To explain this question simply; different types of clothing are ideally suited towards particular jobs in keeping you comfortable when out on the hill, and controlling your body climate. There doesn’t really exist, a single garment that will keep you sweat free, warm, and dry to the best degree. With that in mind, we choose to utilize different layers to perform these functions.
The easiest way to order a layering system is to think about it in the order that you get dressed, as the benefits of layering outdoor clothing start from the layer next to your skin. This is what we call a baselayer. The main purpose of a baselayer is to keep you as comfortable and sweat free as possible. It regulates the amount of perspiration you end up feeling against your skin and so could be described as the layer that provides moisture management. Generally, the materials used are hydrophillic in that they soak up water. These garments are constructed in such a way to allow the water to be transferred from the inside face to the outside face, from which they can evaporate. Unless in really cold conditions, a baselayer is never really intended to insulate. There are different materials that baselayers are usually constructed from, but they always fall into two categories; synthetic, or natural.
The next layer you would typically put on, and so the next layer in the system is the midlayer. The main purpose of a midlayer is insulation, these garments help you regulate temperature. You put them on the stay warm, or take them off to cool down. Pretty simple really. There are some garments that blur the line between this and our next layer, which use ‘softshell’ material. A typical midlayer will be thicker than your baselayer, down to the fact that a thicker material has more space to trap the air warmed by your body. More technical garments do this better, using clever construction techniques such as ‘grid’ or ‘waffle’ patterns on the inner face so that warm air can be trapped in the channels between the pile.
The final layer in a layering system is known as the outerlayer. Now, with the first two layers keeping you comfortable and maintaining a comfortable climate from inside out, there is a need to be protected from outer elements. Wind, rain, snow, hail and sleet can all reduce how effective the first two layers of the layering system perform so whenever any outer elements are present – an outerlayer comes into effect. Sometimes known as ‘hardshells’ or simply, ‘shells’; outerlayers are only there to make sure that what is achieved by the first two layers is maintained.
As more extreme environments are explored, or the demand for lighter equipment increases for example, clothing that crosses the lines between the function of individual layers is developed. A prevalent feature of more technical clothing is how ‘breathable’ it is. So there are midlayers and outerlayers to aid in moisture management. Other garments such as waterproof insulated jackets are also making appearances in recent years, but the core principals of a three layer system still hold true and will provide you with the flexibility and comfort you will need when venturing out.