This is what I wear when it's above 90:

http://www.icebreaker.com/site/icebr...aero_tank.html

Really, I wear this when it's above 75, but I find that when it's really, really hot this works the best for me. It feels so much lighter than any of my poly stuff and way better than the cotton stuff. Icebreaker has buried the link to the really light stuff as they're pushing their fall line right now, but you can still order it.