Layering systems
The theory behind the layering system is that several thin layers of clothing will keep you warmer than a single thick layer.
Layering works by creating air spaces between garments to trap warm, still air creating a microclimate that surrounds your body. You can adapt this to deal with temperature, wind, moisture and exertion levels; if you are feeling cold add a layer, if you feel hot, take a layer off.
Base Layer
The base layer is critical and it should transport moisture away from the skin, dispersing it to the air or outer layers so that it can evaporate. Modern synthetic fibres such as polypropylene and polyester make the best base layers. They are lightweight and strong and are quick to dry.
Mid Layer
The mid layer works as an extension of the base layer, continuing the transportation of moisture from the undergarments and providing insulation to retain your body warmth.
Outer Layer
The outer layer or shell is your principal barrier from the elements. It should allow air to circulate and permit moisture to escape.
For dry conditions an uncoated, breathable wind resistant shell or a smooth-surfaced soft shell is generally sufficient.
When it's wet you will need a waterproof (coated) rain jacket. Ideally the shell should be made in a breathable and waterproof fabric such as Gore-Tex®, which offers protection from wind, rain and snow yet allows water vapour to escape.