Backcountry Science: Staying Warm During Winter Hiking

This weekend is turning out to be one of the coldest periods of the season for the southern and central Appalachians. On top of the cold, several snow events are also bearing down on the region, creating a winter wonderland for outdoor activities. With cold and snow, though, comes the constant threat of high-elevation winter hiking: hypothermia. What should you know before you try to hit the trail? We’ve got some tips and pointers* about winter hiking…along with a little science to boot:

Why do we get cold to begin with? Answering this question on the surface is easy: you never actually “get” cold. Instead, getting cold is all about the transfer of heat, namely from your body (which in winter is far warmer than the air) to the outside environment. In other words, getting cold is more about what you lose than what you gain.

In general, the body loses heat in four ways: radiation, evaporation, convection, and conduction. Radiation occurs when heat generated by your body is lost to the surrounding atmosphere. This makes up just over half of the heat loss experienced under normal conditions, but the other mechanisms listed above can play key roles, too, especially under certain scenarios. Convection, for example, occurs when heat is carried away by air or water flowing across the skin (think of a stiff breeze in cold air), while evaporation can promote heat loss when a liquid such as sweat or water changes to vapor form. The wetter you become (and the quicker evaporation occurs), the more heat you’ll end up losing. Lastly, conduction – the transfer of heat between objects in direct contact – typically isn’t too much of a factor in cold air but can become a much bigger issue when your body is in direct contact with cool water or the cold ground.

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Napoleon Bonaparte & Co. experience their own lesson in backcountry thermodynamics.

Since humans generate their own body heat to maintain a relatively constant internal temperature, our bodies can typically use our metabolism to keep up with heat lost through the above mechanisms. Problems such as hypothermia, however, begin when heat is lost through one or a combination of these mechanisms at a rate to where our body can’t keep up. For example, combine heat lost in the cold backcountry air through radiation with enhanced evaporation from wet or sweaty skin – all coupled with a stiff, high-elevation wind aiding in convection – and you’ve got a recipe for disaster. Without help and with enough heat loss, your body’s metabolism simply can’t keep up.

So, what can I do to prevent getting too cold in the backcountry? That’s where cold-weather clothing comes in. While no piece of clothing can absolutely prevent heat loss in the cold, much of today’s outdoor gear is based around limiting one or more of the above causes of heat loss. Thick, heavy coats, for example, trap some heat that would otherwise escape through radiation and also shield against convective heat loss from the wind. “Moisture-wicking” synthetic fabrics aid in a different way, eliminating moisture from sweat, precipitation, or other sources that would otherwise enhance evaporative cooling or conduct heat away from your body through wet fabric stuck to your skin.

It’s the use of layers, though, that really does the best job of keeping you warm in the cold.  Rather than relying on one single layer like a thick winter coat to keep you warm, it’s often best to go with multiple, smaller layers that each serves its own purpose at limiting one of the heat loss mechanisms mentioned above. Most hikers typically follow the below rules of thumb for layering in the cold:

  • Choose a solid base layer. The “base layer,” or layer of clothing in direct contact with your skin, should generally be a form of moisture-wicking fabric rather than something that will hold moisture against your skin. (If you don’t believe this, try wearing a soaked cotton shirt outside in the cold.) Synthetic fabrics work best, although wool or synthetic/wool blends can also do the same. Whatever you choose, though, make it sure it’s somewhat snug-fitting: the fabric has to be in contact with your skin to wick moisture away effectively.
  • Trap heat before it escapes. Over the top of your base layer should be insulation: something with some bulk that will trap heat trying to escape your body to the surrounding air. This is where more “traditional” cold-weather clothing such as fleeces, down insulation, and sweaters comes in. The colder it is, the more insulation you may need. Most hikers opt for several smaller layers for insulation rather than a single, overly bulky coat.
  • Block wind and water. Finally, many hikers opt for a “shell” as their outer layer. Typical choices here include materials that can shield you from the outside wind and water (like Gore-Tex) while also “breathing ” to let water vapor from your sweat escape.

Layer up, but be smart as you hike. Anytime you’re undertaking physical activity, you’re going to generate more heat as your body burns energy to perform work. To understand why this happens, think of your body as an engine. Just like a car engine generates heat as it powers your car, your body’s cells do the same as they burn chemical energy to perform physical work. In response, you won’t “feel” as cold if you’re exerting yourself…but your body will also begin producing sweat to cool down, too. If you’re layered up, this can become a problem, since becoming sweat-soaked can counteract whatever good those layers are doing.

Being smart with layering also means learning to balance this dance between becoming too hot and too cold: remove some layers if you’re climbing or walking briskly and layer back up when you slow down, stop hiking, or if weather conditions suddenly become colder or wetter. Keeping comfortable with layers is much like what your body does with its temperature naturally – you need to adapt as you move.

Don’t be afraid to call it off. One of the biggest mistakes in winter hiking is biting off more than you can chew. Even if you’re dressed correctly, some conditions just aren’t worth the risk. Trudging through feet of snow can wear you down and bring on exhaustion – especially without specialized gear such as snowshoes – and there’s a point when temperatures drop and winds roar to where even careful layering won’t do much to stop you from losing too much heat. Plan carefully before you go, and consider what conditions will be like at high elevations that typically don’t receive an official weather forecast. It’s likely to be much colder and windier on mountain ridges, and you’ll probably be encountering more snow, too. When those conditions occur, you’re likely to get colder faster in the backcountry, making longer hikes all the more risky. If your gut makes you hesitate a little about heading out, it’s probably a good idea to listen. The mountain will still be there another day.

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*Don’t rely on just these tips to stay safe and warm in the outdoors. Plan smart and prepare accordingly before any trip into the backcountry.

 

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