Perhaps you should consider adding shadows now that you have a sun.
Top left corner trees seem to be a tab bit too blurry. Maybe you were going for that snowy effect, but it doesn't look right.
and alkady's right. There needs to be shadows with the sun.
Well, I'm not very good at such..
Yeah, I know.. Quite bad, I could try to make it better.
(the reason why it is like that is that when I pasted it on the other sky, it wasn't smooth, so I tried to blur it a bit. It got too blurry there)
BTW, I forgot to say; I took the photo, too
EDIT: I edited the photo a bit, tell me if you think it's better
pretty good effect. i know who i'll come to if i need a cloudy day made sunny