Uh... off the top of my head calculations, 40C is equivalent to 104F? I think my summer days get to just below that, on average. Hottest recorded ever was 109F, but that was at an airport, so lots of concrete and very high amounts of heat producing engines may have ticked that up a degree or two.

I think my rowing activities average about 30C when we start, and the courtside fun a hair or two higher. We try to stop by the time it gets close to 39C. Even with a breeze you can't really sustain activity by then. That's when we go to get lunch and then, yep, sleep.