Hardcore cyclists say that there is no such thing like bad weather for bike riding - only bad clothing. And in general, they are right. Even if weather seems to be extremely bad, like a heavy downpour or a freezing snow, you can always find a solution. Wear some heavier clothes, something waterproof and you may find out that all those weather conditions are quite bearable.
But hardcore bikers are wrong. Heavy rain or sub-zero temperatures are not your worst enemy. After all, once you keep moving you produce enough heat to keep you warm. The worst weather to ride your bike is the one that hit us this weekend - humid air with temperatures close to 95F (35C). I know, some of you say "I'd rather be hot than cold" and this statement may be true if you lay down in a hammock, in a shade of some tall trees with a cold beer in your hand. But if you are on your bike, the situation is quite different. There are no clothes that can help you (unless you know about some battery-powered cooling jerseys), there is no shade, there is no A/C, there is no cold beer either. You are on your own, in the heat of the day and your water supplies tend to last shorter than usual.
Yes, I may be biased. I am not designed to the weather like this. I am from the part of the globe where some say that polar bears walk on the streets. Can't wait for this heat wave to be over. 35C is just too much.