The summer before last I cycled in 95 F weather with a heat index of 105. On top of that I could add another 5 degrees from the heat coming up from the asphalt. I cycled with a Camelbak and 2 water bottles that I used to douse myself with during the ride. However, there comes a point in which the skin simply can't cool enough, despite all the water being dumped on it and that's when it's time to get off the bike and head indoors. Your heart rate will increase and your cadence will decrease.

If you get chills, a headache, or nausea, and can't stand the heat and sun, then you are already in trouble.

Cycling in the wee hours of the morning will provide a cooler road surface. If you wait until the evening hours you may still encounter heat from the road.