I read a formula someplace about taking the humidity and the temp and adding them and if it was over a certain number it was too hot to ride. I'll see if I can find it.
I know if the humidity is really high its hard for the horse to stay cool because the sweat doesnt evaporate.
I just figure if the sweat is running down my back and making a little puddle under my butt in the saddle. Or my jeans are so sticky I can't get my leg up to the stirrup its too hot