Running in extreme temperatures is challenging, whether it’s strong winds, rain, snow, or hot days with muggy and humid conditions.
Running when it’s hot outside can make you feel like you’re burning up, and you have to be wary of your core temperature rising too much and of dehydration.
Running when it’s freezing cold can make your body feel stiff and weighted down with tons of layers, and you have to be cautious of hypothermia and frostbite.
But, while runners tend to watch the temperature trends in the forecast and pick the best time of day to get their workout in accordingly, fewer runners remember to pay attention to another critical component of the weather conditions—humidity.
Running in humidity can be even more difficult than running in the heat, and it’s actually the combination of the two, both being high, that really makes running tough.
In this guide, I will discuss why running in humidity is so difficult and offer my expert coaching tips to survive humid runs.
Why Is It So Hard To Run In High Humidity?
When assessing the risk of heat illnesses like heat exhaustion and heat stroke, humidity often plays a more significant role in the “real feel” temperature and the resultant risk.
Humidity makes running in hot temperatures even more taxing on the body because when the moisture content of the air is higher, it feels hotter.
Essentially, the thermal strain of running in the heat increases exponentially—not linearly—with an increase in humidity.
For example, when it’s 88° F (31° C) out with 40% humidity, it will feel like 88° F; when it’s 88° F with 70% humidity, it will feel like 100° F (38° C).
When you bump up to 85% humidity at the same temperature, the heat index jumps to 110° F (43° C).
The heat index helps you gauge the apparent temperature or the “real feel” outside. This metric combines the air temperature with the relative humidity.
Up until the humidity is about 40%, the humidity doesn’t really cause any appreciable increase in the heat index or how hot it actually feels outside.
But, above 40%, humidity rapidly increases the heat index because of the exponential relationship. For this reason, it’s not possible to readily calculate the heat index on your own.
However, there are plenty of online calculators to do so using the relative humidity or dew point, such as the one here.
How Does The Body Respond to Running In Humid Weather And High Temperatures?
When you step outside your air-conditioned home to run on a hot summer day with high humidity, it can feel like stepping into a tropical rainforest or the greenhouse at your local university or arboretum.
Even just sitting around outside in high heat and humidity levels can leave you feeling hot and sweaty in a matter of minutes.
Add running to the mix, and you can quickly become uncomfortably overheated, if not dangerously so.
When you run, the core temperature of your body naturally rises. In response, your body signals your sweat glands to produce sweat droplets.
Sweating aims to reduce your core temperature by carrying excess heat to the skin’s surface through sweat. The evaporation process helps cool the body by releasing heat energy.
However, when you are running in humidity, the moisture content in the air prevents sweat from evaporating readily.1Che Muhamed, A. M., Atkins, K., Stannard, S. R., Mündel, T., & Thompson, M. W. (2016). The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat. Temperature, 3(3), 455–464. https://doi.org/10.1080/23328940.2016.1182669
As a result, the heat energy stays trapped in your body without getting released.
In this way, running in humidity causes heat to build up in your body, increasing the risk of heat illnesses like heat exhaustion and heat stroke.
When running on hot and humid days, the risk of heat exhaustion escalates if you become dehydrated.
The more dehydrated you become, the more your blood plasma level drops. In turn, the body responds by going into “survival mode.”
One of the normal physiological cooling mechanisms is to increase blood flow to the skin’s surface, away from the core of your body, to allow it to cool down and release body heat.
However, when your blood plasma level drops because you have not taken in enough fluids while running, blood flow to the skin’s surface decreases significantly because the body must conserve the available blood volume.
Instead, blood flow is focused on the essential organs to keep you alive rather than spread around to the skin’s surface to cool you down or to the GI tract for digestion.
This latter point is the reason why many runners feel nauseous after consuming energy gels or sports drinks when running in the heat and humidity—digestion slows significantly when you run, but even more so as you become dehydrated.
This leaves the energy gel or carbohydrate-rich sports drink (and even water to some extent!) to sit and slosh around in your stomach.
The key to preventing this is staying on top of your hydration from the beginning of your workout, making sure you’re taking in fluid and electrolytes every 15 minutes on pace with your sweating.
Electrolytes will increase the absorption of the fluid, improving your hydration status in the extreme heat.
The higher your core body temperature rises, the faster your heart will beat to keep up with the demands of your workout and supply enough oxygen and nutrients to your working muscles.
Similarly, your breathing rate will increase because the lungs need more oxygen to fuel the overworking heart and muscles.
Again, dehydration exacerbates the cardiovascular challenge of running in humidity and heat because as your blood plasma level drops, you have less blood volume.
Therefore, less blood is pumped to the muscles and tissues of the body per heartbeat.
The heart has to beat faster to deliver more blood to keep up with the muscles’ demand for oxygen and nutrients and compensate for this reduced volume.
Most runners will experience a heart rate increase of about 10-20 beats per minute when running at 90° F (32° C) compared to running when it is 75° F (23° C).
This increase will be even more severe when running in humidity above 40%.
Finally, an often-forgotten factor that makes it even more difficult to run on steamy, hot, humid days is that your brain is adversely affected as your core temperature rises, affecting your body in a chain reaction.
At a certain point, after your core temperature rises high enough, your brain temperature also increases.
