The Science of Staying Warm

Posted on December 13, 192001 at 12:00:38:

The Science of Staying Warm

Even the most experienced outdoorsman can recollect when a trip was ruined as a result of being cold and uncomfortable. In some cases you may have underestimated the conditions while in other cases what you thought was appropriate dress was not. Any season in Maine can create situations that are life threatening as a result of losing body heat. The closest that I ever came to hypothermia was not in January but in July. If you have an understanding of the principles of heat transfer you will understand some of the mechanisms that create a dangerous situation and be better able to anticipate and dress for them.

Conductive Heat Loss

Heat may leave a body as a result of being in contact with a cooler substance. (2nd Law of Thermodynamics). This is called conduction. When you stand on the ice, swim in cool water, or sleep on a sleeping bag that is in direct contact with the ground you will loose heat through conduction. In addition conductive heat loss may occur if you are not wearing appropriate insulators. The purpose of clothing such as fleece, down, and wool is to provide a layer that the body may heat up that will hold the heat and protect the body's surface from being in direct contact with the cool air. A sleeping pad placed beneath your sleeping bag will also insulate you from the ground preventing it from robbing you of heat. Similarly nobody would argue the necessity of a good pair of insulated boots for a day out on the ice.

Radiative Heat Loss

All warm bodies radiate heat naturally. While we cannot prevent this natural phenomenon we can try to trap the heat . Hats, gloves, and reflective materials can assist in this process. Emergency blankets work on the principle of reflecting radiative heat back to the body. They provide little insulation but work reasonably well at holding heat in.

Convective Heat Loss

Moving air is cold air. Moving air currents next to a warm body carry away heat through a process known as convection. Insulating materials need to trap �dead air� which the body may then heat. This is accomplished through materials that have �loft�. Lofty materials such as fleece or wool will trap dead air in its many voids allowing the body to heat it. A shell or windbreaker over these materials will provide another barrier to convection air currents. If clothing fits too loosely it may allow air to circulate next to the body causing heat loss. Be aware of clothing that is too tight however, this may create barriers to your circulation, impeding blood flow, and making you cold. In addition to carrying away heat, the process of convection can carry away moisture, increasing evaporative heat loss which I will discuss in further detail next.

Evaporative Heat Loss

The body has a way of naturally cooling itself through perspiration. Our body will drive moisture to the surface of the skin allowing it to cool. The reason that this takes place is because the water molecules at the surface of the liquid (sweat) have a higher average kinetic energy( temperature) than those below . These water molecules with the higher kinetic energy leave the liquid phase and enter the gas phase (evaporate). Since the remaining water consists of those molecules with a lower kinetic energy (lower temperature) the surface of the skin becomes cool. Because of the Second Law of Thermodynamics heat leaves the inner body to warm the surface of the skin. This cools the entire body. It�s a remarkable characteristic of human physiology��it can also be deadly. Hypothermia, a life threatening condition where the body is below its normal temperature, is commonly caused in �fair weather� conditions as a result of evaporative heat loss. Imagine climbing a mountain in the summer, perspiring, and shedding layers. As you reach the windy summit evaporative heat loss is in overdrive due to the moving air and your exposed skin. In addition you have used a lot of stored energy ascending the mountain. Hopefully you will have enough energy stored in your body to replenish the massive amounts lost due to evaporative heat loss. Hypothermia results when your body cannot replenish the heat energy lost. It can occur in the summer.
Not everyone chooses to view staying warm as a science but it doesn�t hurt to know the mechanisms for heat loss. In a more practical sense, here are some guidelines for staying warm.

-Wear materials next to the skin that will draw moisture away from your skin, instead of allowing it to deposit. Polypropylene and silk do this. So does wool if you can stand it. Cotton is comfortable but does not draw moisture away from skin.

-Use insulating materials that fit you well. It is best to have several layers of such materials so you can prevent sweating by removing layers. Many of today�s new synthetics are light, good insulators, and they repel moisture. These are good qualities in an insulating layer. Wool is a great natural insulator that will remain lofty even when wet.

-Use a shell over the insulating layers. The shell will reduce the effects of convection with the outside and naturally keep out moisture. The best shells are those that allow perspiration out but do not allow rain in.

-Don�t forget the hands, head and feet. The majority of your heat can be lost from these areas.

The above guidelines will provide you a versatile means to conserve heat in the outdoors. On each outing always anticipate the extremes and don�t be afraid to bring more layers than you may need. You can always remove them. Stay warm.

Registered Maine Guide, Captain Gary D. Scavette is the founder of Northeast Anglers Inc.. You may reach him at or by calling 1-800-558-7658.

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