Also keep in mind that a lot varies from horse to horse. My arab does well in the heat and we ride primarily on trails, even when it's 90+ and somewhat to moderately humid. However we're riding in the shade, with lots of water crossings (up along the hudson river) and he doesn't sweat as profusely and cools off much better metabolically than some of my other horses.
My one OTTB and my clyde cross both do horribly in the heat. My clyde is worse due to body mass and size. With them I don't ride if it's high 80s and humid, let alone 90s.
I also have to be careful as I am prone to heat stroke thanks to having it as a kid. So I know if I start feeling lightheaded at all, or nauseous, that I need to stop and call it quits. Because this happens easily to me even when well- hydrated, it's a good monitor for when I need to call it quits for my horse's sake as well.