If runners waited for perfect weather to run in, we would almost never run. Weather is just something that people who exercise outdoors learn to deal with. (Running in the cold can even be good for you.) But there's bad weather and then there's bad weather, especially in the winter. And knowing the difference could save your life.
So how do you tell when it's too cold to run outside? The wind chill factor is the best indicator, says Brian Schulz, M.D., an orthopedic surgeon and sports medicine specialist at Kerlan-Jobe Orthopaedic Clinic in Los Angeles. The "wind chill" or "real feel" is that little number often listed next to the actual temperature in the forecast. It takes into account conditions like wind speed and moisture to calculate the risk of frostbite to your bare skin. And it's important because the wind moves warm air away from your body and moisture will further cool your skin, making you cold a lot faster than just the air temperature would suggest, Schulz explains. Say the thermometer reads 36 degrees Fahrenheit; if the wind chill says 20 degrees, your exposed skin will freeze as if it were 20 degrees—an crucial distinction for anyone going outside for more than a few minutes.
"There really aren't any warning signs for frostbite—by the time you notice it, you're already in trouble," he says, adding that your hands, nose, toes, and ears are particularly susceptible due to how far away they are from the core of your body (and most of your body heat). This is why he recommends staying indoors if the wind chill drops below freezing. (We've got 8 Ways to Stay Warm During Your Winter Run.)
But frostbite isn't your only concern. The cold, dry air of winter affects your body in many ways. For example, it may feel harder to breathe since your lungs have to work harder to warm the air as you inhale. And your heart may have to work harder too as you expend more energy to stay warm and do your workout.
"Know that your workout isn't going to feel the same [as it would in warmer weather]," Schulz says. "It will take you longer to do the same route and will likely feel harder and you need to plan for that," he adds.
Hypothermia and dehydration are risks for outdoor enthusiasts during any season (yes, even summer!), but are the biggest threat during the winter, says Jeff Alt, an outdoors expert and author. (Here, 4 Tips to Dodge Dehydration This Winter.) The best way to avoid all of those risks is dress appropriately for the weather, Alt says. Just because you feel invincible in your favorite shorts doesn't mean it's a good idea to wear them on a snow run, even if you don't feel particularly cold. Instead, he recommends wearing a base layer that will wick away sweat from your body, a middle layer for warmth, and a water-resistant top layer. And don't forget a hat and gloves.
Proper footwear matters, Alt says. Shoes that are prepped for winter will keep you stable on snow and ice. Yak Ttrax ($39.99; yaktrax.com) are an excellent way to temporarily turn any pair of sneakers into snow shoes.
You also need to be prepared for rapidly changing weather conditions, Alt adds. "Little things can quickly become big problems in the outdoors," he says. So check the forecast and plan routes that keep you close to your home or car so you can get back to shelter quickly if needed. And be sure to leave a note saying where you're going and when you plan to return so loved ones can check up on you if you're not back on time.
The last—and perhaps most important piece of advice, according to both experts—is to use your common sense. "If it hurts and you're uncomfortable, cut your workout short and go back inside, no matter what the thermometer says," Schulz says. (Heading out there? Follow these Cold Weather Running Tips from Elite Marathoners.)