Since hypothermia is the most common cause of accidental death in the back country, proper clothing is essential to every back country user from novice to professional. Hypothermia results when the body loses more heat than it can generate. Effective dressing is the simplest way to avoid hypothermia in the diverse weather of the back country.
Effective dressing means more than simply owning the most expensive parka and the fanciest rain gear. World-class mountaineers have long known the value of specialized techniques in mountaineering dress.
At any time of the year, the most effective way to dress is by “layering”. This method has been proven, not only on Mount Everest but in the summer Eastern Slopes of the Rockies as well.
Layering simply means wearing one thin layer of clothing over another over another. Many experienced winter mountaineers do not carry a heavy down parka into the back country and for good reason. If they become warm underneath a down parka, removing the parka leaves them extremely exposed. Rather, they will carry numerous lightweight layers.
The advantage of layering is that one can add and remove protection from the elements in small increments, thus balancing heat generation with heat loss. In addition, layering traps dead air for additional weight-free insulation.
Composition of Layers
The body is a source of heat, which you want to retain within your clothing. It is also a source of moisture, in the form of perspiration that, in many situations, must be kept away from the skin due to the cooling effect of evaporation. For this reason, the layers of clothing near your body should be thin and porous to hold in heat and wick away perspiration. Middle layers should be thicker in insulating quality to hold in more heat, yet be able to dissipate the moisture further away from the body. Finally, the outer layers should be thick enough to prevent heat loss and still protect the inner layers from the external elements. The most effective outer layer is completely waterproof, yet allows water vapor (perspiration) to escape. Most conventional rain-gear does not allow water vapor to breathe, thus the body’s perspiration is held within the layers of clothing, increasing evaporative heat loss and saturating clothes.
The key to mastering the layering system is to add or remove layers of clothing at just the right times. Remove a layer before you begin sweating; add a layer before you get cold. By doing so, you can balance the amount of your body’s heat generation with heat loss. Conserve your sweat, not your water!
Whenever traveling in the back country, carry additional clothes. This simple suggestion should not be overlooked, since a warm, balmy morning at the trailhead often ends in a cool, windy chill on the summit.
Five Methods of Heat Loss
Just as the body constantly produces heat, it constantly loses it.
There is a simple reason why we wear clothes, besides to preserve our own simple modesty. Since human beings are warm-blooded, we must rely on our own bodies for internal heat. Most warm-blooded creatures are protected from the elements by a coat of fur. This fur helps preserve warmth by trapping air and providing a layer of insulation. Humans, on the other hand, have no such coat of fur. This means, quite simply, that we must maintain a suitable artificial environment close to our skin that allows for the retention of the body heat that we create internally.
Five Ways We Lose Body Heat
There are five mechanisms through which body heat may be lost:
CONDUCTION is the transfer of heat through direct contact. If you were to sit on a slab of ice, your body heat would move through your clothes to the ice below. Wet clothing robs your body of heat through conduction as well. In fact, wet clothing will do so MANY times faster than dry clothing. This is why we should remove a layer of clothing before we begin sweating.
CONVECTION is the transfer of heat through the movement of air. The body tries to create its own insulating layer. This means that the body warms the air close to the skin. If this thin layer of warm air were to remain intact, our bodies would maintain their own insulating layer and clothes would be unnecessary. Unfortunately, wind brushes this warm air aside with little effort. The body must then generate another warm layer of air. In even moderate winds, the body doesn’t have a chance to keep up with this loss of heat.
EVAPORATION occurs when we sweat and the tiny droplets of liquid are converted into vapour. The conversion from a liquid to a vapour results in a net heat loss. That is, the surface on which evaporation occurs will lose heat (which explains why you feel a bit cold when you step out of the shower, before drying off). The body sweats because its internal temperature is too high and it wants to cool down. Sweating will occur even in winter, if the backcountry user does a poor job maintaining ventilation through clothing. When the clothes become wet with sweat, further heat is lost through conduction.
RADIATION is the movement of rays of heat from a warm object to a colder one. At temperatures as warm as 4⁰ Celsius (40 degrees Fahrenheit), fully one half of the body’s heat can be lost by radiation from an uncovered head, since blood vessels in the head lie close to the skin. Hats and balaclavas (facemasks) are essential to prevent this loss of body heat.
We lose our body heat naturally, simply through RESPIRATION. Furthermore, we may burn over 50% more energy in winter than we would in summer. This is partially explained by the fact that we are breathing extremely cold air, warming it and saturating it with water vapour. As much as one-third of our body-heat loss can occur through breathing. Breathing through a scarf or balaclava helps by “pre-heating” the inspired air.
Keeping these mechanisms in mind, and using a good layering system, should keep everyone healthy and happy when out in the wild…