Running in extreme temperatures is challenging.
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 as well as 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 article, we will discuss why running in humidity is so difficult and will offer tips to survive humid runs.
We will cover:
- Why Running In Humidity Is Hard
- How the Body Responds to Running When It’s Hot and Humid
- At What Temperature Does High Humidity Make It Harder To Run?
- 10 Tips for Running In Humidity
Grab your sweat band and let’s jump in.
Why Running In Humidity Is Hard
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 really possible to readily calculate the Heat Index on your own.
However, there are plenty of online calculators to do so, such as the one here.
How the Body Responds to Running When It’s Hot and Humid
When you step outside your air-conditioned home to go for a 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 weather like that 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.
The purpose of sweating is to reduce your core temperature by carrying excess heat to the surface of the skin 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. 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.
The risk of heat exhaustion when running on hot and humid days is escalated 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 surface of the skin, 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 surface of the skin 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 surface of the skin 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.
The higher your core body temperature rises, the faster your heart will beat in order to try 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 to take in more oxygen to fuel the overworking heart and muscles.
Here, again, dehydration exacerbates the cardiovascular challenge of running in humidity and heat because as your blood plasma level drops, you have less volume of blood.
Therefore, less blood is pumped to the muscles and tissues of the body per heart beat.
In order to keep up with the muscles’ demand for oxygen and nutrients and to compensate for this reduced volume, the heart has to beat faster so that more blood is delivered.
Most runners will experience a heat rate increase of about 10-20 beats per minute when running in 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, which affects your body in essentially 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 what temperature 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, 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 slow down due to the buildup of heat and your athletic performance will rapidly decline.
So, at what temperature and humidity does the body reach this critical threshold?
As just mentioned, it varies 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, meaning 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 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.
Here are some tips for running in humidity:
#1: Allow your body time to acclimate
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, as needed, while your body is adjusting to the added stress of running in the heat and humidity.
#2: Run by effort, not by pace
As runners, we tend to love feeling like our training is 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, are often 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 illness cues from your body. When you run by feel, you listen to your body, honoring its needs while still getting a quality workout.
#3: Don’t assume running in the early morning is best
Although running in the early morning before the sun rises may gift you with 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 best time of day to run in the summer.
Oftentimes, the evening is the best bet because you’re spared from the heat of the sun 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 that contributes 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, keeping in mind that you need to drink an additional 16 ounces for every pound lost
#5: Find the shade
Although it won’t take away the humidity, if it’s hot and sunny out, running in the shade can help.
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.
#6: Wear light-colored, breathable clothing
Wear light-colored, lightweight, breathable fabrics for your hot-weather runs, and as little clothing as you’re comfortable wearing (or is appropriate!).
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 still permitting heat to escape from the top of your head.
#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
Okay, so admittedly, when it’s humid, the water won’t evaporate, but cool water can lower your body temperature.
If your run takes you past a safe body of water you’re allowed to enter, 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’s 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 when it’s hot and humid.
To ensure you are well-hydrated for all hot-weather runs, check out our Hydration Guide For Runners.