How Body Heat is Lost

In cold temperatures or winter survival, heat is the currency of the survivor and he does his best to conserve that heat. He does not squander his warmth by exerting himself so hard that he overheats and sweats profusely, which can lead to chilling when he stops to rest. Nor does he allow himself to become so cold that he finds it difficult to function physically and mentally.

When necessary the survivor slows down a bit to maintain a comfortable internal temperature and adjusts parts of his clothing layering system according to conditions. When his activity slows or the temperature falls, the survivor adds layers of insulation in order to keep his precious heat from escaping into the environment.

The normal healthy temperature of the human body is 98.6 degrees F. Go just two degrees below that and you are in serious condition. Go five degrees further and you can die.

There are a number of ways in which heat escapes from the body and into the surrounding environment. Understanding the mechanisms involved in this heat transfer process is vital in order to minimize heat loss and preserve as much body heat as possible in a cold environment.

This knowledge could very well save your life.

Conduction

Conduction is the movement of heat from a warmer object to a cooler one when they are in direct contact with one another. This serves to even the temperature difference between them. For example, when you lay directly upon a cold rock the heat from your warm body will transfer into the rock. You become cooler as the rock becomes warmer.

The rate of heat transfer between two objects of different temperatures depends upon several factors. These include

The temperature difference between the two objects
The total surface area where the two objects are in contact
The efficiency of the insulation that is between the objects
The greater the temperature difference between two objects in contact, the more heat is transferred between them in a given time. For example, when you place your hand on a very hot stove top you will quickly receive a great heat input from the stove to your hand. If the stove top is only warm, it will take much longer to receive the same amount of heat into your hand.

The more surface area in contact between two objects, the more quickly heat is transferred between them. Stick your finger on an icicle for a minute and it feels cold but you will probably not feel too uncomfortable. Strip naked and lay on a block of ice for a minute and you will most likely be very uncomfortable indeed as the ice absorbs heat from your body at at a very fast rate.

The amount of heat being transferred between two objects of different temperatures can be slowed by the use of effective insulation. Insulation retards the movement of heat between them by creating pockets of dead air space which trap the flow of heat.

For example, a spongy foam pad is a excellent insulator between your warm body and a cold floor. However, should you compress the foam pad, thereby removing the small pockets of dead air within it, the pad will become much less efficient at blocking the transfer of heat.

This is why proper insulation beneath you is so important – it must resist the crushing effect of your body weight and be thick and lofty enough for numerous pockets of dead air space to impede the flow of heat as much as possible.

Convection

Convection is somewhat like conduction as mentioned above, but the two objects in contact are also moving relative to one another. Once again, the amount of heat transferred between the two objects is dependent upon their differences in temperature and the amount of surface area in contact. However there is a third important component and that is the speed with which the cold object is moving.

For example, when your warm face is exposed to a blast of cold air the speed of that air matters. If the cold air is moving slowly it may not cool your face very much at all. However if the air is traveling 60 miles per hour you may actually receive a frostbite wound in a matter of seconds.

The blood in your body also transfers heat by convection. As our body cools, its response is to move blood away from the extremities in order to keep the body's core at optimal temperature. The result is that our hands and feet become cooler and may eventually lead to frostbite. This gives impetus to the fact that in order to keep you feet cool you should put on your warm hat.

You can reduce convective heat loss by wearing a windproof out shell. This will stop the air from flowing into your insulative layers of clothing and robbing the body heat that is stored there.

Radiation

Radiation is the transfer of electromagnetic energy between two objects of different temperatures. Since our bodies tend to be 98.6 degrees F, we are often warmer than our surroundings and so we radiate heat to them. In turn we can receive radiative heat from the rays of the sun, fires, and light reflected off from snow, rocks and sand, or water.

Another example of how radiative heat works is the effect of cloud cover on temperature. At night when there are no clouds, the temperature tends to become cooler than on nights when there are clouds. This is because as the earth radiates heat toward the cold outer space, some of this radiation is reflected back by the cloud cover.

Exposed human skin is also a radiator. The more total area of exposed skin, the more energy is radiated to the environment, assuming of course that the body is warmer than its surroundings.

To minimize the amount of radiative heat you lose to your environment make sure all exposed areas of you skin are covered. This includes the head, face, neck, and hands.

Evaporation

When water evaporates its change in state from liquid to a gas takes up a great deal of energy and lowers the temperature of the surface on which it occurs. This is the process of evaporation.

In hot environments evaporation is a welcome process and we may even encourage the process by wetting ourselves down when the need and opportunity arises. In cold environments however, evaporation can be a killer as it consumes a large amount of energy and warmth from your body and transfers it to the outside world. In addition, when the clothing you need to stay warm becomes wet it looses much of its insulative value and exposes you to the risk of hypothermia

Because of the dangers of becoming wet in cold weather, you should carefully monitor your amount of perspiration during times of heavy exertion. If you are becoming overheated and begin to sweat, remove some clothing or reduce your efforts in order to become cooler and keep sweating to a minimum.

Additional evaporative heat loss occurs through breathing. When a dog is hot he will pant. The air he brings into body is filled with moisture that is heated by the body. When the dog exhales he sends this hot moisture laden air out of his body and into the outside world. The dog becomes cooler.

In just the same way as a dog, as you breath in and out you are exchanging heat via evaporation to the outside world. This loss of heat is increased when the air you are breathing in is very cold or very dry. The colder and drier the air and the faster and more deeply you breathe, the greater the loss of heat. In high mountain areas losing heat to the breath rivals that of losing heat through the evaporation of sweat.

Reducing Heat Loss

With knowledge of the heat loss process as part of your survival toolkit, you can better prepare to stay warm in whatever situations arise. To recap:

To reduce the amount of heat you lose through conduction, place quality insulation between you and the object you are touching. The insulation must have plenty of dead air space within its structure and resist compression if weight is to be applied to it.

To reduce heat lost through convection, have a windproof outer shell that you can wear over your insulation layers. This will help prevent wind from penetrating your clothing and removing the body heat you have stored there.

To reduce the amount of heat you lose to your environment through evaporation you need to stay dry. Rain, snow, fog, water, and sweat can make you wet and increase the amount of evaporation the occurs on your body. In addition, when your clothing becomes wet its efficiency as insulation drops off dramatically. Avoid deep heavy breathing as this too will serve to move large amounts of heat from your body and out into your environment via the evaporative process.

Reducing the heat you lose through radiation means covering all exposed areas of your skin so that none of it shows to your environment. This includes the head, face, neck and hands.

And remember: Cool and Dry Stays Alive - survive another day.
 

Original at: http://www.survivaltopics.com/survival/how-body-heat-is-lost/