The layering system is a devised strategy used for regulating your body's temperature while out and about whether it be a gentle walk up to the top of a local hill or ice climbing in winter conditions across a froze glacier abroad.
Depending on who you ask about the best method for layering some will advise a few thick and heavy layers while others will often recommend plenty of thin layers. Either way works in order to sufficiently trap and maintain body heat whilst on the move but in my opinion, I would recommend plenty of thin layers due to the fact they are easier to adjust; giving you more precise control over your body's temperature as well as the fact that once the layers are removed they are lighter to carry and pack down smaller in your rucksack than thicker, heavier layers.
Looking at one of the most generic structures for layering used in the outdoors today below is a list of layers and the purpose is layer is designed to fulfil.
Base Layer- The base layer is designed to wick away any perspiration that would otherwise collect on the skins surface, as well as managing to trap a small amount of the body's heat in doing so. Creating a thin layer of warm air between the base layer and the skin's surface. Avoid using materials such as Cotton as a baselayer because once it becomes wet, it stays wet and cools quickly especially when exposed to wind or air, this will draw away all of the heat that would otherwise be trapped by the layering system. Good examples of base layers are thermal underwear, technical t-shirts and merino wool fabrics.
Mid-Layer or Insulation Layer- The Mid-layer or Insulation Layer is the main layer used in order to completely trap and maintain the body's heat. Sweat is always wicked further away from the body through use of breathable mid-layers. Mid-layers can constitute of a light or mid-weight fleece or softshell material.
Outer Layer- The outer shell works in order to repel any wind and rain and protect your other layers from becoming damp and exposed to the elements. The outer-layer is often made up of a light-weight, packable jacket and trousers that can be packed towards the top of the rucksack for quick access in case of a heavy unexpected downpour. The outer layer should also be breathable so that it continues to wick away any perspiration collected by the other layers and completely remove it from the body and layering system. You will often find the most common fabrics used in the outdoor industry to carry out this purpose are Event, Gortex and Pertex aswell as others such as Paramo.
Figure 1. A diagram of how the layering system works (Backpacking Light, 2015).
Figure Reference: Backpacking Light, (2015). The Layering System. [Image] Available at: http://www.backpackinglight.co.uk/layering-system.html [Accessed 4 May 2015].