Dead air and why wet clothing chills

Air is a poor conductor of heat as long as it does not move. When it moves, it carries heat away by convection. Good insulating materials – fibres, down, foam – solve this problem by dividing the air into a great many small pockets. In these pockets the air can hardly circulate. This is called “dead air”.

A simple rule follows from this: what matters most for warmth is how thick the air-filled layer is – experts call this loft. The material itself matters less than many people think. If the layer is compressed, air escapes and insulation falls. That is why a sleeping bag hardly warms you underneath your body, and why a sleeping pad is so important.

When the material gets wet, water fills the pockets. At room temperature water conducts heat roughly 20 to 25 times better than air. In addition, evaporating the water costs a lot of energy, which is drawn from the skin. Wet clothing therefore loses heat in two ways at once: through conduction and through evaporation.

Not every material behaves the same when wet. Wool and many synthetics keep part of their loft even when wet and still provide some warmth. Cotton, on the other hand, soaks up a lot of water, collapses when wet and dries slowly. That is why mountaineering handbooks advise avoiding cotton on cold, wet trips, for example jeans or cotton T-shirts.

Down is very warm for its weight when dry, but loses a large part of its loft when wet. Protect down jackets and sleeping bags particularly carefully from rain, snow and condensation. A sleeping bag should also be stored loosely, not permanently compressed, so that the filling keeps its loft.

The layering principle follows directly from this physics: a layer next to the skin moves moisture outwards, one or more insulating layers hold still air, and an outer layer keeps wind and rain out. Because individual layers can be put on and taken off, warmth can be matched to exertion.

Sweat wets you just as much as rain, only from the inside. Anyone climbing a slope in a thick jacket sweats through the insulating layer and then gets cold during the break. Better: take off a layer before the climb, open zips, go more slowly. During the break, put on a warm layer immediately, before you cool down.

Put the rain jacket on before the insulating layer gets wet, not when you are already soaked. At camp, swap wet things for dry ones before you cool down. Dry socks and a dry layer for sleeping are often more important than an extra item of clothing for the daytime.

Wet clothing in wind and cold can lead to hypothermia, even at temperatures above freezing. Wind increases both convection and evaporation. Watch for early signs such as violent shivering, clumsiness and confusion – in yourself and in others. With confusion, marked drowsiness or reduced consciousness, call the local emergency number (112 in the EU).

Never dry clothing directly over an open fire or on a stove; synthetics in particular melt quickly and can catch fire. Never run stoves or fires for drying or warming inside a tent or enclosed space – there is a risk of carbon monoxide poisoning.

In very cold environments, some experienced people use a vapour barrier: a thin layer impermeable to water vapour close to the body, so that sweat does not get into the insulation in the first place. This only works with experience, because you easily get wet underneath if you dress too warmly.

The same physics explains many everyday things: double-glazed windows hold a still layer of air or gas between the panes, animals grow a dense winter coat, and a layer of newspaper or dry leaves under clothing can provide some insulation in an emergency. It is always about trapping air and staying dry.