The human body is a marvelous machine, a heat-generating engine that must maintain its core temperature within a very narrow range. If body temperature rises above 106 dogmas Fahrenheit, the brain and other organs will fry, suffering irreversible damage within minutes. On the other extreme, with a core temperature of below 95 degrees Fahrenheit the body begins to sink into hypothermia and may lose its ability to maintain enough heat through shivering or other actions.
Outdoor enthusiasts fight a constant battle to regulate comfortable body temperature for keeping warm in the wilderness. Without clothing, and lacking the warmth of physical activity, most people are only comfortable when the temperature exceeds 80 degrees F. Bring the temperature down, and goosebumps appear. Lower the temperature more, and shivering commences.
The dangers of hypothermia
As hypothermia sets in, the muscles cool and become progressively stiffer, weaker, and lose coordination. Walking on rough terrain becomes difficult, and speech may become slurred. As hypothermia increases, the person loses all interest in whatever goal they were originally pursuing and becomes interested only in getting warm.
In more profound hypothermia, muscular deterioration becomes more pronounced until the victim may no longer be able to stand. Shivering stops at about 92 to 90 degrees F. core temperature and apathy takes over. Mental confusion causes the person to lose hats, gloves, or leave jackets unzipped. Sometimes they will fling off their clothes. And then with increased cooling the victim becomes comatose. Heart and respiration rates slow and eventually heart failure causes death.
Without a thick layer of blubber or fur to prevent heat loss, the human body requires outside help for keeping warm in the wilderness. But instead of thinking of “cold” as the “enemy,” we need to think of “warmth” as the “traitor” that is forever trying to escape and mingle with the outside cold.
The best outdoor clothes are designed to hold our body’s heat in and only let it out to regulate the excess that is generated by vigorous exercise. Keeping warm in the wilderness requires both proper clothing and food intake.
Losing body heat
Body heat can be lost through one or a combination of the following: evaporation, conduction, convection, and/or radiation. To better understand the processes involved, let’s consider the case of Jack, a backpacker who takes the wrong trail, and ends up hiking five miles into the timberline before realizing his mistake.
He is totally exhausted, sweating profusely, and wears only jeans and a T-shirt. After finding a flat rock in the shade, he flings his pack off and sprawls out on the rock to rest.
The air temperature on this sunny September day is in the mid-60s, with high clouds now and then blocking the sun’s rays. When Jack was further down in the valley there were only a few wisps of wind — hardly any wind at all — but up here in the timberline slight to moderate breezes swirl around him.
Now, only minutes after unstrapping his pack and stretching out on the rock, he begins to shiver. All four cooling processes are working on him at the same time. Evaporation of sweat kept Jack comfortable as he lumbered up the trail with the sun beating down on him.
Now, with his heat-generating machine only in idle, evaporation continues to suck heat away. For every liter of sweat that evaporates off his skin, 580 kilocalories of heat are removed from the body. That’s about as much heat as a person produces in an hour of strenuous activity.
While Jack lies on the cold granite rock, conduction is sapping heat away little by little. This method of heat loss may not be as fast as evaporation, but it does take its toll.
Conduction when keeping warm in the wilderness
Conduction occurs when the energy of hot molecules is transmitted by collision to cooler, less energetic molecules. When a warm body is in direct contact with a cooler body, heat always flows to the cooler body.
That’s why you get burned when you touch a hot stove, your cooler temperature is not flowing into the stove, and the stove’s warmth flows into your cooler body. Direct contact is the key. If you are trying to warm a hypothermic person, skin-to-skin contact is best.
With the arrival of microwave ovens, we have learned that our “old-fashioned” ovens are called convection ovens. A convection oven heats food by circulating warm air continuously around the food. What happens when you circulate cold air continuously around a warm body? It depends on how fast the air is circulated.
While Jack lies on the rock, his body warms a thin layer of air next to the skin. This air can be trapped in his clothes and provide warmth and protection from the cold, or it can be blown away by wind or movement. As that warm air is blown away it is replaced by more warmth drawn from the body. This circular path is called a convection current.
If the temperature in the shade where Jack rests is 60 degrees, a 10 mph wind can make it feel like it’s only 40 degrees out. A 20 mph wind drops the temperature right down to a freezing 32 degrees F. Wind chill charts are based on the convection principle of cooling.
Frostbite can happen when high-speed winds combine with only moderately cold temperatures to sear the flesh.
The wind factor
The wind actually attacks us in two ways: through convection by robbing us of the warm air layer next to the skin, and also by dramatically increasing the evaporation rate. Wind protection can have a tremendous effect as you struggle to regulate comfortable temperature.
While Jack was trudging up the trail, he was being warmed not only by his body’s heat-generating engine but also by radiation being emitted directly from the sun and by light bouncing up from the bright reflective rocks along the trail.
