You probably know the recommendations for riding in the heat safely. Acclimate yourself with two weeks of consistent heat riding. You don’t have to ride every day in high temperatures, but you shouldn’t go more than two or three days between heat sessions. Moderate intensity activity is more effective than sitting passively in a sauna or hot area. If you’ve been riding much all summer, your body is likely already acclimated. Your blood plasma volume has increased, in order to produce more sweat. You’re sweating earlier and over more of your body. Of course staying hydrated and well fueled is critical to acclimation. There used to be a theory that you could train your body to work with less hydration and trainers would check a rider’s water bottle after a ride to see that they hadn’t drank it all! This is dangerous and ineffective. Water alone isn’t enough, either. You need to replace you electrolytes; salt, potassium, etc.
Morning and evening are obviously better times of day to ride in the heat, if you have a choice, but so is in a light rain, after a storm, or at least when it’s cloudy. I know some people hunker down on their indoor trainers when the heat soars. You could do the most heat-generating part of a workout with fans on you in air conditioning, and the rest of it outdoors at a moderate pace if you have to. I know someone who rides at 3:30 in the morning- you know who you are. Night-time can be a great time to ride the trails, but use excellent lights and don’t out-ride your beam. At all hours, don’t practice Jack rabbit starts, as you won’t be sweating enough to cool off efficiently yet, so warm up first and get sweating.
Whether it’s a workout, ride, or commute, wetting yourself down is a huge benefit. Dump water over your head, dunk your clothes in water, ride through a sprinkler, even take extra water along just for this purpose. When we were bike touring in S.E. Asia, we’d stop at every gas station to saturate our jerseys and sunsleeves in the ever-present water barrels. That would keep us cool enough until the next station. Commuting here in Lincoln, when I have to be riding at the hottest, sunniest times of the day, I’ll wet down a long-sleeved loose, lightweight over-shirt and have air-conditioning for the next 10-15 minutes. This is also cooler dry than having the sun hit my skin directly, though people don’t believe me until they’ve tried it. If you’re commuting and can’t change your clothes, like me, choose lighter fabrics with patterns rather than solids as they won’t show sweat as much, and wicking fabrics to pull the sweat away from your body with evaporative cooling. I carry a small micro-fiber towel, and try to get there a little early, as I’ll be sweating profusely upon arrival. I cool down fairly quickly, though, so if I can stay outside in the shade for a few minutes, it makes a difference. Those of you with workplace showers and a place to change, enjoy.