Chapter 41 Essentials of Wilderness Survival
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This chapter discusses the general principles of wilderness survival, particularly in temperate and moderately cold (i.e., nonpolar) environments such as the mountains of the American West. For additional information, especially about other environments, please refer to Chapter 1 for high-altitude medicine, Chapter 9 for polar medicine, Chapter 43 for jungle survival, Chapter 44 for desert survival, Chapter 71 for how to live off the land, Chapter 96 for wilderness navigation techniques, and Chapter 112 for certain aspects of environmental changes (e.g., global warming).
Regulation of Body Temperature
The importance of avoiding travel and seeking shelter during storms and extreme cold cannot be overemphasized. The additive chilling effect of wind when added to cold is impressive. Wind-chill charts (Figure 41-1) show the relationship between actual temperature, wind velocity, and “effective” temperature at the body surface. The term wind chill refers to the rate of cooling; the actual temperature reached is no lower than it would be if wind were absent (unless evaporation of liquid is occurring at the body surface). The increase in heat loss as the wind rises is not linear; rather, it is more proportional to the square root of the wind speed.
FIGURE 41-1 Windchill chart.
(From Bowman WD, Johe DH, American Academy of Orthopaedic Surgeons, National Ski Patrol System [United States]: Outdoor emergency care: Comprehensive prehospital care for nonurban settings, ed 4, Boston, 2003, Jones & Bartlett.)
Cold Weather Survival
Decreasing Body Heat Loss
Heat loss from conduction and convection can be prevented by interposing substances of low thermal conductivity, such as clothing made of insulating materials, between the body and the outside air. Clothing creates a microclimate of warmed, still air next to the skin surface. Clothing’s value depends on how well it traps air, the thickness of the air layer, and whether these qualities are reduced by wetting (Table 41-1). Traditional insulating materials are wool, down, foam, and older synthetics such as Orlon, Dacron, and polyester. Wool retains warmth when wet because of a moderately low wicking action and its ability to suspend water vapor within its fibers without affecting its low thermal conductance. It can absorb a considerable amount of water without feeling wet, but is heavier than synthetics, “itchy,” and more difficult to dry. However, its toughness and durability make it a good choice for garments that are subject to hard wear, such as trousers, mittens, and socks. Cotton, particularly denim and corduroy, is a poor insulator. It dries slowly because of its low evaporative ability; high thermal conductance is further increased by wetting. Cotton has no place in the backcountry in cold weather.
Dressing for Cold Weather
First Layer
Long underwear
Wool is an excellent choice for long underwear, but it is itchy and expensive, and it may be difficult to find. Merino wool is less itchy. Polypropylene, acrylic, and the newer polyesters may be preferable because of their lower cost, good insulating ability, and outstanding evaporative ability (see Table 41-1). Avoid cotton. Synthetics, more than wool, tend to retain body odor after washing.
Shelter
Tube Tents
Tube tents are usually about 2.4 m (8 feet) long and provide a tubular shelter that is 0.9 to 1.5 m (3 to 5 feet) high, depending on the brand. They can be pulled over the body to provide a quick shelter or pitched as a “pup tent.” To do this, find two anchors (e.g., rocks, trees) that are the proper distance apart, tie a line to one of them, spread the tent out along the length of the line, run the line through it, and then tie off the other end of the line. The height of the line should be such that the tent can be spread out to accommodate the occupant. To avoid ripping, the tent plastic should be 3 to 4 mils thick (1 mil = 0.0251 mm [0.001 inch]). Tube tents can be improvised from two plastic 55-gallon drum liners, which are 3 to 4 mil thick, or from large, heavy-grade, household trash bags by opening up the closed end of one bag, sliding it into the open end of the second bag, and then duct taping the bags together (Figure 41-2).
Tarpaulins
Tarps can be erected in a number of shelter styles, depending on weather conditions (Figure 41-3). To erect a lean-to shelter, first select a line that is long enough to stretch between two trees that are far enough apart for the tarp to be stretched tight. With the use of a timber hitch, tie off one end of the line to one of the trees at about chest height. Then, rather than passing the line itself through the grommet eyes, insert a small loop of the line through the first grommet eye, and secure the loop with a short stick that is thrust through it on the opposite side (Figure 41-3, B). Repeat this process for each grommet, stretching the tarp tight each time. After the tarp is attached to the line, tie off the other end of the line to the second tree, with the line stretched as tightly as possible. The lower edge of the tarp is then pegged to the ground or anchored with large stones or a length of log. When making pegs, select a length of wood that is 3.8 to 5 cm (1.5 to 2 inches) in diameter and that is twice as long as needed. With a saw, make one 45-degree cut at the midpoint of the stick. In this way, one cut produces two pegs, both of which are sharp enough to be driven into the ground by pounding their blunt ends with the back of an ax head or a large rock. Fill in the sides of the lean-to with vegetation.
To erect a pup-tent type of shelter (Figure 41-3, C), tie a line between two trees, drape the tarp over it, and peg down the sides. Block the ends with vegetation or personal equipment.
A lean-to with an eave (Figure 41-3, D) gives more protection from rain and snow than does one without an eave. Instead of attaching the long edge of the tarp to the line between the two trees, drape the tarp over the line so that several feet are on the other side, and then tie the two corners to pegs for a down-sloping eave.
