Running in hot weather can be tough.
Between the challenges of maintaining hydration and electrolytes, finding what to wear while running in high heat, and regulating one’s core body temperature without overheating, summer running certainly takes some planning and can affect your performance.
However, while we often focus on high temperatures, high humidity can actually have more of an impact on your running performance and how you feel running in hot weather.
When the relative humidity is high, the air feels muggy, and it just seems impossible to finish a run without feeling soaked, even if you are wearing the best moisture-wicking running clothes.
Plus, there are additional challenges with regulating your core body temperature in high humidity because the amount of moisture in the air compromises sweat evaporation trapping your body heat rather than relying on evaporative cooling like you might get in a dry heat.
In this guide to running in humidity, we will discuss how humidity affects how hot it feels, the impact of dewpoint on performance, and tips humidity and running that will not only reduce your risk of heat-related illnesses but also help you feel more comfortable as you work through your summer training plan.
Is It Safe to Run In the Heat and Humidity?
Humidity often plays a more significant role in the “real feel” temperature and the risk of heat-related illnesses such as heat cramps, heat exhaustion, and heat stroke.
High humidity makes running in hot temperatures even more challenging for the cardiovascular system. When the moisture in the air is high, the body’s ability to effectively lower its core body temperature through the evaporation of sweat is poor.
This is because the air is already holding as much moisture as it can.
Essentially, the thermal strain of running in hot temperatures increases exponentially—not linearly—with increased humidity.
The Heat Index helps you gauge the apparent temperature or the “real feel” outside. This metric combines the air temperature with the relative humidity.
For example, when it’s 88° F (31° C) with 40% humidity, it will feel like 88° F, but when it’s 88° F with 70% humidity, it will feel like 100° F (38° C).
When you bump to 85% humidity at the same temperature, the Heat Index jumps to 110° F (43° C).
Up until the humidity is about 40%, the relative humidity doesn’t significantly increase the Heat Index or how hot it actually feels outside.
However, above 40%, humidity rapidly increases the Heat Index.
This is why it is important to look at the National Weather Service heat index chart1US Department of Commerce, N. (n.d.). Heat Index Chart. Www.weather.gov. https://www.weather.gov/ffc/hichart or the heat index when looking at the weather forecast when running in hot weather.
There are also heat index calculations where you can input the air temperature and the relative humidity to calculate the heat index, such as the one here.2Heat Index Calculation. (n.d.). Www.wpc.ncep.noaa.gov. https://www.wpc.ncep.noaa.gov/html/heatindex.shtml
According to the Road Runners Club of America(RRCA),3Hot Weather Running Tips. (n.d.). Road Runners Club of America. https://www.rrca.org/education/for-runners/hot-weather-running/#:~:text=Understand%20heat%20index%20danagers%3A%20Avoid you should avoid running outside if the heat is above 98.6 degrees Fahrenheit and the humidity is above 70-80%.
How Does Dew Point Affect Running In Humidity?
When considering running on a humid day, you must pay attention to relative humidity.
Relative humidity measures the amount of moisture in the air relative to its capacity to hold moisture. And the higher the temperature, the more moisture the air can hold.
Therefore, 100% humidity when it is 40°F outside will feel much less muggy than 100% humidity when it is 80° outside because the high heat day actually holds more moisture in the air, bringing that dial up to 100% humidity.
The same amount of moisture in the air that yields 100% humidity on a 40°F day would be something like 70% humidity on a super hot weather day.
In addition to relative humidity, the dew point will affect your rate of perceived exertion, the effectiveness of sweat evaporation as you run, and how muggy the humid run feels.
The higher the dew point, especially on a hot day, the less sweat evaporation you will get, and the more droplets of sweat will pull on your skin or soak into your running clothes because the air is already saturated with water.
According to estimates4El Helou, N., Tafflet, M., Berthelot, G., Tolaini, J., Marc, A., Guillaume, M., Hausswirth, C., & Toussaint, J.-F. (2012). Impact of Environmental Parameters on Marathon Running Performance. PLoS ONE, 7(5), e37407. https://doi.org/10.1371/journal.pone.0037407 of the impact of high dew point running on performance, you may have upwards of 12-15% decline in running performance when the dew point is above 75-80% F.
What Happens When You Run In Hot and Humid Weather Conditions?
Running in high humidity is taxing on the body, and it can feel uncomfortable quickly. The air can feel sticky and muggy as if you are running in a greenhouse.
Body heat gets trapped in your core because the high moisture content in the air prevents sweat from evaporating readily.5Che 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
The risk of muscle cramps, heat exhaustion, heat cramps, and heat stroke increases when running on hot and humid days if you become dehydrated.
When you become dehydrated, your blood plasma level drops.
This puts a greater strain on the cardiovascular system because as stroke volume decreases, your heart rate increases6Che 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 to maintain the necessary cardiac output to provide oxygen to your working muscles and heart.
