Writing in summer can acually be tough. The weather is nice, the birds are aflutter outside, and there are so many distracting things just beyond the office window. Here are a few things that help me stay focused. Man, I really do have a pathology for lists, don't I? Maybe I should see someone about this.
Allow yourself to write short chapters/sections. Don't set a page limit. Instead, write as much as your mind and body will let you, and when you feel the energy starting to wind down, give yourself a breather.
Incorporate the outside world into your writing. Describing a summer day can actually be inspiring, and soon, you'll be concentrating on the textual breezes and bird-songs of your own design, rather than the enticing ones outside.
Drink iced coffee. Seriously. It's the same as regular coffee, but it doesn't make you sweat. And once the ice melts fully, you end up with a third-of-a-cup of diluted, cold liquid, like a chaser.
Go somewhere air-conditioned. Seating is always the most important consideration, and after that, wifi access. If you can't leave home, find out where the cat's sleeping and set up camp there. Cats always know where the coolest spot in the house is.
ps: the picture of Lafayette Reynolds above is purely to give you an idea of how you should be dressed for summer writing.