Clothing should be selected on the basis of keeping the body at a constant temperature throughout the day. The three most dangerous factors are moisture, wind and extremes of temperature. No Single garment can protect against all three factors. Therefore a layered clothing system is recommended.
The insulating mid layer provides most of the warmth. However, in order for it to insulate properly it must be protected from being penetrated by wind and moisture - which is achieved by the outer or shell layer of waterproof clothing. The base layer is next to the skin, and keeps it dry by wicking away moisture caused by sweating, as well as providing the initial warm thermal layer.
The body has to be kept at a constant temperature. (Boring science stuff) The core of the torso contains most of the vital organs, and along with the brain is automatically kept at 37ºC, its most efficient operating temperature. The outer areas of the body (limbs and extremities, scalp, skin and muscle) fluctuates more, but is usually around 3ºC cooler. Any blood circulating to these areas will return to the core cooler. By altering the balance between the volume of blood circulating the core and the cooler surface, the body can precisely maintain the temperature of the essential organs.
The Layering System - EXPLAINED
There are three layers to this system: The base, mid and outer layers. Each layer is explained below.
It is important to avoid having moisture next to the skin as this causes rapid cooling as well as a clammy and uncomfortable feeling. Therefore Base layer garments should have the ability to wick moisture away from the skin. As well as having this wicking ability they should be fast drying, have flat seams on the shoulders and on legwear, and have a comfortable feel.
Base layer clothing can be found in a variety of fabric weights, from lightweight, stretchy polypropylene suitable for high activity sports where sweating is commonplace, through to mid weight layers which are also suitable as stand alone lightweight active shirts, and are styled accordingly. Finally, there's the thicker expedition weight garments which are suitable for static activities or very cold conditions. These too are most often styled for use as a stand-alone garment when the weather permits. Many Base layer tops can be worn by themselves, in hot weather they are ideal - but remember to carry enough warm clothing for the rest periods or a chance deterioration in the weather.
Mid Layer garments provide thermal insulation, whilst allowing moisture to pass through (helped by the open weave and the low absorbency of the fabric). Air is the best insulator, so the fabrics generally have an open fleecy weave to hold as much air as possible within their structure. This air is heated by your body to provide thermal insulation. However, the wind will easily penetrate the open weave of these garments, replacing the warm air with cooler air. On breezy days this is a continual process, so even in light winds a fleece worn without a Shell Layer will loose a high proportion of its insulation.
The importance of windproofing is often underestimated. For instance at 0ºC a relatively mild 15 mph wind will have the effect of reducing the temperature by 10ºC, giving an effective temperature of -10ºC. Stronger wind can cause even more dangerous cooling. In such circumstances additional warm (i.e. Mid Layer) clothing is not appropriate - adding a windproof shell garment will be far more effective.
The Mid Layer may consist of more than one garment, such as a thin lightweight fleece worn with a conventional fleece garment.
Composite Shell/Mid Wear Garments
Composite garments undertake the functions of two layers. The shelled fleece garments combine the qualities of the insulating mid layer and the windproof/showerproof shell layer in one garment which is ideal for Spring and Autumn walking or casual use. Remember, however, while these are sufficient in low cloud, brief showers, or strong winds, a waterproof outer layer is still required in wetter conditions.
Some waterproof/breathable shell jackets are fitted with additional inward facing zips which permit an insulating mid layer jacket to be attached. The combined garment is thus easier and quicker to put on , remove, and hang on a peg than two separate jackets, making them ideal for more casual use. Seperating the jackets and using them independently gives great flexibility for use on the hill.
Outer (waterproof) Layer
The outer Layer provides essential protection from the wind, rain and the snow, allowing the base and mid layers to work more effectively. Breathable fabrics allow body moisture to escape, whilst keeping the elements out. For these breathable garments to work most effectively, undergarments should be worn, ideally made from synthetic fabrics; natural fibres such as wool and cotton absorb condensation before it has a chance to permeate through the outer shell garment for evaporation.