The two main principles are: firstly, do not let the outside temperature, the wind and the rain get into your body, and secondly, do not lose your body warmth through your clothes. For example: if you wear a thick sweater, not only you will feel clumsy, but also the wind will get through and your body warmth will go out the same way, so you will feel cold. However, if you will wear a few thinner layers and you block the airways by tucking your first layer into your pants, fasten your coat all the way up, and wear a scarf and knee-high socks, you will already feel the difference.
The Base Layer - A base layer is your first layer of protection against the elements and will provide a small amount of warmth. A base layer should also ‘wick’ or ‘transport moisture or sweat’ away from the body. This will help to regulate your body temperature as any moisture build-up can draw warmth away from your body. Base layers need to be worn tightish against the skin. Never wear cotton as a base layer, as it does not wick moisture away, so it retains sweat, loses its warmth, and causes too much evaporative cooling. A good base layer is made out of synthetic fiber and wool. Don’t forget to tuck this base layer into your pants.
The Mid layer - The primary function of the mid layer is to provide insulation. A mid layer will direct any body heat that your base layer didn’t retain back into your body and help stop cold air passing through to your body. A good mid layer will be breathable and also wick any moisture away from the body so any moisture trapped by the base layer is transported out to be evaporated. A thin fleece or merino wool top could be worn during milder conditions.
The Mid and a ½ layer – This is a thin down or synthetic-filled thin jacket. Most of these jackets have the option to tighten the neck opening. If you don’t have a scarf, it is recommended to use this option. Thicker fleece or soft-shell jackets would also be suitable for this layer. Most of the time you will be changing this layer, putting it on or taking it off according to the weather conditions.
The Outer layer / shell layer – This layer keeps away the elements (wind, snow, rain, etc.). The shell layer is usually in the form of a jacket, and it should block the wind and be waterproof. Ideally, the outer layer lets moisture through to the outside (look for jackets that are labeled “breathable”), while not letting wind and water pass through from the outside. There are a multitude of jackets that fit this criterion, but there are no jackets that can are totally waterproof and extremely breathable — there will always be some kind of trade-off. Don’t forget to fasten the edges of the sleeves, otherwise the wind will blow in and your body warmth will blow out.
Deciding whether you will wear a base layer on your lower part of the body too will depend on your planned activity for the day. You can use thermal long johns, leggings or tights. You can also wear socks that go over your knees and a long coat that goes below your knees.
Wearing cotton pants is not recommended. As I wrote above, cotton retains moisture, loses its warmth, and causes too much evaporative cooling. That means that regular denim jeans aren’t recommended either. It is better to wear outdoor pants over the long johns or pants made out of a thicker synthetic material. Ensure that your pants aren’t touching the wet ground.