(above: 1941 Schwinn New World 3-speed bicycle)
Today I took about an hour ride in the afternoon, and it was very, very hot. I will take another, shorter ride this evening, just before dusk. I ride in cargo shorts, a t-shirt, and sneakers. I do have a helmet, which has pretty good ventilation.
I also like to pick out riding areas with more shade.
These are all little things one can do to stay at least a little cooler riding in the summer. It's all very basic, but worth keeping in mind. You can make life easier by being selective about your ride duration, time of day, attire, type of bike, and pace. It may be hot, but I don't want to miss any road time, when I can get it.