Hehe. Gunner and soloman live in Arizona. It's a desert, it gets up to about 120f in the heat of the summer (about 48 C)! They'll claim a "dry" heat, yeah dry like a BLAST furnace! lol
I live in Connecticut, the northeast US on the shoreline. Rarely does it break 100F, and I visit Ontario Canada and take my bike with me in the summer. The temperature isn't as high as Arizona, but the humidity tries to make up for that.
At home, on those really hot days I can ride without a helmet, but I'll keep the helmetless rides to around town, low speed stuff. Canada requires a helmet, so on those hot days and short rides, a half helmet would do the trick for me.