The most important difference between sum­mer and winter camping is the necessity, in winter, of maintaining warmth. For a short time before the winter camper turns in, a wood fire built on green poles laid on the snow, or the small gasoline cook stove in the tent, will make available some heat; but by and large, both on the trail and in camp, the human body must pro­duce sufficient heat to maintain normal temperature. If the interior body temperature drops two degrees, intense shivering results, further lowering of temperature pro­duces sluggishness and coma, and finally, at somewhere between 70° and 75°, death ensues.

We must then consider the human body as a heat-producing machine and determine (1) what can be done to increase the amount of heat manufactured by the body, and (2) what can be done to conserve this heat.

Production of body heat.—A normal male adult at rest, as in sleeping or loafing, liberates approximately 50 calo­ries of heat per hour. This can be increased appreciably by eating or by exposure to cold. The only other means by which skiers may increase heat output is muscular ac­tion.

Violent exercise will increase heat output as much as sixteen times.

Shivering, which is merely a form of muscular action, will if intense, increase heat output several times; in fact, it is nature's method of preventing freezing. The diges­tion of food will increase heat production. The increase is prompt but brief for carbohydrates, much prolonged and higher for proteins, slow for fats.

These facts, applied to the technique of winter moun­taineering, lead to the following conclusions:

1. There is no great difficulty keeping the body as a whole warm while vigorously exercising.
   
   
2. If one is cold during the night, food is desirable.
   
   
3. Vigorous shivering, though uncomfortable, is the most effective way of getting warm inside a sleeping bag.
   
   
4. Another excellent method is to tense the muscles of the body strenuously; relaxing and tensing alternately un­til heavy puffing and warmth result.
   
   
5. Hot drinks are fine only for temporary warming.
   
   
6. To prevent freezing of hands and feet, warm blood must get to them. Tight shoes and mitts restrict circula­tion, and tend to induce freezing. Boots must be reason­ ably tight while one is skiing, but should be loosened im­mediately upon arrival at camp, or before any prolonged rest.
   
   
7. Fatigue decreases the amount of heat produced by the body and should be carefully avoided. It is better to camp or bivouac before becoming excessively tired.

Conserving body heat.—Much more can be done in conserving heat than in producing it. In the first place, the body itself possesses certain mechanisms for this pur­pose. We are all familiar with perspiration and evapora­tion as a means of cooling. We are not so familiar, how­ever, with the fact that the skin automatically shuts off surface blood circulation when exposed to cold, and that this action decreases the heat loss from the skin to one-fourth of normal. This reaction alone goes a long way toward enabling a skier to keep warm. Alcoholic drinks prevent this thermostat from properly functioning and therefore result in rapid loss of heat from the body. For this reason, the inflexible rule of ski mountaineering is that alcohol should never be drunk during or immediately previous to exposure to cold. As the body continues to get colder, the next reaction of the human system is to cut off most of the blood supply which normally goes to hands and feet and thus attempt to preserve normal tem­perature within the torso, where the vital parts of the body are situated. This can reduce the blood supply to the hands to as little as one-eighteenth of maximum. The danger of freezing of hands and feet as a result of such general chilling is obvious. In practice then, if danger of freezing the extremities exists, it is just as necessary for the skier to put on an extra sweater as dry mitts or socks; moreover, the sweater, unlike extra socks, cannot cramp circulation in the feet.

Wind is as important as low temperature in producing chilling. A ten-mile-an-hour wind under some conditions is equivalent to a lowering of the external temperature 60°. On increase of wind velocity above ten miles per hour produces little additional cooling effect if wind-proof garments are worn.

The problem of keeping warm is then primarily the providing of insulation from both wind and cold.

To provide insulation from wind:

1. A sheltered spot should be selected for camp.
   
   
2. The tent should be thoroughly windproof. Because of the large amount of air that filters through ordinary tent walls, coated waterproof fabrics are warmer in heavy winds than the closest woven permeable cloths.
   
   
3. In a heavy wind a snow cave is warmest because it alone can provide still air, although the snow is usually porous enough to allow adequate ventilation.
   
