The Heat Math That Saves Your Summer Runs

Here's the move: before you head out on a hot day, add the temperature and the dew point together. If that number creeps up near 150, plan on slowing your easy pace by about 3 to 5 percent. That's it — that's the whole decision, made before you step outside instead of after you've already cooked yourself at mile 3.

Why dew point matters as much as temperature: it's a decent stand-in for humidity, and humidity is what actually wrecks your cooling system. Sweat only cools you when it evaporates. On a muggy day, it just sits on your skin doing nothing, so your core temperature climbs faster than the thermometer alone would suggest. A 75°F day with high dew point can feel and perform worse than a dry 85°F day.

The other half of the equation is fluid, and timing beats volume. Drink 10 to 20 ounces about an hour before you head out — enough to top off without sloshing. Once you're moving, if you're out longer than 20 to 30 minutes, carry something and take small sips every 15 to 20 minutes rather than chugging when you finally feel thirsty (by then you're already behind).

For beginners especially, this matters because you don't yet have the pattern-recognition to know what "too hot, too fast" feels like until it's happened once or twice — usually badly. Slowing down proactively isn't giving up on the workout; it's the only way to actually finish it intact. If you can shift long runs to early morning, do it — you're fighting less heat and less sun, which means you get to run the pace you actually trained for instead of the pace the weather forces on you.