It seems like the weather in the Mid Atlantic region went from cool and rainy to extremely hot and humid overnight! As the temperature climbs into triple digits it is vital to remember how to keep cooler while riding.
Stay hydrated with water. Yes, I know the marketers of “sports drinks” are good at promoting the necessity of their products, but have you looked at their labels lately? Mostly sugar! Sports drinks should be used sparingly with an emphasis on electrolyte replacement after the ride.
Drink water prior to and during the ride. I usually make a habit of drinking a glass or two of water about hour prior to my ride and then drinking constantly throughout the ride, particularly if a long one. If you are thirsty during your ride, you may already be dehydrated.
Get acclimated and plan your ride. We just got back from 10 days in Jackson Hole Wyoming where the average day temperature was about 65 with no humidity. Tomorrows ride in the 90’s with 95% humidity will be a slower and easier pace. And, I will be riding the C&O Canal where there is plenty of tree canopy. Some trails while beautiful have little tree canopy and can be miserable in full sun. And sunscreen while riding (and sweating) can be a pin.
Dress appropriately. Think loose light-colored clothing that can wick away moisture form the skin. I typically will wear in extreme heat a light-colored cotton t shirt. The multi colored miracle fabric tight biking shirts are not my preference. And as I am prone to heat induced migraines, I am sensitive about keeping cool. The bandannas with water absorbing gel beads help keep me cooler and I never ride in the heat without one. I periodically stop and pour cool water over it and am good to go!
Plan on water breaks and take extra water. The manufactures of water hydration systems Camelbak have an interesting hydration calculator which takes into consideration your age, activity, temperature etc. See it here