Layering clothing system

Your body produces heat all the time and loses it in several ways: through contact with cold things such as the ground (conduction), through air and wind moving past (convection), through radiation and through sweat evaporating. Clothing mainly slows the first two by holding on to lots of tiny pockets of air. Still air conducts heat very poorly. That is why a lofty fleece is warmer than a densely woven shirt of the same weight. When we call a garment “warm”, what we really mean is: it traps a lot of still air.

Two things destroy these air pockets. Water fills the gaps, and water conducts heat many times better than air. Wind pushes through the clothing and keeps swapping the warmed air for cold air. On top of that comes evaporation: when wet fabric dries on your body, it draws heat out of you. It is the same effect that makes you shiver in the wind after a swim.

Layering gives each of these problems its own layer. The base layer keeps the skin as dry as possible, the mid layer provides the air pockets, the outer layer keeps wind and rain out. Because the layers are separate, you can adjust each one on its own. So you do not just have the choice between “jacket on” and “jacket off”, but many steps in between.

What matters is not the single jacket but the regulating. Many people do not get cold on the way because they carry too little, but because they sweat first and then cool down. A well-tried rule is: set off feeling slightly chilly, because after a few minutes of moving you will warm up anyway. Before a climb, take off a layer before you start to sweat – not when your shirt is already wet.

At breaks it is the other way round: put on a warm layer straight away while you are still warm. Then you do not have to win back lost heat with effort; you simply hold on to the heat you have. When it rains, put on the outer layer early, not once the mid layer is already damp. Getting damp insulation dry again is very hard on the way.

Fit counts as well. The base layer should sit close to the body so it can pick up moisture directly from the skin and pass it on. The outer jacket has to be roomy enough not to squash the warm layer underneath – compressed insulation holds less air and keeps you less warm. Head, hands and feet are part of the system: a hat, gloves and dry spare socks often make more difference than an extra jacket.

Synthetic fibres and wool are suitable for the base layer. Synthetics absorb little water and dry quickly. Wool, such as merino, can take up moisture inside the fibre, feels less wet while doing so and stays warmer than cotton when damp. Cotton is unsuitable as the layer next to the skin in cold and wet conditions: it soaks up water, dries slowly and barely insulates when wet.

Mid layers can be fleece, wool jumpers or jackets with down or synthetic fill. Down is very warm for its weight and packs small, but loses much of its effect when wet. Synthetic fill is heavier but keeps more warmth when damp. Fleece is robust and dries quickly but lets wind through – in wind it needs a tight outer layer on top.

The outer layer is a wind- and waterproof jacket with a hood, ideally with vents. In dry, windy weather a light wind jacket that lets more water vapour out is often enough. Specialist clothing is not a must: a wool jumper, a synthetic sports shirt and a rain jacket you already own make a working system. Care, such as renewing the water-repellent finish, follows the manufacturer's instructions.

Hypothermia means that the temperature inside the body drops. It does not only happen in deep winter: rain, wind and wet clothing can become dangerous even above freezing, especially when someone is exhausted or has eaten little. Early signs are feeling cold, shivering and clumsy fingers. If the shivering becomes intense or someone seems confused, drowsy or slurs their speech, that is a serious warning sign.

Then: find shelter from wind and wet, swap wet clothing for dry, or at least put a dry insulating layer and a windproof shell on top, insulate the person from the cold ground and get help – via the local emergency number (112 in the EU). Anything beyond that is best learned on a first-aid course.

Preventing is much easier than treating. Change layers in time, carry a dry reserve layer packed waterproof and remember that children and older people often cool down faster and do not always notice or say so. In bad weather, turning back early is not a defeat but a good decision.

Try out your system on a cool day close to home. Notice after how many minutes of walking you warm up and when you start to sweat. Practise putting layers on and taking them off quickly, including with gloves. Decide which pocket holds which layer so you can find it without searching.

During a break you can compare: how fast do you get cold if you put nothing on, and how does it feel if you put on a warm layer straight away? This experience helps you make the right decision later, before you get cold. With children it can be a game: who notices first that it is time for one layer more or less?