Running in the Heat and Humidity

Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University


For many runners a majority of their run training and race preparation happens over the hot summer months while gearing up for their goal races in the autumn. Maintaining a rigorous training schedule when the temperatures and humidity begin to rise presents a challenge that must be overcome in order to race optimally when the temperatures cool.


As we all know with the summer months comes high heat and often quite high humidity. While many runners know the basics of the importance of staying hydrated, the necessary adjustments to make in the heat for the sake of performance are not as well understood. The summer months and their environmental challenges have significant performance implications for runners.


When you run or become physically active your body generates heat and the ability of that increased heat to escape from your body is reduced in higher temperatures, and severely curtailed in humid conditions. When the humidity is high it can drastically reduce sweat evaporation from the skin, which is the main primary mechanism your body has to stay cool.


It is important to understand that sweat alone is not what actually cools your body. For the sweat on your skin to be effective at cooling your body it has to be evaporated off of your skin and into the air around you. When sweat evaporates it is literally adding a very, very small amount of humidity to the air directly above your skin. And sweat evaporation from your skin occurs most effectively when the environmental humidity is low. If the air is already very saturated with water (i.e., the humidity is high), it becomes much harder for the sweat on your skin to evaporate into an already humidified environment.


High tech running clothes help to keep you cooler by utilizing fabrics that wick sweat droplets off of your skin, and then spread them out into the fibers of the fabric. By wicking sweat off of your skin and into the high tech fabric you dramatically decrease the size of the sweat droplets, and a smaller sweat droplet evaporates more rapidly than a larger one. All of this equals faster evaporative cooling and assists with cooling your body temperature. Cotton clothing holds onto the sweat, becomes sarurated, and slows the evaporative cooling process. Simply put, investing in long lasting, high tech running clothing will help to increase your performance by assisting in keeping your body cool.


However, even when wearing the appropriate high tech sweat wicking running clothing everyone tolerates high heat and humidity differently. The size of your physical body plays an important role in how well individuals are able to deal with increased heat and humidity. For example, the larger your body the greater your weight and frequently this also means there is more insulation; increased body fat. A larger body generates more heat when active and so it becomes easier for that body to overheat.


We also know that age influences your heat and humidity tolerance. During the aging process our bodies naturally adapt in such a way that heat and humidity are tolerated less, and one reason for this change is that sweat glands generally produce a lower volume of sweat as we get older. Less sweat equals less chance for evaporation, and a subsequent lowered ability to cool oneself effectively.


It should be emphasized that running ability, or how experienced one is in the sport has little influence on their ability to cool themselves effectively. Said another way, less experienced or less proficient runners may mistakenly believe their faster more experienced counterparts are better at cooling themselves during the high heat and humidity. Indeed, more proficient or experienced runners likely run at a faster pace, and the faster you run the more heat you generate; more heat that must be dealt with in order to stay cool.


Most endurance athletes agree the ideal conditions for running are when the day presents us with an air temperature of approximately 50°F, low humidity, a light wind, and an overcast or cloudy sky. These conditions will allow you to run your best during your training, and in a race situation provide the best opportunity for establishing a new PR. As you begin to have variation in any of these factors running performance can be negatively affected. Performance decrements are modest at first, but significantly more pronounced as environmental conditions turn hotter and more humid. The table below provides general guidelines based on research we have done in the laboratory for how the environmental temperature influences a running pace of 8:00 minutes per mile:


Air Temperature                      Running Pace Per Mile                       Influence on Pace

50 degrees F                            8:00 minutes/mile                                None

60 degrees F                            8:12 minutes/mile                                2% - 3% increase

70 degrees F                            8:31 minutes/mile                                6% - 8% increase

80 degrees F                            9:06 minutes/mile                                12% - 15% increase

85 degrees F                            9:31 minutes/mile                                18% - 21% increase


Your actual performance degradation will vary, as environmental temperature is only part of the equation. If it is a sunny versus a cloudy day, low humidity versus high humidity, and how acclimated you are to the increased heat all factor in to how much your individual performance will suffer in warm conditions.


My advice is to not just simply pay attention to the temperature when you look at the local weather forecast. Be cognizant of the relative humidity and in the summer months a calculation termed the Heat Index. Meteorologists focus on the Heat Index as a way to factor in both temperature and humidity and provide a measure of how it actually feels when you are outside. If you are not able to locate a Heat Index reading when viewing your local weather report, there are a multitude of weather websites and apps that will easily calculate it for you. Generally speaking, the humidity really does not have an influence on athletic performance until it is greater than 35%.


Importantly from a performance perspective your heart rate will be elevated for the same intensity of running (same pace per mile) when you are in a hot and humid environment. An increase in heart rate of approximately 12 - 15 beats per minute will occur when the temperature elevates from 70 degrees to 90 degrees, and your effort will feel greatly increased. However, if it is also above 35% relative humidity the increase in heart rate will likely be even more severe. 


Training in hot and humid weather can be an extremely humbling athletic experience if you are not accustomed to hot weather. Your increased effort may lead you to believe you are unfit and in need of a lot of additional training. This is likely not the case. Your body needs approximately two weeks to fully acclimate to the heat and increased humidity. When hot weather first arrives preemptively slow your pace using the table above as a guide. You may find you need to slow your pace even further at first.


The goal for your running when you are first exposed to the heat of summer should be to just spend time on your feet. Do not be overly concerned with your regular pace per mile or planned workout. I advise you to let go of any particular goal with regard to time or pace, and be aware of the early signals of heat exhaustion, which include dizziness, being disoriented, extreme fatigue, and nausea. The most prudent choice for your run if the heat and humidity are very high is try and utilize a treadmill in an air-conditioned room.


The long term reward for the training you will do in the heat and humidity is that when autumn arrives and the race season is here, your fitness will be high and you will find that your pace per mile will likely be faster with less perceived effort.

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