This, in turn, can affect your focus, concentration, decision-making skills, and energy. You also start to feel dizzy, disoriented, and uncoordinated with your running form.
At What Temperature Does High Humidity Make It Harder To Run?
There’s no clear answer to the temperature at which running in high humidity starts to get challenging.
It really depends on your own body mechanics, heat acclimatization, body size, age, hydration status, and training status.
Essentially, it’s at whatever point your core body temperature rises to what is deemed a “critical threshold.”
Beyond this threshold, your nervous system will induce a total-body slowdown due to the buildup of heat, and your athletic performance will rapidly decline.2Che Muhamed, A. M., Atkins, K., Stannard, S. R., Mündel, T., & Thompson, M. W. (2016). The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat. Temperature, 3(3), 455–464. https://doi.org/10.1080/23328940.2016.1182669
So, at what temperature and humidity level does the body reach this critical threshold?
It will depend on several factors and even from a weather standpoint, it’s not totally clear nor easy to calculate.
What we do know is that the body’s response to increasingly higher temperatures, even in the absence of high humidity, is non-linear.
This means the percentage by which your running performance will decline will be significantly more moving from 75 to 90° F (23-32° C) as compared to increasing from 55 to 70° F (13 to 21° C).
Moreover, as explained, humidity greater than 40% increases the heat index exponentially, so it will feel hotter running when it’s 88° F with 75% humidity than at 93° F with 50% humidity.
10 Tips for Running In Humidity
Unfortunately, no runner has the power to control Mother Nature and the weather conditions, so it might be inevitable that you’ll have to run on hot and humid days.
As a running coach who trains athletes in all types of weather, here are my best tips for running in humidity:
#1: Allow your body time to acclimate
Can you get used to running in humidity?
Most exercise physiologists agree that it takes about 10-14 days for your body to acclimate to hot-weather running.
Be patient with yourself and adjust your workouts and training plan as needed while your body adjusts to the added stress of heat training.
#2: Run by effort, not by pace
As runners, we tend to love feeling like our training is as precise as possible, and that often involves nailing certain paces and splits.
However, summer running, or days when it feels like an oven or greenhouse outside is better served by running by feel, using effort and not pace to guide your workout.
Not only are you less likely to hit specific paces in the heat, but you’re also more likely to ignore heat-related illness cues from your body. When you run by feel, you listen to your body, honoring its needs while still getting a quality workout.
You might need to lower the intensity or run a shorter run than planned to adjust to the conditions.
#3: Don’t assume running in the early morning is best
Although running early before the sun rises may give you cooler temperatures, the humidity is often highest in the morning.
Look at the combination of air temperature and humidity (the heat index) when planning the coolest time of day to run in the summer.
The evening is often the best bet because you’re spared from the sun’s heat, and the humidity tends to be lower.
#4: Hydrate enough before and during your run
Staying well hydrated is crucial for staving off the dehydration contributing to heat exhaustion when running in the heat and humidity.
Depending on your sweat rate and the environmental conditions, aim to drink at least 4-8 fluid ounces of water and/or electrolyte-infused sports drink every 15-20 minutes during your run.
The goal is to hydrate on pace with the fluid you lose through sweat such that your weight on the scale after you run is within a pound or two of your weight before you head out the door.
If it’s not, adjust your hydration plan moving forward, considering that you need to drink an additional 16 ounces for every pound lost.
Drink water throughout the day to start your workout already hydrated.
#5: Find shade
Although it won’t remove the humidity, running in the shade can help if it’s hot and sunny outside.
Dark-colored asphalt radiates heat, adding to the cloud of hot air you’re running through.
Trails are a great option for summer miles because they are usually shaded from the natural canopy of the trees.
Bike paths and rail trails are often more sheltered from direct sunlight as well and can help you from overheating.
#6: Wear light-colored, breathable clothing
Wear light-colored, lightweight, breathable fabrics for your hot-weather runs.
Dark colors absorb heat from the sun.
It’s also a good idea to wear a visor and running sunglasses to keep the sun off your face and out of your eyes while permitting heat to escape from the top of your head. (Also, don’t forget your sunscreen!)
#7: Shift your training schedule
If your training schedule calls for a long run or a hard workout during a heat wave or dreadfully humid day, see if you can shuffle some workouts around so that your rest day coincides with the least favorable weather conditions and your tough workouts fall on tolerable days.
#8: Douse yourself with water
Admittedly, the water won’t evaporate when it’s humid, but cool water can lower your body temperature.
If your run takes you past a safe body of water, jump in before or halfway through your run to lower your body temperature.
Alternatively, soak a bandana in ice water at home and tie it around your neck before you head out for your run.
#9: Run indoors
Running indoors in an air-conditioned space with a treadmill is advisable whenever the heat index is considered at an elevated risk for developing heat exhaustion.
Sure, it might not be your favorite thing to do, but it beats canceling the run altogether.
#10: Adjust your goals
It’s simply a scientific fact based on human physiology that your physical performance declines when it’s hot and humid.
Accordingly, it is unreasonable to expect yourself to nail split times and maintain the paces you can hit under more reasonable weather.
This is another reason why focusing on effort is more important than running pace in hot and humid conditions.
To ensure you are well-hydrated for all hot-weather runs, check out our next guide:
