Like there would be a greater chance for rain and fog in that area but not always. I try to remember which game it was but there were sandstorms and what not and Dead Space 3 had snow storms in some areas. Not knowing where I go or what is in front of me makes it more fun to explore a new area. Hope the map will be grey in places we didn't visit yet.
Yeah, I'm sure there are areas that are more prone to certain weather types (again, they could have altered the swamp simply for the videos). The demo from E3 last year featured a forest in Skellige (an area that is often quite cold and murky, from the looks of it), and the forest that the leshen inhabited got gloomier and gloomier the deeper you progressed.
One thing I will add, though, is the difference in 'grittiness' between the earlier trailer footage and this new gameplay. For example:
I love the aesthetics of this scene, slightly 'washed-out' and gritty look.
But compare that with the same fish market we see again during the Dijkstra quest in the demo (1:50 mark):
The colour scheme looks entirely different. Although, this could just be the time of day, angle, or other factors.