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It strikes with only the faintest hint that it’s there. Like a snake in the savannah, it slithers across your skin in the form of salty sweat beads. Its tentacles slowly grip your socks until you are swamped in your own moisture. Before you even know it you’ve been overwhelmed by the quicksand of summer: humidity!
If Stephen King wrote a novel for runners, he would base it upon this mythical environmental phenomenon. While there are a cast of side characters who also contribute to the horror, humidity is the star of the show. It’s the primary culprit soaking your clothes until they can no longer absorb another drop of fluid. It’s the reason your eyes sting, almost as if they’re being pickled by salty sweat because your eyebrows fail to catch the cascade of perspiration coming off your forehead. And worst of all, it’s an accelerant for dreaded hot spots and blisters on your feet.
How do we decode and effectively combat the monster that goes by humidity? With over 30 years of personal running and 7 years of coaching experience in the Gulf of Mexico’s muggy armpit—also known as south Texas—I’ll break down how to successfully train in high humidity conditions.
First, let’s start with a primer on humidity, and how it impacts your running performance.
Humidity is the concentration of water vapor present in the air. There are three types of humidity measurements; absolute, relative, and specific humidity. In this case, we only need to concern ourselves with relative humidity. When you open the AccuWeather app on your phone and see a humidity percentage, they are predicting what the relative humidity will be in the future. The complicated way to describe relative humidity is that it is the “present state of absolute humidity relative to a maximum humidity given the same temperature.”
But a better way to break it down is with a baking analogy. Imagine that temperature is the volume of your measuring cup and the volume of ingredients in the cup is the absolute humidity or the amount of moisture in the air. When there are higher temperatures (bigger measuring cups), there can be a higher maximum humidity (more of the ingredients can fit in the cup). Relative humidity is the percentage that your measuring cup has been filled. Ready to bake a moist cake now?
Focus on the Dew Point
My favorite ways to get a quick glimpse into how it feels outside are to check the heat index and dew point on your weather app. Heat index is a pretty simple combination of relative humidity and temperature to give you what it legitimately feels like outside. When you hear your die-hard amateur meteorologist running friend give you the “feels like” temperature forecast in 15 minute increments for the next three hours, this is what they are referring to. Dew point is a bit more complicated, but still a fantastic indication of just how dry or humid it will feel when you step outdoors.
So when you see heat index and dew point forecasts, what numbers should you be looking for how to mentally prepare yourself for your upcoming run? In general, dew point is what you are going to want to be looking for. Anytime you see a dew point below 55 degrees, it’s going to be pleasant sailing out there on your run.
When 55 to 65 degrees is in the forecast, you want to start tempering your expectations ever so slightly. Think about a 1 to 3 percent pace adjustment, or about 5-10 seconds per mile for many of us. This is especially true when running faster than an easy effort. Once you ascend to a dew point above 65 degrees, the oppressiveness really makes itself apparent. The effort you have to put forth to maintain pace will creep up exponentially, especially when the dew point enters the 70s where you should be adjusting your pace expectations slower by at least 5 percent, if not 8-10 percent.
How to Train in High Humidity
So what is the best way to fight back against humidity?
This is where it gets interesting! There are some significant differences in how we can manage high humidity conditions compared to a higher temperature but low humidity environment. A common adage going around right now is to keep yourself as wet as possible when the going gets hot. That is fine if the humidity is low and it will evaporate quickly, but significantly less desirable in high humidity conditions as the moisture that sits on your skin, soaks your clothes, and overloads your socks and shoes will not evaporate quicker than it accumulates. A better way to combat this would be to wear an ice bandana. Instead of cooling yourself via evaporation of fluids on your skin, you will be cooling yourself by way of conduction, when the heat from your body transfers to the cool ice and melting water of the bandana.
Maintaining hydration levels is the biggest key here. The general recommendation is to replace approximately 75 percent of the fluids and 100 percent of the electrolytes that you are losing. With the athletes I coach, I like to dial this in with a sweat test, which would specifically measure your fluid loss on an hourly basis and the composition of your sweat. Of particular importance is how much sodium you are losing. The neat part is that once you have these measurements for yourself, you can periodically do sweat tests on your own in changing seasonal conditions knowing that your sweat composition remains remarkably similar year around.
For myself in particular, I did a sweat test with Levelen in June and found that I had a fluid loss of 1.68 liters (56 ounces) per hour and a sodium loss of 1,816 milligrams per hour. I’ve noticed an improvement in my running performance in humid conditions now that I am aiming to take in 40 ounces of fluids per hour to go along with 1,600 to 1,800 milligrams of sodium per hour, especially later on in long runs when dehydration really takes its toll.
We all need to adjust our pace expectations and maintain a positive attitude when running in high humidity. I promise that your running fitness is still in line with where you were a couple months ago in the spring—if not even better than before—even if your paces at all intensities slow down running in humid conditions. Keep your training paces in perspective, and you will set yourself up for a breakthrough fall. When the weather conditions normalize at the end of summer, the gains you have made over the summer will bring the faster paces out of hibernation and ultimately lead to optimal racing results.
In order to minimize chafing everywhere and hot spots on my feet, I make shoes, socks, and running short changes mid-run during my summer long runs. My long run routine is to go for a route that takes about an hour that ends where I started at my so I can quickly towel off in my car or a nearby restroom before putting on dry clothes and shoes and then continue on for another lap or two. This is also beneficial for reloading on the significant amount of fluids and electrolytes you want to carry and consume for your longer efforts.
One common misconception about training in high humidity is that if you run later in the day, the humidity is lower and, thus, you won’t be as affected. And while the notion that relative humidity being lower later in the day is true, the heat index will be so brutally high by mid-afternoon that you’ll be feeling the heat and associated rise in core body temperature significantly more (especially if in direct sunlight) than if you had run earlier in the day. Running earlier in the day or later in the evening once the heat index and humidity have dropped is still your best bet.
So when Stephen King’s newest bestseller, “Suffocating Shroud: The Lethal Mists” is released, the main character will have read this article and triumphantly overcome 80 degree dew point conditions as they slay the humidity monster. Just as you will be the conquering hero in your own battle with these conditions!
Ryan is the head coach of the Maverick Running Community and a professional ultra trail runner based in San Antonio, Texas. He has notable results winning the 2021 Bandera 100k in his first ultra trail race to go along with numerous podium performances in national level races. In addition to his ultra trail experience, he is a two time United States Olympic Marathon Trials qualifier with PR’s of 2:14:27 in the marathon and 1:03:45 in the half marathon.