For me, and of course YMMV, the only important number is humidity.
Riding in Utah with 100 degrees and 2% humidity was wonderful. The sweat evaporates as soon as it comes out and does its little cooling trick. Just had to make sure to keep drinking water.
In St Louis, any temperature over 76 with 80% or higher humidity and the evaporation stops along with the cooling effect.

I haven't traveled a lot in my life, but I finally figured out how powerful an effect humidity has after my second trip to Utah. The difference is amazing and I'm really looking to get the heck out of the Midwest before I get too old. Utah is the dream, wifey is open to change.

Out here, on the B, best bet to survive hot weather is put the stock windshield on during the summer months to get some evaporation. And, as always, seat beads!