Building a Story from Atmosphere

You may not know this about me, but I rarely get to enjoy any part of autumn.  I have vicious seasonal allergies that turn my eyes into bloody, oozing, swollen slits and I generally can't breathe except for really noisily through my mouth.  So naturally, when everybody's skipping around, exclaiming over cider donuts and artisanal squash, I am generally uninterested.  I have my own private world of pain to inhabit, and I don't need any cider donuts there, just Allegra and Benadryl, thanks.

But this year I managed to outlast the ragweed somehow, and when I came to in mid-October I realized something--I would get to see the fall!  And the thing about fall is that it's the perfect time to experience atmosphere.  Leaves blushing into bright red and orange, fog creeping between the trees, the sounds of dead leaves skittering on the sidewalk--for about three weeks now, I've been walking around in awe, like have you seen this?  It's SO COOL!  And then everybody is like, yes, we know.

This fall has made me aware of how in love with atmosphere I am and how it impacts my writing.  Atmosphere is the mood or feeling evoked by the setting and dialogue and themes, although when I'm gestating a new WIP, the atmosphere comes first for me, before anything else.  I know whether I want a world to feel lovely or haunted and gritty and then everything else starts falling into place.  What kind of world would give the impression of being haunted?  What kind of characters would live in that world?  How would this characters interact with it?

For some writers, the character comes first or the world or the story, but for me, every time, it's the mood.  The feeling I want to leave a reader with.

What comes first for you?