This should help, but I even disagree with this. I think the numbers are too high.
When is it too hot to ride a horse? - eXtension
Except for the time I lived in SoCal, I never have ridden once it's past 85 degrees because of the humidity --- especially if there's no chance finding shade.
In SoCal I could ride until around 10 in the morning during the hot months even if the temps were in the 90's because they don't have humidity.
Well, they think they do, but 4% humidity doesn't get on the scale when a person is used to big double digit numbers
Hope the link helps