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How to Run When Humidity is High

By Liz Plosser, Runner’s World
7/26/2013 7:14:20 AM

Runners often obsess over weather reports, tracking the coolest time of day in which to run. But as anyone who's ever tried to finish a five-miler in steamy conditions knows, it's not just the temperature that matters, it's the humidity.

"Of all the climate measurements we take to assess heat risk for our runners, humidity is the biggest factor," says George Chiampas, D.O., the medical director of the Chicago Marathon. Humidity makes warm summer runs even more taxing because the higher the moisture content of the air, the hotter it feels. An 88-degree day with a relative humidity just under 40 percent, for example, will feel like 88 degrees. Hot, yes, but when humidity reaches 70 percent, that same 88 temperature feels like 100 degrees.

Image by Mark Matcho

Unless you're lucky enough to live in Paradise, Nevada-the least humid city in the U.S.-here's how to cope when running in steamy conditions.

Plus: 3 Heat Wave Survival Tips for Runners

Why Humidity Matters

When you run, your core body temperature naturally rises, and your sweat glands produce droplets that carry excess heat to the surface of the skin, where it evaporates. But humidity prevents sweat from evaporating, so the heat stays put. "On a hot, humid day with no breeze, you have lost a key way to get rid of your building body heat, which can make running dangerous," says Michael Bergeron, Ph.D., a professor at the Sanford School of Medicine of the University of South Dakota.

If your body heats up and gets more and more dehydrated, it goes into survival mode, maintaining blood flow to your essential organs (to keep you alive) and to your skin (to regulate temperature). Less blood will flow to your GI tract, which will make the digestion of sports drinks or gels difficult, and you may feel nauseous as a result. You may also find you are more prone to side stitches when you are overheated-especially if your breathing becomes shallow and uneven. And your heart rate will escalate as your ticker and lungs work overtime trying to deliver oxygen throughout your body, Dr. Chiampas says.

TRY THIS: 8 Ways to Cool Down Your Summer Runs

But wait, there's (ugh!) more. If you continue to gut it out, your brain temperature will rise, which makes matters worse: Your ability to assess your own body temperature will become difficult (runners often report feeling chilled or goosebumpy when they're overheating). You can also start to lose control over body mechanics (your form and footing will get sloppy), and your mental abilities may start to break down (you may feel dizzy or disoriented).

"Your temperature can spike in minutes," Bergeron says. "If you're running a 5-K or a 10-K on a hot day, you can jack up your body temperature quickly." Also, it's a myth that newbies or not-fit-enough runners are the ones who suffer in hot, humid conditions. In fact, competitive athletes may be more prone to heat-related illnesses because the faster you run, the more body heat you generate. "As humidity increases, thermal strain and premature fatigue increase exponentially, and so running at your normal pace will feel very difficult," Dr. Chiampas says. It's also important to recognize that feeling sluggish on a sticky day doesn't indicate a lack of fitness or a lapse in mental toughness-it's your body's physical response to a stressful environment.

Continue reading on Prevention.com to find out what you can do.

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