A couple of things to note about the Weather Schema:-
It is not a xAPWeather Schema, ie it is not a formal core xAP Schema with a specification like for instance xAPBSC
If you look at the schema, most of the keys are optional. Also you will see that most data 'types' eg wind, are available in both imperial and metric forms
This schema originated many years ago and I think was based on the data either from a one wire weather station or the data aquired from a page scrape, or perhaps both.
I can think of 3 readily availbale sources of Weather Schema data
mi4's weather connector, which grabs metar data like
EGNJ 081020Z 00000KT 9999 BKN026 00/M02 Q1044
The WeatherDisplay program which interfaces with a variety of weather stations
Cumulus which similarly interfaces with a variety of weather stations
The implementation is based on the schema, but which options are selected depends on the inclinations of the implementor and the data available. In addition other keys may also be added. So the Weather Display program produces messages like
So you can see here that he has added rain (in inches) and humidity, but left out for instance wind direction in degrees (regrettably).
The 'extra...' keys come from someone elses extention to the schema, I think for one-wire, which have been slavishly copied here but are not used.
James' (mi4) xAPWeather connector produces messages like
CloudK.0=Broken cloud at 0.8km
CloudM.0=Broken cloud at 0.5M
Which just decodes the METAR data and adds an extra section for Sunrise and Sunset based on Latitude and Longditude. Regrettably since the BBC has changed their Weather site and / or the location indexes the non metar data sources no longer work and so the Weather Forecast messages are not available
I don't have examples of the Cumulus message to hand, perhaps you could post an example here?
For completeness, there is another, very different, version of the schema on Patrick Lidstones site, but this does not seem to be the common usage, or adopted as the schema on the official xAP site and so probaly should be avoided.
So to answer your main question, from the above, I would advise that you base your extension to the schema on what has gone before
I have a spare Dockstar here so I would be very interested to hear if you make any progress with that
I am sure there may be other comments, but you can see above what has gone before
Hope this helps
(The other kevin)