Wind chill: why wind makes it feel colder

In still air, a thin layer of motionless air warmed by the body lies right next to the skin. Because still air conducts heat poorly, this layer acts like a wafer-thin insulation. Wind keeps stripping it away and replacing it with fresh, cold air. Physicists call this forced convection: the air movement is driven from outside, not just by warm air rising.

The stronger the wind, the faster exposed skin loses heat. But the effect does not grow without limit: in light wind each extra bit of wind makes a big difference, while in already strong wind the additional effect gets smaller. The protective layer of air is then almost entirely gone anyway.

The wind chill index converts air temperature and wind speed into the temperature that would feel the same on dry, exposed skin in still air. Since 2001, the US and Canadian weather services have used a common, revised index based on models of heat loss from the face.

The most important protection is surprisingly simple: a windproof outer layer. Even a thin wind jacket holds in the warm air in your clothing that the wind would otherwise blow through the fabric. Thinner warming layers can then go underneath. A thick fleece without wind protection, on the other hand, loses much of its effect in a gale.

The parts most affected are those often left uncovered that have a lot of surface relative to their size: face, ears, nose, fingers. A hat, neck gaiter or face protection and gloves make the biggest difference here. Take breaks in the lee of rocks, buildings or forest, and choose a sheltered spot for your tent as well.

If wetness is added, things become even more critical. Wet skin or clothing loses extra heat through evaporation in the wind. The wind chill index does not account for this; it only applies to dry skin.

A common misconception is that wind can make water freeze when the air is above zero, just because the wind chill is below zero. That is not true. A water container, an engine or a pipe has no heat source of its own. Wind only makes it reach the air temperature faster – it does not get colder than the air.

The feels-like number is not a law of nature either, but a model. Different countries and weather services calculate it differently, so values from different apps are not always directly comparable. Sun, your own movement and clothing are not included in the index. Even so, wind chill says more about frostbite risk than air temperature alone.

In strong wind and cold, exposed patches of skin can freeze, often without the person noticing, because the spot goes numb. In a group, watch each other for white or waxy-pale, numb patches on the face, especially on the nose, cheeks and ears. Anyone who notices this gets out of the wind, covers the spot and gets somewhere warm.

Treating frostbite belongs in knowledgeable hands; this entry gives no instructions for it. With extensive or deep frostbite and with signs of hypothermia such as confusion or marked drowsiness, call the local emergency number (112 in the EU). Children and older people cool down faster and need particular attention.

On summits, ridges, open plateaus and at the coast, the wind is often much stronger than in the valley or in the forest. Anyone starting out below in pleasant conditions easily underestimates how cold it will be higher up. The wind jacket therefore belongs at the top of the backpack, within reach, not right at the bottom.

When cycling, riding a motorbike, skiing or on a boat, your own speed also creates a headwind. For your skin it works just like natural wind. That is why face and hands get cold so quickly on a fast descent.