Ron's dirt-smudged nose

[macfotuk at yahoo.com]

No: HPFGUIDX 111330

Someone MUST have posted on this because you are mostly very clever 
and mostly very observant but it isn't something I've seen anything 
on:

Please tell me if I'm being thick and/or have missed something (a 
joke for example), but exactly why is Ron's nose smudged when Harry 
first meets him on the Hogwart's express (SS/PS)? Mrs Weasley makes 
a point of noticing it and so does Hermione (also meeting Ron for 
the very first time) and it was in the film (sorry spit I hate them -
 well, no I don't actually, but yes they're not 'canon'). 

I think in this same chapter (and again in the film) there was the 
sunshine, mellow spell failing to work on Scabbers (i.e. turn him 
yellow) which becomes understandable only later in PoA when we find 
out that Scabbers=Pettigrew - it is as likely that the spell was 
countered as Ron having got it wrong or that it was a lousy spell on 
Fred and George's part - the explanation offered by Ron in the book.

Any comments or theories about Ron's dirt-smudged nose would be 
appreciated here. Is it just, perhaps, that JKR is saying 'typical 
boy doesn't even know his nose is dirty because he doesn't care 
about his appearance'? Or something else?