Winter Camping (chapter 2)

Clothing System

Mechanisms of Heat Loss

To choose proper clothing and sleeping systems, it is necessary to first understand the basic heat loss pathways.

Conduction

Conduction is defined as the process by which heat is transferred from a warmer to a cooler object when the two things are in direct touch. When a backpacker's body comes into direct contact with the chilly ground, conductive heat loss occurs. While trekking, the largest source of conduction heat loss is through the soles of the feet. While sitting or sleeping on a cold ground surface, conductive heat loss happens. Conduction is also a significant source of heat loss in wet clothing, owing to water's exceptional conductivity.

Convection

Convective heat loss happens when a fluid or gas moves. Convective heat loss happens in outdoor garment systems when warm air adjacent to the body and within the clothing is displaced by cool air from the surrounding environment. Naturally, the primary component leading to convective heat loss is wind.

Along with wind-induced or "forced" convection, "passive" convection happens as a result of the "chimney effect," which draws cool, dense air into our garment system through the cuffs and waist hems, displacing warm, light air that departs through the neck hems and other vents.

Radiation

The majority of heat loss from the human body occurs as a result of infrared emissions. Radiative heat loss is most visible after sunset on cold, clear nights. Cloud cover mitigates some of the impacts of radiative heat loss by reflecting a substantial amount of radiant heat back to the earth's surface. A traveller wearing an appropriately constructed cold weather clothing system will not incur considerable radiative heat loss unless he is extremely thinly dressed.

Evaporation

Evaporative heat loss is the best friend of desert hikers and the biggest enemy of winter travelers.

When a liquid (such as sweat) changes phases to a vapor, this is called evaporation (sweat vapor). This phase transition necessitates the presence of heat. Regrettably, your body heat is responsible for this phase transition. Evaporative heat loss is particularly visible during the "flash-off" effect, which happens following a period of high physical activity and sweating in cold temperatures, followed by fast evaporation and chilling after resting.

Perspiration-induced evaporative heat loss can occur in one of two ways. Sensible (or "active") perspiration occurs when liquid sweat droplets form on the skin's surface in reaction to a stimulus.

overheating. This extra heat is typically caused by dressing too warmly for the exercise level. Sensible (or "passive") perspiration is the direct emission of sweat vapor from the skin in response to a humidity gradient (i.e., when your skin "dries out"). While insensible perspiration is more prevalent during periods of rest or sleep, sensible perspiration is most prevalent during moments of exertion.

Respiration

Respiration is a technical term that refers to the process of evaporation (of moisture in the lungs) and convection (displacement of warm air in the lungs by cold air from the outside environment).

Due to the fact that the lungs are completely humid, respiration acts as a significant heat sink in cold, dry environments.

When moist air is exchanged with much drier outside air, significant moisture (and consequently body heat) can be lost. Additionally, a portion of your body heat is lost during the process of warming the chilly air that enters your lungs.

Insulation

Clothing's thermal insulation is proportional to the thickness of the dead air space it contains. Dead air is defined as any enclosed volume of air that is too small to support natural convection currents. These currents have been found in units with a diameter of as little as 2 millimeters. The body heats the dead air adjacent to the skin, which creates a layer of warmth surrounding the body. It is not the clothing that keeps you warm; it is the dead air. This is because the denser a material is, the faster it can conduct heat; the density of air is plainly negligible in comparison to a piece of fabric.

Thermal conductivity, denoted by the symbol k, is a characteristic that describes a material's capacity to conduct heat. Heat transmission between materials with a high thermal conductivity is faster than between materials with a low thermal conductivity. As a result, high thermal conductivity materials are frequently employed in heat sink applications, while low thermal conductivity materials are used as thermal insulation.

Thermal resistivity, R, is calculated as the product of the thickness of the material and the reciprocal of its thermal conductivity. Thermal insulators with a high R value are considered to be good. When the thermal conductivity of an insulating material is low and the thickness is large, the R value will be high. R-values can be applied to layered materials.

The Layering Principle

Having multiple layers of clothes is crucial to establishing this dead air space.

Each layer gives some dead air space. This allows you to add or remove layers as the temperature or activity level changes. Remember, your body is the source of heat, and garment layers merely serve to trap it and slow its loss to the cold. Too much clothing causes overheating and sweating. The quantity and type of layers you wear must be balanced with your activity level.

Why not just layer up and sweat? Wet surfaces lose up to 25 times more heat than dry surfaces (due to the higher density of water). Sweating and being saturated lose heat quickly due to evaporation. Due to the dry air, you sweat a lot and lose a lot of water. Dehydration raises the risk of hypothermia. So you want to regulate your layers so you are warm but without sweating profusely.

Traveling in the cold means constantly altering clothing to stay warm. This means having layers you can add or remove, and allowing for layer flexibility.

In most cases, convection causes the largest heat loss. To properly insulate, you need a windproof outer layer.

The "bellows action" of garments is also convective. As you move, a bellows effect develops, releasing warm air through clothing openings and sucking in chilly air. This action can diminish your body's personal insulation by up to 50%. Thus, all layers must be "sealed" effectively (i.e. buttons, zippers etc.). Layers with holes allow you to ventilate and open the "chimney damper" without having to remove a layer. Opening and shutting a jacket's zippers or armpit zips allows you to vent if you're too hot or seal up if you're too cold without having to add or remove layers. The bellows motion pumps heated air out of loose clothing.

You need garments that fit well but not too tightly. Excessive clothing compression reduces dead air space in layers below and restricts bodily movement.

The efficiency of clothes is also proportional to the diameter of the body part covered. Each bodily part has a preferred insulating thickness. Beyond that, the extra mass seems to hinder movement more than the extra insulation does. Have you ever noticed that wearing thin gloves makes your hands colder? Because insulation wrapped around a curved surface has a larger cross-sectional area, it also has a larger surface area to lose heat.

Thin insulation around small cylinders, like fingers, actually increases heat loss until it reaches 1/4 inch thick. Note: A windproof layer reduces convective heat loss.

To be continued ...