The human body, too, although not as intensely as the sun, radiates energy known as infrared “heat” waves. Out in the open, when the air is still, and you are reasonably dry, radiation can account for two-thirds of your heat loss.
Most of this heat pours out of your head because great volumes of blood pass just under the skin with little or no natural insulation. As soon as Jack sat down in the shade, he lost the sun’s warming effect, and he himself started radiating heat into his colder surroundings.
In Jack’s case he could immediately reverse this dramatic four-prong attack of heat loss by stripping off his sweat-soaked clothes, putting on a few thin layers of dry clothes including a wool cap and windjammer, stepping out from the shade and into the sun, finding a wind-protected spot of ground, and sitting on a piece of closed-cell foam and not the cold hard ground.
The best clothing to wear for keeping warm in the wilderness
Certainly, a large down-filled parka will keep you warm and protect you when temperatures are below freezing. But what happens when you attempt to wear that parka during a vigorous climb up a mountain?
In a few minutes, the sweat will be pouring, and the parka will come off. Then you’ll be back to freezing as the sweat evaporates off your skin. To avoid these extremes you can regulate your temperature by using the layering method.
It is useful to classify clothing into layering categories according to their principle function:
- the underwear layer for maintaining a comfortable climate next to your skin;
- the insulation layer for keeping heat in;
- the shell layer which protects against wind, rain, snow, and sun.
The best materials for the under-wear layer are synthetics. What about those cotton-waffle long johns stashed in your drawer?
In most situations, cotton is the worst material for winter underwear and should be avoided.
All fabrics hold water by surface tension. Cotton, however, actually absorbs water into its fibers, and when it does, it loses all of its resiliency. That’s what makes wet T-shirt contests so popular. Wet cotton fabrics collapse and cling to the skin forcing all the water the fabric is holding to evaporate directly off the skin — not good in the wintertime!
Advantages of synthetics
New synthetics are constantly being developed, but they share many of the same characteristics. Their main feature is the ability to wick moisture away from your skin. This wicking process subtracts moisture without subtracting heat the way evaporation does.
Synthetics also dry very fast and won’t cling to the skin when wet. Polypropylene underwear was the pioneer in this field.
For the insulating layer, you can use a few wool sweaters, a camping shirt, or even a down-filled vest. Even though wool absorbs water like cotton, it does not totally collapse when wet, so it still provides insulation. Goose down, on the other hand, loses all insulation powers when wet. Down parkas are best used for inactive times around camp and when temperatures are below freezing.
As with underwear, the synthetics are gaining popularity over natural materials for the insulation layers. For best temperature control, stick to several thinner layers of insulation.
Outer layers may have to be bought in larger sizes to accommodate the additional bulk of all your inner layers. Otherwise, the layers may squeeze together, reducing their insulation properties and effectiveness at trapping warm air.
The shell layer is your main defense against wind and rain. Depending on environmental conditions, you may want to go with a totally waterproof rain outfit, including rain pants or chaps. When dealing with the shell layer, ventilation is the big problem. If you totally block out the water from the outside, you have problems with inside sweat and vapor not getting out.
Many good shell systems contain zippers and vents to help with ventilation problems. Just remember that if you are getting too warm, one of those inside insulation layers may have to come off.
Vapor barrier system for keeping warm in the wilderness
Using a vapor barrier layer (VBL) is a fairly good way of reducing the amount of moisture that evaporates off your skin. This system will stop fully 15 percent of your heat loss, yet it is one of the lightest layers you can wear — only about eight ounces for a VB shirt, pants, and socks. It is ideal for keeping warm in the wilderness.
Designed to be worn next to the skin, and usually made of waterproof coated ripstop nylon, a vapor barrier works best during sedentary periods when you are not likely to sweat. Otherwise, it can get to be a sauna inside!
If you don’t like the feel of the VBL next to your skin, you can wear a thin synthetic layer underneath. Many people say that a VBL is too warm for temperatures above 40 degrees. It really proves its worth, however, when temperatures dip well below freezing.
You can improvise a VBL vest by punching head and armholes in a large plastic garbage sack. VBL socks can be made from plastic bread bags. A sleeping bag VBL liner can be made by duct taping together a sheet of black plastic available at your local garden nursery. More permanent items can be sewn together using coated ripstop nylon.
When dealing with all of these clothing layers, don’t forget your head, hands, and feet. Since your head radiates heat like a light bulb, trap that heat in with a wool knit cap or a synthetic fleece balaclava.
For hands, don’t go out and buy an expensive pair of ski gloves. They may be warm when you are pounding down the mogul field, but nothing beats a warm pair of mittens. Use the layering system here too. An inner layer for wicking moisture and providing insulation, an outer layer for wind and water protection.
