The high today in Phoenix, Arizona was 112*F. I didn't ride today but I rode for about an hour earlier this week when it only hit 109*. I ended that ride because of time available and not the conditions. In the interest of full disclosure the humidity today was all of 4% and that does make a big difference. Still, there is no denying it is BLOODY HOT! My observation is that the human body's cooling system is remarkably effective, particularly in a dry environment. I find that as long as I keep the water flowing in as fast as I'm sweating it out, I can tolerate the heat reasonably well. Avoiding sunburn is very important though. In more humid environments the evaporative cooling i not nearly as effective but the moving air produced by cycling will increase the amount of evaporation. Even though I've been drinking water while riding, it is not beyond my capacity to consume around 48 ounces of water at the end of a ride. But to reiterate, I try to keep the the water level stable rather than replenish it in one massive gulp at the end of the ride. BTW, I'm 49 years old.