Lupin and the Full moon

[Kristin]

No: HPFGUIDX 17350

As I reread PoA again it brought up an interesting question about the 
Shierking Shack sequence. Why does Lupin only transform when the full 
moon came out from behind the clouds?  I always had the impression 
that a werewolf transformed no matter what, it didn't matter whether 
or not the moon was obscured. Is this a mistake on JKR's part or just 
her idea that werewolves only transform when they are in the light of 
a full moon.  Any thoughts on this?

Cheers,
       Kristin