Heat transfer: conduction, convection, radiation, evaporation

Heat is the energy of motion of the tiniest particles. In a warm object the particles jiggle more vigorously than in a cold one. When two materials touch, the fast particles bump into the slow ones and pass on energy – that is conduction. How well a material conducts varies greatly: metal and stone conduct well, water also much better than air, while still air conducts very poorly.

In convection the material itself moves. Air that your skin has warmed becomes lighter and rises or is blown away by the wind; cold air moves in and has to be warmed again. Radiation needs no contact at all: every object sends out invisible infrared light, the more the warmer it is. This is how the sun warms the earth across empty space, and how you feel the heat of a campfire on your face.

Evaporation is a special case: turning liquid water into vapour takes a lot of energy. The water draws this energy from the surface it is lying on – your skin or your clothing. That is why sweat cools you, and why you get cold so quickly in wet clothing. Every breath also costs some heat and moisture, especially in cold, dry air.

In real life several paths are almost always at work at once. Someone sitting on a rock in the wind wearing a sweaty jacket loses heat through conduction, convection and evaporation at the same time. A good solution therefore closes several doors at once rather than just one very well.

The ground is the path most often forgotten. A thick sleeping bag helps little if you lie on cold, damp ground without a pad, because the filling is squashed under your weight and then holds hardly any air. Sleeping pads, dry leaves in a thick layer or a backpack as a seat interrupt conduction.

The emergency blanket is a good example of the limits of a single item. It reflects heat radiation and keeps wind off, but because it is thin it hardly insulates against conduction. It works best combined with a pad, dry clothing and wind protection.

In heat you want to get rid of heat, not keep it. The sun and hot surfaces such as tarmac or rock radiate heat onto you; if the air is warmer than your skin, it heats you as well. That is why shade is so effective: it blocks most of the radiation from above.

When the air is warmer than the skin, the body is left almost only with sweating. But sweat only cools if it can evaporate. In humid, muggy air that is harder – sweat drips off instead of evaporating and then hardly cools. That is why muggy weather feels so draining.

The same physics explains everyday cooking tricks: a lid on the pot reduces evaporation and convection, and a windscreen around the stove stops the wind from carrying the heat away. Both save fuel and time.

This entry explains physics, not medicine. You cannot judge from these basics alone whether someone is hypothermic or has a heat illness. With signs such as violent shivering, confusion or drowsiness in the cold, or dizziness, nausea and confusion in the heat, call the local emergency number (112 in the EU).

A common mistake is working so hard in the cold that you sweat heavily. The wet clothing then cools you as soon as you stop. Better to take off a layer before you start sweating and put something on again straight away during breaks.

Water conducts heat much better than air. Anyone who gets wet or falls into cold water therefore cools down much faster than in air at the same temperature. Never run heat sources such as stoves or fires in tents or enclosed spaces – there is a risk of carbon monoxide poisoning.

Why does a metal railing feel colder in winter than a wooden one, even though both are equally cold? Because metal draws the heat from your hand away much faster. Your skin does not measure temperature but how fast it is losing heat. That is exactly why the feeling is often misleading.

Frost on clear nights is also explained by radiation: a cloudless sky sends back hardly any heat, so ground, cars and tents cool strongly, sometimes even below the air temperature. Under trees or with cloud cover it often stays a little milder. That is a good reason not to pick a camp spot completely open under the sky.