I just got back from a 20 miler. It was tough. My lunch was sprayed along the side othe rode at about the 14 mile mark, and by mile 16 I was only able to muster a 14mph pace. When I got home, I all but fell off the bike, because I didn't have the strength to completely lift my leg over the top tube. I had plenty of water, was dressed appropriately, and have ridden in weather that was hotter. BUT.... today felt hotter than I can ever remember. The temperature was only 95 and the humidity about 90%... typical summer weather for the Philadelphia area. I think what made it feel so hot was that the entire ride was in direct mid-day sun and along a blacktop road that was hot enough to fry eggs. The combination of all of these factors made today feel like the most heat intensive ride I've ever been on.
It was probably a mistake to ride with the combination of conditions I just described. I still don't feel all that well. Next time, I'll find some shade and/or wait until later in the day. So, what combination of things makes it too hot for you to ride?