I’m sure there are standards, e.g. WMO, that could be adopted, but you’ve got to get through the multi-national discussion first. For example, US users really won’t like using Celcius and having to convert to get to the real temperature 
Perhaps a starting point is propose WMO units, allow a 4 week discussion/argument period then enforce the WMO units
NWS XML schema…is it published. Last time I looked all I could find was an example snippet. From memory the definition of temperature seemed to run into about 400 lines. Either I didn’t look hard enough, or perhaps they’ve published the scheme since I went looking (about 4 months ago I think).
Sorry for dipping in and out of this discussion. I’m a little busy with various things at the moment, but I am really interested in the definition or adoption of a reasonable XML weather schema.