Originally Posted by DrIsotope

Basically, you just won't feel very hot. The evaporative cooling effect is working as well as it possibly can-- and without noticing any changes, you can blow your electrolyte balance and overheat your core pretty quickly. This I know from unfortunate experience.

The mention of 50oz of water is because of what I call "unsustainable cycling." Even severely moderated, water requirements for this heat will exceed 2 bottles per hour. So what do you do? To prove you're a hardman, do you stop at a convenience store every 45 minutes to fill up with water? Do you just ride laps around the house so you can refill as often as is needed? Any day where 2 bottles won't last an hour, it's just not worth riding. There are no awards, no bonus points for it. It's suffering for the sake of suffering.

Bottom line, if you're cycling for recreation, there is absolutely zero reason to be out there when it's 109º. Two years ago the Redlands Bicycle Classic was unseasonably hot for the first 2 days of the competition-- so much so that attrition due to heat-related illnesses took out about 20% of both the men's and women's fields, and temps were only in the mid-90s. So the following year they moved the event two months earlier in the year... and the TT got snowed out.