How the body loses heat

Conduction means heat passes directly into anything you touch that is colder than you. Ground, rock and snow are large, cold masses that can soak up heat almost without limit. Anyone sitting or lying still on them keeps losing heat through the contact area – and often only notices once hips and back are already cold.

Convection means moving air or water carries away the warm layer of air right next to your body and replaces it with cold air. That is why a windy day feels much colder than a still day at the same temperature. Radiation is heat your body gives off as invisible light. On clear nights it radiates into the cold sky; a roof, a tree or a sheet above you intercepts some of it.

Evaporation costs heat because water needs energy to turn into vapour – and it takes that energy from your skin. Sweating is useful on hot days but becomes a problem in the cold. Wet clothing combines two paths at once: water conducts heat far better than air, and it also evaporates.

Which path costs the most heat depends on the situation. Lying on cold ground, it is often conduction. In wind with damp clothing, it is usually convection together with evaporation. So it helps to ask yourself briefly: where am I losing the most right now?

In an emergency there is a simple order: first stay dry and get out of the wind, then put a layer between you and the ground, then add a roof against rain and radiation. A roof alone is not enough if you are lying on wet ground underneath it. Even a thick sleeping bag helps little underneath you, because its fill is squashed flat by your weight.

Acting early is crucial. It is much easier to stay warm than to get warm again. Put on a jacket before you feel cold, and take off a layer before you start to sweat.

Hypothermia means the temperature inside the body is dropping. It does not only happen in snow: rain, wind and exhaustion are enough even in mild temperatures. People who sit still or are injured, very tired, hungry or soaked are particularly at risk.

Warning signs are intense shivering, clumsy hands, stumbling, confusion, slurred speech and unusual drowsiness. Important: the person affected often does not notice it themselves, because thinking slows down too. So keep an eye on each other in a group.

At warning signs, act immediately: get out of wind and wet, insulate from the ground, replace wet clothing with dry clothing if possible. If things get worse, call the local emergency number (112 in the EU). Medical treatment belongs in the hands of emergency services – this text only explains how to prevent chilling.

A break on a cold rock after a steep climb: you are sweaty (evaporation), it is windy on the ridge (convection) and you are sitting on rock (conduction). Remedy: sit on your backpack, find the sheltered side, put on a dry, windproof layer straight away.

A clear, still autumn night under the open sky: radiation plays a big role here, and dew or frost often forms on meadows. A tarp over the sleeping spot and a good sleeping pad make the biggest difference.

Drizzle in mild weather: many people underestimate this situation because it is “not really cold”. Yet slowly soaked clothing and a light wind can cost a lot of heat over hours. Putting on rain gear early is the most important step here.

You often hear that people lose most of their heat through the head. Put that generally, it is not true: the head loses roughly as much as its share of the body surface. But because it is often the only uncovered part of the body, a hat still makes a noticeable difference.

Shivering is a protective reaction: the muscles produce heat as they do it. But it costs a lot of energy. So anyone who is cold should eat and drink something as long as they can think clearly and swallow.

Clothing does not produce heat itself. It only holds on to the heat your body makes by trapping still air. That is why loose, dry clothing in several thin layers often keeps you warmer than a single tight, thick garment.