I write outdoors every chance I get -- to the extent that when the weather is nice, I run an extension cord and my office chair out into my yard where I work under the shade of a gigantic pine tree. We're very fortunate with this yard in that it has several mature trees, some very nice hedges and a couple of flowerbeds. There are a lot of squirrels and tons of small songbirds, and we're planning on planting some good butterfly and hummingbird-attracting flowers as well. It's kind of hard to explain just what being outside does for me -- it puts my mind at rest, clearing it for the task at hand in a way that I've never felt indoors. When working inside I tend to feel drowsy and work is often difficult, but outside I can work for hours fully alert, even during times of the day when I might otherwise give up and take a nap. In addition, I sleep a lot better on the nights following my outdoor workdays, which sets up the next day to be even better.
I have been diagnosed with ADHD, and suffer badly from SAD when the weather is gray, so the theory is that the sunshine and wonderfully calming atmosphere outside counteracts both of those. When I'm inside, it's nearly impossible to keep my mind from jumping all over the map in terms of topics, none of which are ever what I'm trying to work on. Outside, if my mind wanders then it tends to fall on the surroundings -- the bumblebees, the breeze in the trees, the smell of the lilacs -- which then just relaxes me and allows my thoughts to quiet so I can get back to work.