Some of us seem to have cold feet constantly and for us, keeping warm in the wilderness is a challenge. It can be 72 degrees out, and our feet feel like ice. I have found that using insulation, insulation, and more insulation is the only way my feet will stay warm. Two pairs of socks and a pair of waterproofed leather hiking boots just doesn’t cut it in the snow. My feet require double thermal inner socks, thick wool outer socks, and boots with thick wool felt liners to stay warm.
Besides not getting enough warm blood down to the toes, conduction is the big enemy. My thick leather hiking boots would freeze hard as ice and then simply draw away all my feet’s last bit of heat.
If you are snow camping, try making a pair of snow booties. Use a thick-coated nylon material for a shell and line the booties with closed-cell foam on the bottom and open-cell foam on the sides. A simple drawstring can be sewn in the top to keep the snow out. These are nice to wear around camp and are warm as can be. If your feet get cold add more foam insulation.
What to put into your body
Moving about in a winter environment is more difficult and takes more energy. Crunching through snow or just fighting off the cold can cause the body to demand more calorie intake. If you’re not performing jumping jacks to in-crease heat production, the body will automatically kick in the shivering reflex, causing the muscles to twitch and burn calories.
For keeping warm in the wilderness and keep that heat production from falling, you need to eat. Eating a meal increases metabolism by about ten percent for the next four to six hours. You should eat at least that often. Many experienced travelers eat every two hours or so.
For every ounce of protein or carbohydrates, you consume you get about 110 calories. For fats, that’s about 250 calories per ounce. It may seem easier to increase fat intake if you’re worried about how much food you can carry in your backpack, but fatty diets are bad for your long term health and quickly become unpalatable.
Carbohydrates will digest faster than fats, providing the energy a lot sooner and will help keeping warm in the wilderness. Try making your own trail mixes by including cereal grains, nuts, and dried fruit. Munching on trail mix can provide a steady stream of calories for increased winter demand. For longer trips, you’ll need to combine protein, fats, and carbohydrates to get adequate vitamins and minerals as well as calories.
For more information on making trail foods and keeping warm in the wilderness, check these articles:
And don’t forget water. Although dehydration doesn’t directly cause hypothermia, it can greatly aggravate its effects. If you don’t drink enough water, the blood becomes thicker and harder to pump into the extremities. It is bad enough that the body automatically shuts down blood vessels near the skin and saves the heat for the core. Dehydration will further reduce circulation and blood volume to oxygen-starved cells.
If you happen to be at higher altitudes where altitude sickness is a problem, you may have to force yourself to drink enough liquids. If your urine is dark yellow, you need to drink a lot more. When you drink that cup of hot chocolate, you may be actually needing quarts of liquid.
What don’t you want to put into the body?
Both alcohol and cigarettes cause constriction of the blood vessels in your extremities — exactly the opposite you want to have happen for keeping warm in the wilderness. Also, alcohol depresses your system and makes it more difficult to shiver. Avoid taking tranquilizers or sleeping pills for the same reason. Don’t assume that beverages containing alcohol are useful for keeping warm in the wilderness, that’s just a myth!
Even with the latest and most expensive winter clothing, there comes a time when shelter is the only thing that will save you. Blizzard conditions, extreme cold, and long durations in cold environments demand an adequate shelter. Knowing how to plan and build a proper shelter is mandatory for keeping warm in the wilderness.
For relatively mild conditions, you can construct a simple lean-to using a poncho, space blanket, and some paracord. Tape the space blanket to one side of the poncho and then erect the poncho lean-to between two trees using your paracord.
Stake out the bottom for a tight, flap-free setup. A small fire in front will radiate heat toward you, and the space blanket will bounce it back, increasing the effect.
If the wind is a problem, try finding a hollow log or a deadfall and huddle inside or underneath where you may find a pocket of air surrounded by snow
Snow caves generally require lots of snow and energy to build. Even if there are only a few inches of snow on the ground, you can construct a snow cave. What you do is form a mold using pine boughs. Form a pile of boughs about three feet high and then throw snow on top of the pile a good eight to ten inches thick.
Once this snow has set for two to three hours, you can pull the branches out and crawl inside. If you throw a tarp or a blanket over the branches first, before piling on the snow, the boughs will come out quite easily.
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Keeping warm in the wilderness and staying comfortable in the winter isn’t really that difficult. Get the right clothing and enough of it. Watch your body for signs of chilling. If you feel cold, either put on more clothing or get active. Have a positive mental attitude.
The more time you spend thinking about how cold and miserable you are, the colder and more miserable you’ll be. Acclimate yourself. Over a period of time, you can get used to almost anything. Start out with short winter trips to test out gear and new techniques, then later, you’ll be able to handle multi-day treks on up to full-blown winter expeditions!
Useful resources to check out: