Running in heat and humidity: how much to slow down.

What heat actually costs you

Every July I get the same message, worded almost the same way: "My easy pace is 40 seconds slower and my heart rate is 10 beats higher. Am I losing fitness?" No. You are paying a tax.

Running is a heat engine with terrible efficiency. Most of what your muscles produce is heat, not motion, and your only real way to dump it is evaporating sweat off your skin. Hot air shrinks the gradient. Wet air stops the evaporation, so sweat just runs off you without cooling anything. Your body then makes a choice: it sends blood to the skin to cool you instead of to your legs to run. Less blood to the legs means less oxygen per beat, so heart rate climbs to compensate. Same effort, higher cost, slower pace. Nothing is wrong with you.

The mistake is not running in heat. It is running your printed paces in heat and calling the result a fitness problem. That is how a good summer block turns into six weeks of failed workouts.

Dew point is the number, not humidity

Most runners check temperature and relative humidity. Relative humidity is close to useless here, because it is relative. 90% humidity at 50°F is a cool damp morning. 90% humidity at 85°F is a sauna. Same number, completely different run.

Dew point is the absolute measure of moisture in the air, so it tells you directly whether your sweat has anywhere to go. Every weather app shows it, usually one tap down from the main screen. Rough field guide for how it feels on the run:

The method I use adds air temperature and dew point together in Fahrenheit for a single combined stress score. 78°F air with a 62°F dew point is 140. The same 78°F with a 45°F dew point is 123. Same temperature on the app, very different day. The approach comes out of Coach Hadley's Maximum Performance Running work, and it has held up across every summer I have coached through.

How much to slow down

Here is the scale I hand athletes. Add air temperature and dew point, find your band, apply the percentage to your normal pace.

Put real numbers on it. An 8:00 per mile runner gives back 10 to 15 seconds a mile on a 140 day, closer to a minute on a 165 day. A 6:00 runner on that same 165 day gives back 25 to 30 seconds. The percentage is the same for everyone. The seconds are not, which is why "just add 30 seconds in summer" is bad advice for anyone who is not already an 8-minute runner.

Each band is a range on purpose. Acclimation moves the whole scale, which is why the first 72° day in April wrecks people and the same day in September feels like a gift. Wind, shade and your own sweat rate shift it too. Start mid-range and let effort make the final call.

Run the effort, not the watch

The adjustment gets you in the right neighborhood. Your body decides the exact address. In 15 years of coaching, the athletes who handle summer best are the ones who make effort the primary control in June and go back to pace in October. They still look at the watch. They just stopped letting it argue with them.

Three checks that beat a pace number when it is hot:

1. Breathing rhythm. You should be able to speak a full sentence on an easy run. If easy pace is 9:20 instead of 8:40 and you can still talk, that is the correct easy run. See why easy pace matters more than most runners think.
2. Late-run drift. Heart rate rising 8 to 12 beats over the back half at unchanged pace is normal cardiac drift in heat. Rising 20 or more means you are behind on cooling or fluid, and the run should get shorter.
3. The chill test. Goosebumps, a cold flush, or stopped sweating on a hot day means stop running. Not slow down. Stop, get shade, get fluid. This is the one signal I never let an athlete negotiate with.

How to move the workout, not cancel it

Heat does not mean the quality work disappears. It means the shape changes. Roughly 7 out of 10 athletes I coach need their summer workouts restructured rather than just slowed. What to change, in order of how much it helps:

• Shorten the rep, keep the pace. Instead of 4 x 1 mile at threshold, run 8 x 800m at the same pace with the same total rest. Same stimulus, less accumulated heat per rep. This is the single best summer substitution.
• Lengthen the recovery. Add 30 to 60 seconds to every jog recovery and use it to cool down. Walk it if you have to. Rep quality is what matters, not the rest.
• Move the session, not the effort. A tempo at 5:30am in a 135 score is a real workout. The same tempo at 5:30pm in a 165 score is a slog that costs you Thursday too. Tempo work is the most sensitive to this because it is long and continuous.
• Pick the loop, not the point-to-point. A two-mile loop with water stashed on it beats an out-and-back where mile 9 has no shade and no options.

What I do not recommend: pushing the printed pace and calling it mental toughness. A threshold session run 15 seconds per mile too fast in a 160 score puts you at VO2 max effort for 20 minutes and costs you the next two days. The workout was supposed to be repeatable.

Acclimation is real, and it is faster than you think

Summer eventually stops feeling impossible because your body adapts to heat specifically, and fast. Plasma volume expands, you sweat earlier and more, and your sweat gets more dilute so you hold on to sodium. Most of that shows up inside two weeks of consistent exposure, and it fades about as quickly once you stop. I wrote up the full mechanism in the physiology of summer running.

Two practical consequences. Do not hide from every hot day, or you will arrive at a warm September race with no adaptation at all. And do not chase the adaptation with hard efforts, because easy running in heat is already a heat stimulus.

Fluid and sodium, without the overthinking

Hydration advice has gotten complicated for no good reason. Three rules cover almost everyone.

1. Start topped off. What you drink in the 12 hours before a hot run matters more than what you carry during it. Pale urine before you head out is the whole test.
2. Drink to thirst, and put sodium in it. For anything over an hour in heat, plain water is the wrong choice. You lose salt in sweat, and replacing only the water dilutes what is left.
3. Weigh yourself before and after one long hot run. Once is enough to learn your sweat rate. Losing more than about 2% of body weight means you need a plan for fluid on route, not more willpower.

If your kit dries white with salt after every run, you need meaningfully more sodium than the runner next to you. That is an individual setting, not a rule you read somewhere.

Racing in the heat

Racing is where the adjustment matters most and gets ignored most, because you picked the goal time months ago in cooler weather. Set race pace off the conditions on race morning, then run the first third slower than that. Heat compounds. A marathon 3% too fast in cool weather costs you a few minutes at the end. The same 3% in a 155 score is the difference between a finish and a walk from mile 20.

Race-day habits worth real time in heat:

• Take water at every station from the first one, before you feel like you need it.
• Ice in the hands, down the front of the singlet, and on the back of the neck. Cooling the skin lets you hold pace longer.
• Run the shaded side of the road even if the line is longer.
• Reset the goal out loud before the gun. Deciding at mile 18 that today is not the day costs far more than deciding it at the start.

Athletes hate that last one. But every runner I have coached who reset the target on a hot morning ran a better race than the version of them that refused to.

What to do tomorrow morning

The short version, in the order you should run it:

1. Check air temperature and dew point. Add them. That is your score.
2. Run the number through the heat pace calculator to get your adjusted easy, tempo and interval paces for today.
3. Set the adjusted pace as the ceiling, not the target. Effort still makes the final call.
4. Go early if the score is above 150. Shade and time of day beat any gear.
5. Above 180, move indoors or make it easy and short. No hard session at that number pays for itself.
6. Log the conditions next to the pace. In October, that log is how you will know summer did not cost you anything.

Most runners skip step six, and it is the one that saves the block. Pace written without context reads like a bad summer. Pace next to a 158 reads like a runner who trained well in hard conditions.

If you would rather not manage all of this alone, that is what 1-on-1 coaching is for. You can apply here and we will talk it through on a call.

Written by

Coach Johnny Crain

2:12 marathoner. 4× US Olympic Marathon Trials qualifier. 4× NCAA national champion and 4× NCAA runner-up. Spoke at the RRCA National Convention. Founder and head coach at RunFitCoach, where he coaches every athlete personally.