I rode once last year in 104F temps. It was quite the adventure. A few things I noticed:
- I didn't have power and rode slower.
- My HR was higher (as goldfinch said).
- I needed food (I don't normally eat on this 25-mile route).
- I drank one of my bottles of water, and used the other bottle for dousing my head, neck & shoulders.
- I stopped about half-way to get some cold water from a shop, ate my PowerBar, and that helped alot. But I needed more. I should have been munching little bites throughout the ride.
I've been riding this year in 100F+ temps this week, and watching my water & food intake and stopping refilling more frequently for refills and haven't had any problems.