An Overview of Heat Loss Mechanisms and Practical Guidelines for Staying Warm with Lightweight Gear
by Ryan Jordan | 2003-10-24
The literature on physiological thermoregulation and clothing’s effect upon it is already vast. If you’re interested in a comprehensive review of thermoregulation in an outdoor context, refer to the excellent Secrets of Warmth (Hal Weiss, The Mountaineers, 1992). This article focuses primarily on specific classes of apparel, shelter, and sleep systems for cold, wet conditions. We’ll cover clothing systems for hot and dry weather elsewhere. Specifically, this article covers three major topics: mechanisms of heat generation, mechanisms of heat loss, and the design of equipment to minimize heat loss.
Mechanisms of Heat Generation
Even in cold conditions, heat generation can be a problem : excess heat generation resulting from vigorous activity can result in sweating that leads in turn to rapid evaporative cooling and the eventual breakdown of the body’s thermoregulatory capacity. We’ll consider this problem later. For now, let’s assume that conservation of every bit of the body’s heat is a desirable condition, and address the primary means by which the body gains heat in cold conditions.
Your best source of heat is generated internally by your body’s metabolic engine, fueled by calories gained from the metabolism of food. When you reduce caloric intake, you can reduce your body’s ability to produce heat. In a recent alpine climb in Wyoming’s Teton Range, Alan Dixon and I spent 38 hours without sleep and with only enough calories for a twelve-hour climb as we engaged in a difficult descent out of a summit snow and ice storm. By the end of the route, as we were approaching our car on a sun-warmed trail in sixty-degree temperatures at the valley floor, we were still shivering – while wearing our PolarGuard 3D-insulated belay parkas. Our metabolic engines, which had been fueled by more than 300 calories per hour for the first twelve hours of the climb, had slowed to a crawl.
Trying to calculate the proper number of calories needed for a given level of exertion while simultaneously accounting for environmental factors such as temperature, wind, and precipitation, is a futile exercise. The National Outdoor Leadership School (NOLS) has devised elegant predictive models for caloric consumption that significantly overestimate the amount of food required for most individuals. This results in extra comfort, but not necessarily extra safety. Consider that the two to five pounds of extra food could be better spent on warmer insulation, or dropped from a pack altogether to reduce the energy expended to carry the extra weight: such conservative predictions do not necessarily make sense from a safety standpoint.
It is well known (via any of a variety of textbook formulas) that the basal metabolism of an average build male (e.g., 150 pounds, 5’8” in height, and 30 years of age) expends approximately 1,600 to 1,800 calories, with an average level of active metabolism adding 400 to 800 additional calories. It can fairly be assumed that a hiker carrying a light pack and who understands the processes of (1) walking efficiently and (2) regulating both their metabolism and activity level should be able to hike several miles a day across moderate terrain and expend as little as 2500 to 3000 calories per day. One must keep in mind that caloric needs will increase in response to colder temperatures, higher altitudes, a faster hiking pace, more body mass, and a change in body mass to a higher muscle : fat ratio that occurs on a long-distance hike. However, most people make the mistake of overestimating these needs, especially on hikes of less than two weeks in length.
Lightweight backpackers wishing to reduce their food weight must learn to hike efficiently, carefully monitoring their exertion rate (by monitoring heart rate, for example), maintaining its constancy, and thus, expending energy that takes maximum advantage of the body’s walking momentum rather than expending excess energy to fuel rapid changes in momentum that result from acceleration and decelerating the body.
Another approach is to design a balanced diet with a high caloric density, a subject that is addressed adequately in many other texts.
Radiative Heat Absorption
Sunlight provides some ability to add heat to your body. You can easily notice this if you are wearing dark exterior clothing on cold, sunny day. Due to weather variability, however, you can’t rely on the sun as a heat source in the backcountry.
Activity (exercise) creates heat. Increased use of muscles leads to increased metabolism. While some of the caloric burn is used to fuel muscles, much of it is wasted as excess heat, which is then subject to loss through conduction, convection, radiation, respiration, or evaporation.
In the winter, you may want to consider artificial heat sources. Chemical hand and toe warmers are among the most popular gear items in the overnight kit of a backcountry skier, snowshoer, or mountaineer. Firebuilding of course, is the classic example of external heat and remains a skill that has probably saved more individuals from hypothermic death than we care to acknowledge in an outdoor culture that increasingly eschews the skill, art, and joy of firebuilding as part of a no-trace ethic. Finally, it is well known that core treatments of hypothermia include warm drinks (to warm the body core from within) and shared body heat another person (to warm the body from the outside).
Mechanisms of Heat Loss
In order to design appropriate clothing and sleep systems, we must first understand the primary mechanisms of heat loss.
