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Interesting METAR phenomenon

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Interesting METAR phenomenon

Postby Fritz » Sat Nov 12, 2016 11:58 pm

Approaching Tokyo in rather nice weather conditions, the sky suddenly became very dark.

Image

Because there was no volcanic eruption nearby, I figured this to be some kind of bug. But re-initializing the weather system didn't change anything, so I continued the flight.

Image

The METAR string that caused it was submitted by Narita Airport, and it was the same in the internet, i.e. not "damaged" by FG:

RJAA 122230Z 33004KT 4000 BCFG BR FEW001 SCT/// 10/09 Q1025 BECMG TL2300 6000 NSW

I finally found that the part "SCT///" (whatever this means) causes the phenomenon, even if added to very simple METAR strings which don't contain any other cloud information.

Edit: This only happens when ALS is turned on. It seems that there is no atmosphere to scatter light...
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Re: Interesting METAR phenomenon

Postby sanhozay » Sun Nov 13, 2016 1:01 am

That part of the METAR means scattered clouds but the sensor at the AWOS station could not determine the altitude. A complete reading would be something like SCT020. The slashes indicate unknown or temporarily unavailable.
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Re: Interesting METAR phenomenon

Postby Thorsten » Sun Nov 13, 2016 7:13 am

Try looking at what the parsed METAR info in the property tree says - the weather systems aren't directly using the string, there's a parser stage running over it, and likely that can't deal with the /// - the only question is what part of the info it then zeroes/NaNs.
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Re: Interesting METAR phenomenon

Postby Fritz » Sun Nov 13, 2016 8:33 pm

BKN/// does the same, as I just found out - again at Narita:

RJAA 131930Z 31004KT 260V320 4500 MIFG BR FEW030 BKN/// 11/10 Q1022 NOSIG RMK 1CU030 A3021

There are so many weather-related properties that it is difficult to find anything that is affected, at least for somebody like me who doesn't have any knowledge of this part of the software. But, to make a start, the properties "local-weather" and "local-weather/METAR" (including the layers) aren't affected in any way by adding or removing the BKN/// part.

What is (obviously) affected is "environment/metar/clouds": Layer 1 (the "broken" layer), has one strange property: elevation-ft: -1e+20. This is 5141 when changing BKN/// to BKN050. So /// translates into -1e+20, but changing this value manually to -1e+20, doesn't cause the black sky phenomenon on its own. -9999 is the value for unused layers, so this could be a start.

I didn't find other affected properties, but there probably are. Is there a way to export the whole property tree into a text file so that it can by compared by a diff tool?
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Re: Interesting METAR phenomenon

Postby rominet » Mon Nov 14, 2016 10:50 pm

You can put this in a binding:
Code: Select all
     <desc>Dump the global property tree to the console</desc>
     <binding>
       <command>nasal</command>
       <script>props.dump(props.globals)</script>
     </binding>

and then redirect the FG output to a file.
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Re: Interesting METAR phenomenon

Postby Fritz » Tue Nov 15, 2016 11:42 pm

Ok... I'll try this tomorrow.
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Re: Interesting METAR phenomenon

Postby Fritz » Mon Jan 09, 2017 8:51 pm

Sorry, I didn't do FG for some time. I had a hardware problem and several real life issues caused by the old year ending and the new year starting. And it probably won't get better over the next weeks. I'll come back to this.
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