Two different humidities
Almost all the confusion about winter skin comes from a single conflation. There are two ways of describing how much water is in air and they behave completely differently.
Absolute humidity is how much water vapour the air actually contains, expressed as a mass per volume of air. It is a quantity.
Relative humidity is how much water the air contains as a proportion of the maximum it could hold at that temperature. It is a ratio, and the denominator moves with temperature.
The maximum amount of water air can hold rises steeply with temperature. Warm air can carry a great deal. Cold air can carry very little. This is why breath fogs in winter and does not in summer: warm saturated air from your lungs meets cold air that cannot hold that quantity of water, and the excess condenses into visible droplets.
The consequence for skin is direct. Your skin does not care about a ratio. What drives water out of it is the difference between the vapour pressure at the wet interior of your body and the vapour pressure of the air outside. A cold day at ninety per cent relative humidity can still have a low absolute water content, and therefore still pull water out of you briskly.
Why a wet British winter is still a drying one
This is the point at which people who live in Britain object, reasonably. It rains constantly. The forecast says the relative humidity is high. How can this possibly be a drying environment.
Two answers. First, liquid water on your skin is not water vapour in the air, and it does not reduce the vapour pressure gradient once it has evaporated. In fact evaporating surface water cools the skin and leaves the outer layer softened, which is a different problem covered in the article on wet and dry cycling.
Second, and more important, the air you spend most of your time in is not the air outside. It is that air, heated. Which brings us to the mechanism that does most of the damage.
Heating cold air is a dehumidifier
Take outside air at close to freezing and at high relative humidity. Bring it inside. Heat it to a comfortable room temperature. You have not added a single molecule of water, but the air can now hold several times as much as it did. Its relative humidity has therefore fallen a long way.
That is the state of the air in most heated British buildings in January. It is why wooden furniture and musical instruments crack in winter, why static shocks are a winter phenomenon, and why people whose skin is fine in a Scottish gale in November find it is worse in a warm office in the same week.
We are not going to give you a number for the relative humidity of your living room, because it depends on your heating, your ventilation, your cooking, your drying of laundry and the construction of your house. What you can rely on is the direction: heating cold air without adding water lowers relative humidity, and the colder the source air, the more it lowers it.
Cold plus wind is not additive
Cold air and wind together do more than either alone, and the reason is worth understanding because it changes what you do about it.
Still air next to your skin picks up water and becomes locally humid. That local humidity reduces the gradient at the skin surface, and so reduces further loss. It is a small, free, self generated protection.
Wind removes it. Moving air replaces the locally humid film with fresh dry air continuously, holding the surface gradient at its maximum. In effect, wind converts your local microclimate into the ambient one, permanently, for as long as it blows.
This is why a hood, a buff or a closed cuff is not merely insulation. It is a device for maintaining a still air layer, and in barrier terms that is a more important function than warmth. It is also why wind deserves its own article.
| Relative humidity | Absolute humidity | |
|---|---|---|
| What it measures | Water present as a proportion of the maximum at that temperature | Water actually present per volume of air |
| What happens when you heat the air | Falls sharply | Does not change |
| What the forecast usually gives you | This one | Rarely, though dew point is closely related |
| What your skin responds to | Only indirectly | This one, via the vapour pressure gradient |
| Winter outdoors in Britain | Often high | Low, because cold air cannot hold much |
| Winter indoors, heated | Often low | Low, and now with a much larger gradient |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
What cold does to the skin directly
Separately from the humidity story, cold changes the skin itself.
Blood flow to the skin is reduced in cold conditions as the body preserves core temperature. Reduced surface blood flow means reduced delivery of everything the skin needs to build and repair, at exactly the moment demand is highest.
Lipids behave differently at different temperatures. The ordered sheets in the stratum corneum have a physical structure whose properties change with temperature, and a cold layer is a stiffer layer. A stiffer layer under bending load, which is what a hand does every time it grips something, is more likely to split.
And sebaceous gland activity is generally lower in cold conditions, so the surface film that would normally sit over everything is thinner in winter than it is in summer.
All three point the same way. In winter, the skin is being asked to do more with less.
What to do with this
The reason this article exists is that once you hold the absolute humidity idea, several bits of common advice stop being arbitrary.
Covering exposed skin is not vanity. It restores a still, locally humid air layer.
Heating your house less is a skin intervention, not just an economic one, and it is the largest single indoor lever most people have.
Applying something occlusive before going out in cold wind does more than applying it afterwards, because the exposure is the event.
And the fact that your skin is worse in a heated office than on a cold beach is not a puzzle. It is the expected result.