Moreover, as heat builds up in the body because the high humidity prevents sweat evaporation from the skin, your core body temperature continues to rise.
A secondary physiological cooling mechanism 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 kept your hydration intake up to your increased sweat rate while running in the heat, blood flow to the skin’s surface decreases significantly because the body must conserve the available blood volume.
Instead, blood flow is concentrated on your vital organs 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 why many half-marathon and marathon runners feel nauseous after consuming energy gels or sports drinks when running in the heat and humidity—digestion slows down significantly when you run,7Khodadadian, S. (2017, July 18). How Exercise Affects Your Digestion – Manhattan Gastroenterology. Manhattan Gastroenterology. https://www.manhattangastroenterology.com/exercise-affects-digestion/ but even more so as you become dehydrated.
This leaves the energy gel or high-carb sports drink (and even water to some extent!) sitting around and sloshing in your stomach rather than hydrating your body and increasing your blood sugar levels.
Together, the challenges of regulating your core body temperature, the increases in heart rate that come with dehydration, and the inability for sweat evaporation to help you cool down can increase the rate of perceived exertion,8El Helou, N., Tafflet, M., Berthelot, G., Tolaini, J., Marc, A., Guillaume, M., Hausswirth, C., & Toussaint, J.-F. (2012). Impact of Environmental Parameters on Marathon Running Performance. PLoS ONE, 7(5), e37407. https://doi.org/10.1371/journal.pone.0037407 decrease running performance,9Che 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 decrease time to exhaustion, and increase the risk of heat illnesses during hot and humid runs.
How Should You Adjust Your Training When Running In the Humidity?
The weather conditions are out of our control, so you will likely experience high humidity and high temperatures with summer running, especially if you live in a hot and humid climate.
As a certified running coach and a competitive distance runner myself, I have found some fairly effective strategies for sticking with your marathon training plan and staying safe running when it is hot and humid:
Here are some tips for running in humidity:
#1: Allow your body time to acclimate
Most studies10Tyler, C. J., Reeve, T., Hodges, G. J., & Cheung, S. S. (2016). The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis. Sports Medicine, 46(11), 1699–1724. have found that it takes about two weeks for your body to acclimate to hot weather running.
Be patient and adjust your training plan as needed while your body acclimates to heat training and the added stress of running in humidity.
#2: Run by effort, not by pace
To avoid frustration and reduce the risk of heat cramps and heat exhaustion, your best bet when running in high humidity coupled with high temperatures is to train by effort or heart rate rather than pace.
This will help you gauge the strain and stress on your body due to the summer weather conditions rather than try to push through and stick with your training plan workouts to a T.
Take walk breaks if needed to lower your heart rate and keep your rate of perceived effort appropriate.
#3: Don’t assume running in the morning is best
Although air temperatures are usually cooler when running in the early morning before the sun rises, the relative humidity is generally highest in the early morning.
Running in the evening is often the best time to run in the summer because the sun has set, causing the air temperature to drop and the humidity to be lower.
Use the heat index rather than just the air temperature to determine the best time of day to run in hot and humid weather.
#4: Focus on hydration
Drinking enough fluid to keep up with your sweat rate is crucial when running in the heat and humidity.
You can determine your sweat rate by weighing yourself before and after your run and subtracting the number of ounces of fluid you drank.
If you can’t do this, aim to drink at least 4-8 fluid ounces of water or sports drinks with electrolytes every 15-20 minutes during your run.
Electrolytes and a 6-8% carbohydrate solution11Research, I. of M. (US) C. on M. N., & Marriott, B. M. (1994). Use of Electrolytes in Fluid Replacement Solutions: What Have We Learned From Intestinal Absorption Studies? Www.ncbi.nlm.nih.gov; National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK231118/#ddd0000025 will increase fluid absorption, improving hydration status.
You should carry a water bottle or hydration pack, depending on how far you run.
#5: Try trail running
Although it won’t lower the humidity levels, if it’s hot and sunny out, running in the shade can help.
Trail running is a great summer running option because the tree cover protects you from direct sunlight and reduces perceived effort.
#6: Wear light-colored, breathable clothing
Wear light-colored, lightweight, moisture-wicking fabrics 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.
You also need sunscreen with at least SPF 30 to protect your skin from sunlight’s UV rays.
#7: Cool your body with water
Even though water won’t evaporate in high humidity, I love running through sprinklers, carrying an extra water bottle to pour water over my head, or soaking a bandana in ice water and tying it around my neck before running in hot weather.
Even if it doesn’t do much to lower my core body temperature, it can be a mental boost that decreases my rate of perceived exertion when running in hot and humid conditions.
#8: Run indoors
Although most distance runners would rather run outside, your best bet on super hot days is to run indoors on a treadmill with air conditioning or a climate-controlled environment.
Remember, it is always better to be safe than to put your body at risk of heat illness or struggle through a miserably humid run.
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