My horse is allergic to flies (or fly saliva or something about flies!), so he has to live in a fly sheet during fly season, which is pretty much year round where I live. Our normal in the summer is 100+ degrees and at least 60% humidity. My horse is stalled during the day under a fan and stays just as cool as the other horses in during the day. And supposedly light colored sheets actually help reflect light off dark colored horses therefore keeping them a little cooler.
I don't know where you live, but around me horses sweat in summer just standing around, so even if my horse went out during the day, I'd rather have him a little sweaty (which would happen anyway) and content, rather than sweaty and miserable and covered in welts/hives.
Sully ~Sullivan's Fly Supply~ [17.1 TB] RIP 2/24/14
Rio ~Camperio~ [18.0 Oldenburg]