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- Why does heat slow down running pace?
- How much does heat affect running pace?
- How can you deal with heat making running so much harder?
- When should you turn to indoor runs?
Higher temps mean one thing for runners: A slower pace.
Every runner is susceptible, regardless of fitness or experience. In fact, a study of the 2018 London Marathon—at 75.4 degrees Fahrenheit, it was the hottest in the competition’s then-37-year history—reveals that the average finishing time was 20 minutes slower than all the previous years. And 75 degrees doesn’t even compare to temps reaching the 80s, 90s, or even 100s, as cities across the country experience this time of year.
We know slowing down can feel difficult for runners vying for goal times and how seeing your pace fall well below your average (while still feeling difficult!) can be mentally tough to handle. So we’ll reiterate that everyone slows down in the heat and it’s definitely okay—actually encouraged—to do so. With this guide, we also help you understand just how much heat can slow you down, plus how to make running in high temperatures more tolerable.
Why does heat slow down running pace?
Your body is constantly responding to the environment in an effort to regulate its internal temperature. It has specific processes for staying cool when running in the heat and humidity. Often, those processes make you move more slowly.
“The body’s main source of cooling itself during exercise is through the evaporation of sweat,” says Todd Buckingham, Ph.D., an exercise physiologist with PTSportsPRO in Grand Rapids, Michigan. But the body can’t evaporate sweat as efficiently as it gets hotter and more humid. When sweat can’t evaporate quickly, your body’s core temperature increases. As your core temperature rises, your body tries to cool itself by pushing more blood to the arteries and veins near the surface of your skin. This cooling effort makes the heart work harder and diverts energy from working muscles.
The result: Every step takes more effort, causing your pace to slow.
Running in the heat and humidity also causes you to lose more fluid via sweat, which increases your likelihood of dehydration.
Dehydration brings another set of issues that can slow you down. This includes leading to a loss of fluid (plasma) in the blood. “Less plasma in your blood means your heart can’t pump out as much blood with each beat,” Buckingham says. This causes your heart to beat faster. “To provide the muscles with enough oxygen to provide the body with energy, the heart must pump more times to meet the same oxygen requirement,” Buckingham explains.
How much does heat affect running pace?
Heat and humidity affects each runner’s pace differently. However, as a general rule of thumb, the typical runner can expect to add 20 to 30 seconds per mile for every 5-degree increase above 60 degrees Fahrenheit.
In one study, researchers examined the relationship between running performance and weather conditions using data from more than 382,000 Boston Marathon finishers from 1972 to 2018. They found that a 1-degree Celsius bump in average temperature slowed running performance by an average 1 minute and 47 seconds for all finishers.
An older study published in PLoS ONE gathered results from six major marathons (Paris, London, Berlin, Boston, Chicago, and New York) from 2001 to 2010. The optimal temperature for male and female runners varied from 3.8 to 9.9 degrees Celsius (38.8 to 49.8 degrees Fahrenheit), with every 1-degree increase above the optimal temperature causing pace to slow by 0.03 percent. So, if you run a 10-minute mile in 9.9 degrees Celsius (49.8 Fahrenheit), expect to run a mile in 10:18 when the temperature increases to 19.9 degrees Celsius (67.8 Fahrenheit) and a 10:36 at 29.9 degrees Celsius (85.8 Fahrenheit).
Expect to add about 20-30 seconds per mile to your pace when running in heat.
How can you deal with heat making running so much harder?
Unless you’re willing to log all your warm-weather miles on the treadmill, you’ll have to learn to make friends with heat. The typical adjustment period for heat acclimation is about one to two weeks, according to the Cleveland Clinic, so runs should start to feel slightly easier after that (depending on how much time you spend outside and the heat level).
In the meantime, running coach Nicole Gainacopulos, C.S.C.S., founder and head coach with RunMomentumMKE, offers these tips to help you cope:
Be strategic about your routes
Opt for shorter loop routes so you can head indoors if the heat is too much. Other good route options include ones with plenty of shade and those that go through parks with restrooms and drinking fountains where you can refill your bottle, Gainacopolus says.
Slow down
As mentioned, you can’t expect to hit the same paces you normally hit when running in cooler temps. If you usually track your heart rate, adjust your pace until it matches your target heart rate, Gainacopolus suggests. Otherwise, use the talk test to gauge your effort—if you can talk but not sing, you’re working at a moderate intensity.
Hydrate well
Gainacopulos recommends bringing water with electrolytes every time you run in the heat. This can help prevent dehydration and overheating. “If it’s super hot and you’re sweating a ton, try to take one to two sips every five minutes or so,” Gainacopulos says.
Be sure to drink plenty of water before and after your run, too. You can gauge your hydration levels by checking your urine color. Aim for pale yellow, Gainacopulos says.
Wear appropriate clothing
Wearing clothes made of moisture-wicking fabrics like polyester, nylon, and bamboo can help you stay cool and dry on hot runs. Avoid cotton, which soaks up sweat, Gainacopulos says. Don’t forget sunglasses and a hat.
Think of the perks
As your body gets used to running in the heat, you’ll reap benefits that carryover into the colder seasons. Your body begins producing more plasma, which helps delay dehydration and allow your heart rate to remain stable, Buckingham notes. Getting through hot runs also makes colder efforts feel easier. “Think how all the hard work you’re putting in during these hot weather months is going to make fall racing that much better,” Gainacopulos says.
When should you turn to indoor runs?
Some days, it’s best to stay inside. Exercising in hot, humid weather increases your risk for heat exhaustion and heat stroke. These heat illnesses are potentially life-threatening, causing symptoms like headache, nausea, dizziness, confusion, seizures, and eventually, loss of consciousness, per the CDC.
To stay safe, move your runs indoors (or do a cross-training session) anytime there’s a heat advisory or you’re worried about the temp.
Lauren Bedosky
Lauren Bedosky is a freelance health and fitness writer who specializes in covering running and strength training topics. She writes for a variety of national publications, includingRunner’s World,Prevention, Experience LifeandWomen’s Running.
