I do short rides early in the AM, before work, a couple of days a week. I go whether it's warm or cold, but today, at 5:30, it felt positively awful out there. It must be 90% humidity. Should have taken my inhaler.
My best riding temps are between 55 and 70. I have to acclimate to anything higher. Last summer, I had to; we had 25 days over 90 degrees.
I lived in Florida for 5 years and AZ for 16. I can't imagine riding in either of those places during the "hot" months. I used to walk at 5 AM in AZ, and it was already 105 degrees by then, in the summer. All of my exercise was done in a gym.