Heat & Humidity: Why it's Harder & How to Adjust Your Pace (Calculator)
- Cory Smith
- Jun 20, 2015
- 4 min read
Updated: Mar 10
Key Takeaways
Why pace drops: your body redirects more blood to the skin to cool you, leaving less blood and oxygen for working muscles, which reduces performance.
Run by effort, not the watch: use perceived effort to guide intensity and let pace be slower while keeping the same training stress.
Timing matters: run early morning or late evening to reduce heat load and limit pace decline.
Gear + conditions matter: wear light, moisture-wicking clothes (avoid cotton); dew point/humidity often impacts you more than air temperature.
Adjust expectations: summer isn’t peak-performance season—reset goals, focus on consistency, and trust that paces will rebound when it cools down.
Aren’t summers great? Weekends spent at the beach, pop-up parks, street festivals, and outdoor dining galore make it a great place to be. Enjoying the outdoors is just plain easier in warm weather. Except, perhaps, if you’re running.
🔥 Want to see exactly how much you need to adjust your pace? Use our free heat-adjusted pace calculator to get your adjusted splits.
Summer for the average runner means overheated runs, dripping sweat, and the constant desire for water. And, as if that’s not enough to deal with, odds are you may find yourself struggling to keep a pace or finish runs that seemed easy just two months earlier, which can cause you wonder, “What’s wrong with me, am I getting worse?”
I’m here to tell you, no, you’re not getting worse. Running in the high heat and humidity has negative effects on your running pace, and there is no way around it. Extreme heat and humidity can have an impact on your average mile pace of anywhere from 5 to 30 seconds, depending on the distance, pace, and weather.
Frustrating, right? However, understanding why your pace slows can help you deal with the frustration of running at a slower pace. Here's the deal: As you run, your core body temperature rises. In an effort to maintain a safe core temperature, your body adapts by directing a greater percentage of blood flow to the skin surface.
The higher the temperature, humidity, and dew point, the more blood will be directed to the skin's surface to be cooled. This increase in blood towards the skin causes a major problem for endurance athletes because it reduces the amount of blood available for the working muscles, and blood is a key ingredient in the process of generating energy.
As if that’s not enough to fight through, your body also has trouble taking in as much oxygen when it’s hotter, which means less oxygen is going to your muscles. Having less blood and oxygen flowing to your muscles is like trying to water your garden with a leaky hose. Sure, you’ll get it done, but it will be slower. The same goes for your running pace, sure you’ll be able to do it, it will just be slower.
So, now that hopefully you accept the fact your pace will suffer, here’s what you can do to deal with it and run the best you can on gruelingly hot and humid days.
1. Run by perceived effort versus actual pace. "Just run by effort" is something you’ll hear experienced runners say all the time. What does that mean, though? Running by perceived effort means judging your running pace by how hard it feels rather than how fast you're moving. As we just learned, our bodies have to work harder in higher temps and humidity. Using the perceived effort scale in the summer accounts for the additional stress heat places on our system.
For example, if running eight-minute miles felt like a four (moderate effort) on a one to 10 perceived effort scale in 50-degree weather, you’ll need to adjust your pace for 90-degree days with 90-percent humidity in order to maintain that same perceived effort. So your actual running pace in the heat may be 8:20-minute miles, it’s placing the same amount of stress on your body as the eight-minute miles did in ideal conditions. Get it?
2. Run in the mornings or late evenings when it’s cooler. This one is pretty straightforward. Running when it’s cooler — say at 6:30 a.m. or 8:30 p.m. — means your body won’t have to adjust to the heat quite as much and your pace won’t suffer quite as much.
3. Dress properly. This means wearing light-colored, moisture-wicking clothing and avoiding cotton. Remember how I mentioned, as we run our core temperature rises? Well, the main way our body cools itself isn't through the actual process of sweating but through the evaporation of that sweat. If the sweat cannot evaporate from your skin, it inhibits your ability to cool down. Technical wear is designed to wick the moisture away from the body so it can continue to sweat and cool itself.
This explains why humidity is the lesser-known but more important factor. Dew point affects performance more than the actual air temperature. The higher the humidity and dew point, the harder it is for sweat to evaporate off your skin. So do yourself a favor and wear the right gear.
4. Adjust your goals. As I tell my clients, summer is not the time for amazing performances, especially in the longer distances like the half-marathon and marathon. (This is why it's hard to track down longer races in July and August.) Your body just doesn’t operate as efficiently as it does in ideal racing temperatures. The key here is to run shorter races, look to have fun, and use the perceived effort scale until the temps start to drop, and odds are, you’ll be surprised by how paces that felt hard in the summer feel very easy again once it cools down.
And please, please, please accept that your pace will suffer and understand this is not a reflection of your actual fitness level. Lastly, enjoy it while it lasts, because soon enough it will be 12 degrees with a -5 degree wind chill and you’ll be begging for this 90-degree weather to return.