   
4. Clothing should consist of a thin, windproof outer shell and as many inner garments as may be required for warmth. This windproof shell usually consists of a parka, a face mask (under extreme conditions), outer mitts of leather or fabric, and windproof trousers, either wool gabardine ski pants or special trousers over usual ski pants. Ski boots complete this shell. Windproof joining of these articles of clothing should be provided: a sewed-on parka hood, adequate parka drawstrings for face and waist, straps on cuffs of outer mitts and the usual tight-bottom ski pants.

To provide insulation from cold:

1. Warmth of clothing and sleeping bags is directly proportional to thickness and independent of weight. Warmth results from small bodies of dead air enclosed by cell walls or held between numerous fibers. For this reason several layers of fluffy, loose-woven sweaters are better than hard-woven garments weighing several times as much.
   
   
2. Down, feathers, and wool, in the order named, are the best insulators. This is true for both sleeping bags and clothes.
   
   
3. Sleeping bags should be covered with tightly woven cloth to prevent warm-air permeation as well as to con­tain the down. A complete hood extending over the head prevents loss of warm air around the shoulders and in­sulates the neck and head. Heat loss from an unprotected head and neck exceeds that from the remainder of the body when protected by a sleeping bag. Form-fitting de­sign with the outer shell cut larger than the inner shell gives most insulation for least weight.
   
   
4. Placing sleeping bags close together conserves body heat. A sheet of cloth over all the bags helps.
   
   
5. Insulation is necessary between snow and sleeping bag; the wool, down, or feathers are crushed by body weight to a thin layer having little insulating value. Fir boughs under the tent floor shingled in the usual matter are thoroughly satisfactory. Above timberline some sort of mattress is desirable. Mattresses made of wool and kapok provide ample insulation but are bulky. Modern closed cell foam plastics are also bulky but provide much better insulation than air mattresses, where internal air movements can cause rapid heat loss. Open cell foam is also useful but must be placed in waterproof covers to prevent absorption of moisture. A mattress, air or otherwise, 18 by 36 inches, is wide enough to sleep on and long enough to extend from shoulders to hips. Food bags, boots, and clothing may be placed under the head for a pillow; a rucksack, parka, climbing rope, or other insulating material at hand can be placed under the legs and feet.
   
   
6. Damp articles of clothing, whether wet or frozen, are poor heat insulators. Every possible means should be taken to keep all of the clothes dry, especially the inner mitts and socks.
   
   
7. Oily skin is a much better heat insulator than wet skin. Too much washing and the elimination of natural skin oils is not desirable.
   
   
8. It is better to change to dry mitts at the end of the day before the hands become cold. Even though mitts which have been worn during the warmth of the day do not feel cold, they are probably quite damp from per­spiration and are poor insulators. The same is true of socks.
   
   
9. Pressing the sleeping bag against the wall of a tent or pulling it very tightly over one's body decreases the thickness and resulting insulating value of the bag. This is one of the principal causes of a cold back. For the same reason the sleeping bag should be fluffed up by shaking well before using.
   
   
10. As many layers of dry wool clothes should be worn inside a sleeping bag as are necessary to keep warm. These include shirts, sweaters, pants, and socks as well as underclothes. To keep weight to a minimum, every item must serve as many uses as possible. Thus, clothing should serve as insulation both day and night. A sleeping bag that is adequate for sleeping in the near-nude at the lowest expected temperatures is unnecessarily heavy. Even damp clothing will dry during the early, warmer part of the night if there is enough insulation to maintain warmth and if temperatures are not so low as to freeze the moisture into frost within the sleeping bag.
    
    
11. Heat loss from a small portion of the body can result in chilling the entire body. The blood acts as a circulating heating and cooling system. Therefore, it is important to cover hands and head, even when not dis­tressingly cold, if there is any indication or threat of body chill.

The reverse is also true. If a sleeping bag is too hot, exposure of head, arms, and chest will provide adequate cooling for the entire body.

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