Conduction is defined as the transfer of heat from a warmer object to a cooler object when the two objects are in direct contact with each other. Backpackers experience conductive heat loss anytime the body is in direct contact with cold ground. While hiking, the primary source of conductive heat loss is out of the feet via soles of your footwear. While at rest, conductive heat loss occurs while sitting or lying on the cold ground surface. Conduction is also a major source of heat loss in wet clothing, due water’s excellent conductive properties.
Convective heat loss occurs in response to movement of a fluid or gas. In outdoor clothing systems, convective heat loss occurs when warm air next to the body and in the clothing is displaced by cool air from the outside environment. The biggest factor contributing to convective heat loss, of course, is wind.
In addition to wind-induced or “forced” convection, “passive” convection occurs via the “chimney effect” that draws cool, dense air into our clothing system from pants cuffs and waist hems, displacing warm, light air that exits out of our neck hems and other vents.
Radiative heat loss from the human body occurs primarily due to infrared emission. Radiative heat loss occurs primarily on cold, clear nights, and is readily noticeable after sunset. Cloud cover dampens the effects of radiative heat loss somewhat, by reflecting a significant portion of radiant heat back to the earth’s surface. A backpacker carrying a properly designed cold weather clothing system will not experience a significant amount of radiative heat loss unless he is thinly clothed.
Evaporative heat loss is a desert hiker’s best friend and a winter traveler’s worst enemy.
Evaporation occurs when a liquid (such as sweat) changes phase to a vapor (sweat vapor). This phase change requires heat. Unfortunately, your body heat drives this phase change. Evaporative heat loss may be most noticeable in context of the “flash-off” effect, which occurs after a period of intense physical activity and sweating in cold conditions, followed by rapid evaporation and chill after stopping to rest.
Evaporative heat loss from perspiration can occur in one of two ways. Sensible (or “active”) perspiration is caused by the formation of liquid sweat droplets at the skin surface in response to excess heat. This excess heat is usually a result of being dressed too warmly for a given activity level. Insensible (or “passive”) perspiration is the direct emission of sweat vapor from the skin in response to a humidity gradient (i.e., your skin is “drying out”). Insensible perspiration is most significant while at rest, or while sleeping, while sensible perspiration is most significant during periods of activity.
Technically, respiration combines the processes of evaporation (of moisture in the lungs) and convection (displacement of warm air in the lungs by cold air from the outside environment). Because humidity in the lungs is 100%, respiration is an important heat sink in cold, dry conditions.. Significant moisture (and thus, body heat) can be lost when that moist air is exchanged with much drier outside air. In addition, some body heat is lost to the process of warming the cold air entering your lungs.
Minimizing Heat Loss with Apparel and Equipment
By now you should have already thought of obvious ways to use your clothing and other equipment to minimize the various mechanisms of heat loss. We’ll explore some of the principles for preserving your body heat below.
Minimizing Conductive Heat Loss
Recall that conductive heat loss occurs when your body is in contact with the ground surface (footwear, while walking; or ground, while sleeping) or other cold objects. These can include ice axes or trekking poles (hands), sunglasses or goggles (face), and even metal zippers, which have been known to induce frostbite on the chin of more than one winter mountaineer.
Minimizing conductive heat loss through the feet (and specifically, through the soles of your shoes or boots) simply requires a barrier between your bare feet and the ground. This barrier will include socks, insoles, and the sole of your shoe or boot. Thick, minimally compressible socks made with a high-density wool, synthetic, or blend, combined with insoles made of closed cell foam or loden (felted) wool provide good in-shoe protection. Shoes with thick mid-soles and those with lugged soles (which minimize direct contact with the ground surface) provide the basis for good winter footwear. Of course, sole design to prevent conductive heat loss through the feet is only part of the story. Convective and evaporative (and to a lesser extent, radiative) heat loss also occur in the feet.
To preserve heat while sleeping, an insulating ground pad is a must. Ground pads that maximize dead air entrapment by minimizing passive convection within the pad are typically the most effective. Consequently, one usually sleeps warmer (albeit, not as cushioned) on a closed cell pad relative to an inflatable pad. This is because significant passive convection can occur through the inflatable pad’s open-cell foam insulation. To minimize pad weight while still maintaining comfort, consider combining a torso-sized closed cell (or inflatable) foam pad with a backpack for the lower legs. Rest the head and neck on shoes or clothing to prevent conductive heat loss in those areas.
Conductive heat loss through the hands is well known among backcountry skiers and climbers. This occurs when the blood vessels in the hand constrict while gripping an already cold tool or pole. In general, the best prevention against this type of heat loss is the use of minimally compressible insulation in the palms of your gloves or mitts. Some manufacturers have recognized the need for this, and are offering handwear insulated with easily compressible insulation for the back of the hand (to minimize weight and maximize warmth) and with minimally compressible insulation (such as fleece) for the palm, or “grip” side of the glove or mitt.
