The hazard faced is a condition known as hypothermia, which occurs when a person's body temperature drops to the point where it can no longer sustain life. This is a very deadly condition. Temperature, humidity, wind chill and skin/water contact area are just some of the factors that play into if, when and how quickly hypothermia will set in. Another related hazard is frostbite, where the water in the cells of extremities freezes and damages the cells.
Layering can help with both of these issues, but for it to be effective it must be used correctly. While thickness equals warmth (the more insulation between you and the cold, the more heat will be retained), layering works on the fact that during the course of activity, different insulation layers will be necessary. First thing in the morning, more insulation may be needed. As the day warms and the person's activity increases, layers of insulation can be removed until the body is at a comfortable equilibrium between heat generation and heat loss to the cold. If the activity level or the temperature drops, layers can again be added.
THREE COMPONENTS OF LAYERING
By this principle, multiple thinner layers are preferable to one very thick layer, but the concept of layering is more involved than that. In fact, layering can be thought of as having three components: the base layer, the insulation layer and the shell.
The base layer is the layer directly next to your skin. Even in the cold your body will perspire. The base layer should be a non-absorbent material that will wick the moisture away from your skin. While wool can accomplish this, synthetic materials may be a better choice. Wool tends to irritate on direct content with skin. Do not use cotton. Cotton absorbs water and will keep the moisture next to the skin, which will draw heat away.
The insulating layer is composed of multiple thin layers. Again, if possible this layer should be synthetic materials or wool. The synthetic materials will continue moving the moisture of perspiration away from the skin and allow it to be evaporated into the environment. If cotton is used, try to keep it away from the skin.
The final layer is the shell. This is the outermost layer and is protecting every layer under from the elements. For snow and rain it would have to be waterproof and serves as your wind shield. This is your most important layer as it may keep water or wind from all the other layers and increase your heat retention.
For the extremities, the story is slightly different. A few factors account for this. First off, your core temperature is generated from your torso, so hypothermia is not as much of an issue when dealing with the extremities. Also, the legs and arms tend not to perspire as much as the torso and require one or at most two layers (base and one insulation layer). The head is very important from a heat loss perspective and any type of layering will involve a hat or cap. Don't forget the ears. They are soft cartilage and can suffer severe frostbite
. By taking these factors into account, layering can be one of your tools to prevent cold weather injuries. ip