"Zero wind" (00000KT) scenario not completely handled

[SkySails]

Hello! First of all, thank you for your awesome work on this family of parsers 🙌

I am debugging an issue in the Typescript port of this project, and it seems like the same thing occurs over here and since that other project is a port, I think it's best to work with this upstream first!
The following METAR report incorrectly parses to the wrong unit:

KATL 270200Z 00000MPS

The result, as per https://www.metar-taf-decoder.com, is Wind: Speed : 0 KT Direction : North | 0° - KT instead of MPS.
This is a really small issue, but it has other consequences in the Typescript port which you can read about in the linked issues at the bottom!

Related issues

• "Zero wind" (00000KT) scenario not completely handled aeharding/metar-taf-parser#80
• Visibility incorrectly parsed as wind speed, visibility not included in output aeharding/metar-taf-parser#79

More details

The culprit seems to be this regex, as it results in the following incorrect matching:

python-metar-taf-parser/metar_taf_parser/command/common.py

Line 82 in 164900f

regex = r'^(VRB|00|[0-3]\d{2})(\d{2})G?(\d{2,3})?(KT|MPS|KM\/H)?'

It seems to be there to satisfy this test case, which includes an invalid METAR string according to the specifications I can find. As usual, most are not too explicit on the matter but this is one example:

Wind speeds (ff) are two digits (or three digits if required), in knots.
https://meteocentre.com/doc/metar.html

So I guess this is a case of choosing which case to support. If we change the regex by adding an additional required zero:

(VRB|000|[0-3]\d{2})(\d{2})G?(\d{2,3})?(KT|MPS|KM\/H)?

The unit is now correctly parsed and the speed is correctly registered (not that it matters, it's all zeros anyway)

The downside is that the test case will start to fail. Is there a reason behind supporting 0000KT (four digits) specifically? Is it a common occurrence in the real world?

[mivek]

Hello @SkySails

Sorry for the delay.
The support for the 0000KT token actually comes from a PR from the Typescript project:

• Change minimal visibility regex aeharding/metar-taf-parser#66
  The original METAR was KATW 022045Z 0000KT 10SM SCT120 00/M08 A2996 however after checking in the Federal Meteorological Handbook, it seems that 0000KT is invalid.
  Here is an extract of section 12.6.5 regarding the Calm wind

d) Calm Wind. Calm wind shall be coded as "00000KT"
The first three zeros represent the wind direction and the last two zeros represent the speed.

(VRB|000|[0-3]\d{2})(\d{2})G?(\d{2,3})?(KT|MPS|KM\/H)?

is the right regex.

Thank you for your analysis of the bug, I will push a fix.

[SkySails]

No worries at all!

Ah, I should have looked through the issue history too, that's my bad. Thank you for referencing it!
You're welcome, I'm happy to help!

[mivek]

Hello @SkySails

I merged the fix and released a new version 1.8.2. I will report the fix in the Java version in the next few days.

Thanks again for reporting the issue

[SkySails]

That's great news! Thank you for fixing it!