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wmo-im / iwxxm

XML schema and Schematron for aviation weather data exchange
https://old.wmo.int/wiswiki/tiki-index.php%3Fpage=TT-AvXML
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Missing temperatures #165

Closed mgoberfield closed 4 years ago

mgoberfield commented 5 years ago

During testing and validation of IWXXM v3.0 TAFs, a common validation error occurs when only one temperature extrema is provided in the TAF. For example, max temperature and time of occurrence is provided, but not minimum temperature/time -- it's missing entirely (no use of solidus). The missing temperature element cannot be nil'ed.

Should the TAF be considered invalid in such cases or iwxxm:min(max)imumAirTemperature element type be changed to be nillable?

blchoy commented 5 years ago

Currently the temperature forecast block is optional, but when present it must contain a maximum AND a minimum temperature forecast. This is our interpretation of the 'O's and 'M's in the Annex 3 template. We checked the TAFs in 2017 and noticed that only one aerodrome reported a single max or min, but there are a lot reporting 2 max + 1 min or 1 max + 2 min. The latter suggests that the majority of the cases consist of typical bell (or inverted bell) shaped profile. To straightly comply with Annex 3 we may want to repeat the single peak or trough value but obviously this is not what people is doing with TAC.

I agree with you that we may want to take this last opportunity to make the max and min nillable (equivalent to solidi in TAC), or even optional in IWXXM 3.0 to align with existing but not quite Annex 3 compliant practices. If there is any discussion telling us that the former interpretation is indeed the pragmatic (or even correct) way of representing one or three high-lows that should surely give us more confidence to make the change.

blchoy commented 5 years ago

Tim Hailes had given us his opinion on this matter:

I believe there are 3 things that each indicate that temperatures should be provided in pairs of maximum and minimum. These being:

  1. Table A5-1 – The "temperature" (group) element is optional, not the individual temperature forecast fields;
  2. The temperature examples always show pairs of temperature forecasts. If single temperatures were allowed then these should be included in the examples; and
  3. Appendix 5 section 1.2.5 states the maximum and minimum. It doesn't say maximum and/or minimum or similar.

1.2.5 Temperature

Recommendation.— When forecast temperatures are included in accordance with regional air navigation agreement, the maximum and minimum temperatures expected to occur during the period of validity of the TAF should be given, together with their corresponding times of occurrence.

So it is a coding issue which applies to both TAC and IWXXM. It is also a potential problem for TAC-to-IWXXM translation: Shall we treat odd numbers of TX/TN in TAC untranslatable and make partially translated IWXXM counterparts?

blchoy commented 4 years ago

I am closing this at the moment as there is nothing to be done from TT-AvXML perspective until the relevant requirements in Annex 3 has changed.