After reading this title, most people would advise on staying home, turning AC on and serving plenty of cold drinks. However, for us, cyclists, this may not be an option. Here is what you can do to survive the heat, such as one that hit us this week in Boston.
Leave your bike home and drive your car or take a bus instead. This assumes you are willing on giving up cycling and switching over to another vehicle, where you can benefit from air conditioning. Again, not an option for every hardcore biker but if outside temperature hits 90F (32C) and over 70% humidity, you will likely be ready to say "screw it" and enjoy AC inside your car.
Stay indoors and ride on a trainer. Good option for those who desperately want to ride their bikes without giving up the cool indoors. Main problem - you won't get anywhere this way.
Go for a ride very early in the day. By early I mean like 5am to 7am. This way you can avoid the heat and enjoy your ride the best time of the day - the coolest and least humid. The problem with this approach is obvious - it will work for recreational rides but don't expect to run many errands at 5am.
Don't ride with a helmet on. I can't emphasize this one more. I still see cyclists on a quiet bike greenway, such as our Minuteman Bikeway riding with helmets on. Seriously, in a weather like this the risk you damage your head by falling off your bike is significantly lower than getting a heat stroke. Helmets are good for competitive sports but shouldn't be needed in a well-designed city. Sorry, I value my brain too much to let it cook under a helmet.
Buy a "pissing helmet". If you are one of those believers who think that bike and helmet is the same as sushi and soy sauce - they should always go together, consider this crazy invention. It looks as weird as it works and I am not going to try it, but if you don't mind your helmet to pee on your head then who knows - maybe it will keep you cool?
Build an AC for your bike. If everything else fails, try equipping your bike with your own "AC unit". All you need is one of those handlebars-mounted baskets. Next, fill it with ice to the top and go for a ride. Melting and vaporizing ice will create a very cool fog in front of you that will be next blown over your face and body as you ride through it. This way you can ride your bike and have air conditioning at the same time (patent pending!).
For those who have generator hubs on their bikes, try to connect a small desktop fan to your hub and install it on your handlebars. This will enhance the cooling effect dramatically. Main problem here - riding your carbon road bike with a huge basket full of ice is not exactly aero.