Running in Heat & Humidity: Adjusting Pace Without Losing Progress

It’s June in Knoxville. You walked out the door for your morning run and the air hit you like a warm wet blanket before you made it to the end of the driveway. You’re half a mile in, your heart rate is already elevated, your pace is slower than it should be, and some part of your brain is convinced your fitness has evaporated overnight.
It hasn’t. But we need to talk about what’s actually happening—and what you need to do about it.

When temperatures rise, your cardiovascular system has to work harder to cool your body. Blood that would otherwise be directed toward your working muscles gets rerouted to the skin to help dissipate heat. Your heart rate climbs. Your perceived effort goes up. Your pace goes down. Humidity compounds it. Your body cools itself primarily through sweat evaporation, and when the air is already saturated with moisture—which in Knoxville from June through August it very much is—that evaporation slows down significantly. Your cooling system becomes less efficient, your core temperature rises faster, and your body has to work even harder to keep things under control.

The result is straightforward: the same effort that produced a certain pace in April will produce a slower pace in June. That’s not failure. That’s physiology.

This is worth saying plainly. Knoxville summers are not just hot—they’re humid in a way that is genuinely oppressive for runners. The combination of heat and humidity here creates one of the more challenging warm-weather training environments in the region. If your summer runs feel brutally hard, they are. You are not being dramatic and you are not losing fitness. You are running in conditions that make the same effort objectively more difficult. Acknowledge that. Adjust accordingly.

What Adjusting Actually Looks Like

Stop running by pace. For the summer months, pace is not a useful metric. What matters is effort—and effort in the summer needs to be governed by feel and heart rate, not by the numbers on your watch.
Easy runs should feel easy. Conversational. If you can’t speak in full sentences, you’re going too hard. If your heart rate is sitting where it normally sits during a tempo effort, you’re going too hard. Slow down. It doesn’t matter what the watch says.

A general rule of thumb: expect to add anywhere from thirty seconds to two full minutes per mile to your easy pace on hot, humid days and still be working at the same relative effort as a cooler day. Some days it will be more. On the worst days in July and August, it will be significantly more. This is normal. Adjust without ego.

Time of day matters more in summer than any other season. If you’re running midday in Knoxville in July, that’s a choice with real consequences. Early morning gives you the lowest temperatures and the least direct sun. Evening gives you lower temperatures than midday but often higher humidity. Neither is perfect—pick the lesser problem for your schedule. Route selection matters too. Shade, surface, and airflow all affect how hard a run feels. A shaded trail at the same pace as an exposed road run will feel meaningfully different. Use that.

Go into every run hydrated. Not drinking water as you head out the door—actually hydrated, starting the day before. Summer running on a hydration deficit compounds every other challenge the heat creates.

The Work Is Still Work

Heat training has real physiological benefits. Your plasma volume expands, your cardiovascular system becomes more efficient, and your body gets better at thermoregulation. Every sufferfest mile you log in June and July is making October’s race day easier—even when it doesn’t feel remotely like training.

Slow down. Run by effort. Control what you can control—time of day, route, hydration. Stop measuring summer miles against spring miles. And trust that every run you complete in this heat, however slow, however ugly, is doing exactly what it’s supposed to do.

October will remind you why you did it.

-Brittany Kellogg, CPT, CSNC