A triangular tarp shelter can be erected rapidly and provides good protection (Figure 41-3, E). It requires three pegs and an anchor point on a tree.
Plastic Bag Shelters
Large, heavy-grade (3 to 4 mil), orange, plastic, 208-L (55-gal) drum liners make good short-term emergency shelters. Having one to crawl into quickly will speed warming and drying as well as shelter the survivor from further wetness and chilling. However, total enclosure in a plastic bag is both uncomfortable and dangerous as a result of increased wetting from the condensation of water vapor in exhaled air and perspiration plus poor ventilation, with a lack of oxygen and buildup of carbon dioxide. To minimize these problems, cut an opening in the bottom end of the bag that is just large enough for your head, and then pass the bag over your head so that your face is at the opening (Figure 41-4). If you get too warm, you can push your head through the hole. When creating the hole, cut the plastic at 90 degrees to the fold to reduce the likelihood of the bag tearing along the seam.
Winter and Cold Weather Emergency Shelters
Snow is a good insulator (Table 41-2). Its heat conductivity is 1/10,000 that of copper, and its insulating ability superior to wool felt, so snow shelters may be warmer than other types of constructed shelters as long as the inhabitants remain dry. Contact with the snow or cold ground is avoided by sitting on a foam pad, dry leaves, grass, a backpack, or (in survival conditions only) a bed of evergreen boughs.
Substance | Conductivity* | Temperature Measured (° C) |
---|---|---|
Air | 0.006 | 0 |
Down | 0.01 | 20 |
Polyester (hollow) | 0.016 | — |
Polyester (solid) | 0.019 | — |
Snow (old) | 0.115 | 0 |
Cork | 0.128 | 30 |
Sawdust | 0.14 | 30 |
Wool felt | 0.149 | 40 |
Cardboard | 0.5 | 20 |
Wood | 0.8 | 20 |
Dry sand | 0.93 | 20 |
Water | 1.4 | 12 |
Brick | 1.5 | 20 |
Concrete | 2.2 | 20 |
Ice | 5.7 | 0 |
* Conductivity is the quantity of heat in gram calories transmitted per second through a plate of material that is 1 cm thick and 1 cm2 in area when the temperature difference between the sides of the plate is 1° C.
Natural Shelters
Caves and alcoves under overhangs are good shelters and can be improved by building wind walls with rocks, snow blocks, or brush. A fire should be built in such a way that heat reflects onto the occupant. The fire should be 1.5 to 1.8 m (5 to 6 feet) from the back of the shelter, with a reflector wall of logs or stones on the opposite side of the fire; the occupant should sit between the fire and the back of the shelter (Figure 41-5). Adjustments may have to be made to prevent too much smoke from reaching the occupant.
FIGURE 41-5 Example of two large plastic bags used to form a one-person survival shelter.
(Courtesy Peter Kummerfeldt.)
Constructed Shelters
When no snow is available, shelters can be built of small trees, branches, brush, and boughs. In cold weather with minimal or absent snow cover, the most satisfactory type is a lean-to covered by a tarp, with the two sides closed with brush or piled snow, a fire at the open front, and a wall of logs or stones on the far side of the fire to reflect heat into the interior of the lean-to (Figure 41-6). Walls or a roof of brush, branches, or broad leaves should be thatched (i.e., each layer should overlap the one below it).
Snow Shelters
A snow trench is the easiest and quickest survival snow shelter and the one that is least likely to make the diggers wet. It can be dug in most areas that are flat or on slight to moderate inclines as long as the snow is 0.9 m (3 feet) or deeper or can be piled to that depth. A 1.2 × 1.8-m (4 × 6-feet) trench can be dug in 20 minutes, with one end roofed over with a tarp or boughs and a fire built at the opposite end (Figure 41-7). Again, adjustments may need to be made to avoid excessive smoke exposure, which can be prevented to some extent by setting the long axis of the trench at a right angle to the apparent wind direction.
If a large (2.4 × 3 m [8 × 10 feet]) tarp and a stove are available, a trench can be dug that is as comfortable as a snow cave; this will hold two or three people. The object is to keep the maximal amount of snow around and over the trench. The trench is dug as narrow as possible at the surface while still providing sufficient room to shovel; a suitable size for the top is 1.2 m (4 feet) wide by 2.4 m (8 feet) long. It is undercut at the back and sides so that the bottom is 1.8 to 2.1 m (6 to 7 feet) wide by 2.7 to 3 m (9 to 10 feet) long (Figure 41-8). A narrow entrance helps to contain heat and can be closed with a small plastic sheet or a pack. Four or more skis or thick limbs are laid from side to side over the top of the trench, with ski poles or branches interwoven at right angles. A tarp is then laid on top of these and the snow piled around its edges to hold it down. In very cold weather, the entire tarp can be covered by a layer of snow; at least 20 cm (8 inches) is needed for proper insulation. When the entrance is closed, a small stove and the occupants’ body heat will raise the interior temperature to −4° to −1° C (25° to 30° F). Higher temperatures should be avoided so that clothing and bedding will not become wet from melting snow.