Thought of this thread yesterday. It was 32 C (about 90 F) and 38% humidity. According to the chart, this is no problem for horses and riders - not even in the yellow! My daughter was supposed to haul to her coach's for a lesson, but the other rider's mom pulled out because of the heat. She's a nurse, and said the ER was full of people struggling with the heat. So we didn't go (the whole point of hauling there is for my daughter to ride with someone else). I would have gone, but I admit, that heat was pretty unbearable! I don't know how people cope with higher temps. Again, it has a lot to do with what your body is used to. Here, we rarely get this kind of heat so everyone is dying!
But we ride when it's -15 C no problem.