO had MAG VAR 15 degrees or greater but I was wrong. I know SQL does 20 degrees for sure. Maybe its a hold over from when they used to be 15 degrees and no one ever questioned it.

[Mike Fries]

just did some recon:

SEA subtracts 20
LAX keeps it the same
SAN adds 10 ???

[Charan Kumar]

I would think 13 degree variance (KLAX) would mean a lot when wind speed is higher, but that is even more confusing they don't do it at KLAX.

So Tom, if you are wondering, for now continue using 20 degree variance as that is what all local controllers are doing. We will find out what will eventually be the case :)

[Chuck Barlow]

The other thing that is going on here is that the METAR is already rounding a precise heading to the nearest 10. So if you consider:

A. Actual wind true direction at KSFO is 284.


METAR rounds this down to 280.
ZOA tower controller rounds this down to 260.
Magnetic variation at the airport is 15 degrees.
"Real" or "correct" wind direction is 269 -- our adjustment is off by 9 degrees.

B. Actual wind true direction at KSFO is 285.


METAR rounds this up to 290.
ZOA tower controller rounds this down to 270.
"Real" or "correct" wind direction is 270 -- our adjustment is spot-on accurate!

So partly how well we are doing here is the luck of what the actual wind direction is relative to any rounding in the METAR.

[Tom Seeley]

Just for the sake of the discussion, at LAX they DO subtract for the wind:


Note that in ZLA, the magnetic variation is roughly 13* to the east. This means that there is roughly ten degrees difference between the wind readout on the METAR and the actual magnetic heading with such a value. This requires us to reduce 10* from the readout when issuing it to the pilots. We cannot reduce 13*, as METAR direction readouts are incremented only by 10*

[David Carman]

Thanks Tom. I think Mike was referring to real-world LAX ATCT, as he used to work there.

[Mike Fries]

oh sorry my recon i posted was by calling real world ATIS and looking up the current METAR

[Tom Seeley]

Thanks Tom. I think Mike was referring to real-world LAX ATCT, as he used to work there.

Oh duh, that's what I get for posting anything that early in the morning. Thanks Dave.