As the runner’s rule of thumb goes, you should always dress like it’s 20 degrees warmer than it is. That’s why you see racers in singlets and shorts when it’s 45 degrees out: Once your furnace gets going, winter turns to spring and spring turns to summer.
But what do you do when it’s already summer? When a heat wave sets records seemingly every month, and when the humidity rises along with the temperature, making the ambient air feel like an oppressive sweater? Well, for one, you reset your expectations. You’re going to be slower, and everything’s going to feel harder. You should also think a little more about your running gear—make sure you have enough water and are as comfortable as possible.
There’s no across-the-board truism for running in the heat, and acclimatization is a hell of a drug: Some folks don’t leave the house without enough water for an overnight camping trip, while some camels can happily lope along under the blazing sun for an hour without noticing. Either way, make sure you’re prepared, from the best shoes to sunscreen to hydration packs. Anything to stay off the dreadmill, right?
Updated July 2021: We removed older picks, like Supergoop sunscreen, and added new ones, like the Coros 2 and UE Fits.