Your night clouds feel too bright, I think. Tried a bit of tweaking with levels and curves in PS: http://dl.dropbox.com/u/778345/westmark-paintover.jpg
night time shot needs work. the colors and tones seem washed out and the clouds are weirdly high contrast. id have a look at what cryengine does for their time of day for inspiration.
I think that night time clouds shouldn't be that opaque. Being able to see through all of them looks a little off. I would try to smooth them out (contract and colour wise) and don't make them as much see-though.