I think a little common sense goes a long way in this kind of situation. I wouldn't make my decision based on a thermometer. The way the heat effects a person/animal depends greatly on many factors. IMO, the best determining factor is how you feel when you are at the barn for a little while and what kind of riding you intend to do. Most importantly though, listen to your horse. It's in the high 90's here and very humid, so if I'm going to ride it needs to be early or late in the evenings. I did not make this decision based on the thermometer, though, I made it based on the fact that during the time it takes me to put out feed, I work up a sweat. If I were to go solely on thermometers though, I'd say anything in the 70's most certainly, 80's highly likely that your horse can do it, and 90's proceed with caution and take all factors into consideration. Horses are not the wimps we make them out to be.