By Coach Adolfo Salgueiro

As summer temperatures rise across the Northern Hemisphere, runners in warmer regions like South Florida already feel the heat. You don’t have to wait for June 21st. In many places, summer conditions arrive much earlier. Now is the perfect time to revisit how heat and humidity affect running performance so you may adjust your training and remain strong, safe, and consistent in hot weather.

In previous years, I have written about tips for summer running and guidelines to run in extreme heat. You can click the links in this paragraph to read those posts, which I highly recommend. This year, I am writing about a different aspect of this subject. While intuition tells us heat and humidity affect our running, this year, I want to dig a bit deeper into it.

I recently read about a study measuring the effects of those two elements on marathon performances. My goal today is not to show you how an 80º temperature or 70% humidity will adversely affect your marathon time. What I want to do is project what these difficult weather conditions affecting a marathon mean for you, training every day in the heat.

Let’s start with one of the study’s most interesting takeaways: runners don’t slow down simply because their core temperature gets too high. They slow down before that happens. In other words, heat doesn’t just tax your body; it messes with your brain. The rate of perceived effort rises early, and that alone is enough to force a drop in pace. You feel like you’re overdoing it, even though your physiology is within range. This is not just psychological, it’s your body protecting itself. When the humidity is high, the fatigue effect compounds.

According to John Davis’ analysis of nearly 4,000 marathon performances across 754 races, the optimal temperature range for top performances is narrow: 35–55°F (2–13°C). Performance peaks around 48°F (9°C). As conditions warm beyond 65°F (18°C), things deteriorate, and they do so fast, especially when humidity becomes part of the equation. A heat index of 80°F (air temp and humidity combined) can slow your marathon pace by roughly 3%, and it only worsens.

In extreme cases, like 75°F and steamy, you’re looking at 6–8% slower marathon times. That’s 10–15+ minutes lost for someone aiming for 3:00, 14-19+ if you aim for 4:00, and 18-24 if you are shooting for 5:00.

But here’s where it applies to your day-to-day running: that slowdown isn’t just for race day. You need to adjust mentally and physically if you’re doing tempo runs, long runs, or any effort-based session in hot, humid conditions. Your body is under extra fatigue. It’s not about being weak, it’s about heat load. Think of it as carrying an invisible weight that gets heavier the longer you carry it. You don’t need a calculator to tell you this. If it feels harder, it is harder.

Humidity alone is not a significant issue until the temperature crosses 65°F (18°C). After that, humidity and heat multiply their effects. So, if you’re in a place like South Florida, where hot and humid is the baseline from May through September, your training isn’t just slower, it’s under constant stress. Knowing and accepting it allows you to reframe your expectations, adjust your pace, and be kind to yourself when the stats on your watch don’t match your effort.

Davis recommends using the heat index rather than raw air temperature to judge effort. The “feels like” measure is a more useful parameter. If you’re training in an 80°F heat index, expect that same 3% drop-off, even on a regular weekday run. The longer the effort, the more critical it becomes to respect this adjustment. And this doesn’t just apply to marathoners. For any longer sustained effort, these principles apply.

Heat adaptation, hydration, and fitness all play a role in adjusting to the reality of the new weather. If you’ve been out there, you know the stress is real, and if you’re not changing for it, you’re not training smart. Use this validated science to listen to what your body’s been trying to tell you. The heat isn’t just uncomfortable; it’s performance-changing. If you ignore it, it can become dangerous and even life-threatening.

Share your experiences running in the heat in the box below, so other runners can benefit from them.