Minimizing Convective Heat Loss
Convective heat loss occurs in response to wind (active convection) and the chimney effect of air movement in clothing (passive convection). Windproof clothing, worn over insulating clothing capable of trapping dead air in its thickness, provides reasonable insurance against convective heat loss. The value of a very thin wind shirt for outdoor activities cannot be emphasized enough. The wind shirt should be highly breathable (to minimize overheating and speed moisture vapor movement out of the clothing system) and very thin (so as not to contribute significantly to the insulative value of a clothing system, since insulating weight is most efficiently spent in other areas).
Clothing should be evaluated for its ability to control (not just prevent or minimize) passive convection (the chimney effect). Clothing with adjustable cuffs and hems that can be fully opened or fully closed provides the most versatility – tighten the cuffs and hems to preserve heat and loosen them up to vent heat and cool down and minimize overheating.
You can minimize active convective heat loss due to wind in a sleep system by using a bivy sack, pitching a low tarp, or sleeping in a completely walled shelter (e.g., a pyramid shelter staked to the ground, or a tent). Passive convection in a shelter is usually desirable. Many are designed with strategically placed low and high vents to promote the chimney effect, which minimizes accumulation of humid air in the shelter and thus reduces condensation.
Minimizing Evaporative Heat Loss
Vapor barrier clothing is the only sure way to prevent evaporative heat loss. Vapor barrier fabrics are not only waterproof, but completely impermeable to moisture vapor as well. Consequently, evaporative heat loss cannot occur, because the microclimate inside the vapor barrier is a closed system that approaches equilibrium between water and vapor phases. Because vapor barrier clothing requires a high level of attentiveness to your body’s physiology and frequent fine-tuning of your clothing system to prevent sweating, however, it is not very popular among mainstream outdoorsmen. Accumulation of liquid moisture in a vapor barrier microclimate creates a new and very significant method of heat loss: conductive cooling as heat loss to water in the system. Because of this vapor barrier clothing tends to be wholly impractical for high exertion activities.
|Radiative heat loss is most significant between sunset and sunrise, when the atmosphere loses tremendous amounts of heat that was absorbed by sunlight throughout the day. The best defense against radiative heat loss is thick insulation.|
Minimizing Radiative Heat Loss
Radiative heat loss is assumed by many to be negligible relative to other heat loss mechanisms (i.e., wind-induced convective heat loss and evaporative cooling) in inclement conditions. In windless conditions when the body is not active, however, it can be significant, especially at night. Radiative heat loss can be minimized by one of two methods. The first is by wearing a reflective barrier (such as aluminized nylon or mylar) near the skin capable of reflecting infrared radiation back to the body. The second is by wearing thick clothing (e.g., down or high-loft synthetic fill garments). The latter strategy is effective because infrared radiation cannot travel through thick insulation, and thus, most of the infrared radiation lost by the body can remain entrapped in the clothing system rather than exiting out to the environment.
Minimizing Respiratory Heat Loss
Respiratory heat loss can be significant in cold, dry conditions. Simply not breathing, however, is not a very practical heat conservation strategy. In theory, respiratory heat losses can be minimized by breathing air that has been pre-warmed and/or pre-humidified prior to taking it into the lungs. High altitude climbers and arctic travelers know well that breathing through a fleece balaclava or face mask can improve respiratory comfort by increasing the humidity and warmth of air being breathed prior to its entry to the lungs. Some innovative products are now appearing on the marketplace specifically designed to magnify this effect, and are informally known as “heat exchange face masks.”
Warm When Wet ?
This chapter would not be complete without addressing a common marketing claim prevalent among manufacturers of synthetic insulation: warm when wet.
The presence of liquid moisture in any insulating material results in :
- more rapid conduction of heat from the body than in a dry material,
- displacement of air volume that is normally used to house effective insulation (dead air), and
- evaporative cooling resulting from the use of body heat to induce a phase change from liquid to vapor. In addition, the absorption of liquid moisture into a knit, woven, or high-loft insulation results in a reduction in insulation resiliency caused by fiber structure collapse.
This effect reduces the thickness of the insulation further and magnifies the effects of (1) to (3).
While there are both natural (e.g., wool) and synthetic fibers (especially, relative to down) that are more resistant to the fiber structure collapse induced by the presence of liquid moisture, there is no fiber in existence, be it natural or manmade, that can fairly make the claim that it is warm when wet.
Conclusion: The Impact of Thermoregulation on Lightweight Backpacking
The most important impact of thermoregulation on lightweight backpacking is rooted in the skill of the hiker. Hikers who understand thermoregulation, the body’s physiological response to metabolic and environmental stress, and the response of apparel and sleep systems to moisture and heat resulting from physiological activity and environmental conditions, can effectively and safely reduce their clothing and sleep system weight in the backcountry. On the other hand, failure to understand the complicated relationship between thermoregulation, physiology, environmental conditions, and equipment can result in a failure of the hiker’s body, the hiker’s gear, or both. This can lead to hypothermia and death. You must understand that as you reduce the weight of gear that keeps you warm and dry, you must also accept some reduction in safety margin.
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