Cold weather skin: the physics of winter dryness and what to do about it

Why lips crack in winter: cold air holds little water, heating drops indoor humidity into single figures, and the vapour pressure gradient across skin rises.

Winter skin is usually explained with the phrase "the cold dries you out", which is true but not informative. The mechanism is worth understanding properly, because once you see it you can tell which interventions will work and which are theatre. It comes down to how much water air can hold at a given temperature, and what happens when you take cold air indoors and heat it.

Cold air holds almost no water

The amount of water vapour air can hold rises steeply with temperature. This is not a linear relationship, and the steepness is the whole story. Outdoor air at minus 10 C is close to saturated at a vapour pressure of under 3 hectopascals. Take that air indoors, heat it to 21 C, and its capacity has risen almost tenfold while its water content has not changed at all. The relative humidity collapses.

Nine percent relative humidity is drier than most deserts on an average day. A well-sealed modern building with mechanical heating and low air exchange holds more moisture than that because people, cooking and washing add water, but in a draughty house on a cold week, indoor humidity in the teens is routine. That is the environment your skin spends sixteen hours a day in, and it is nothing like the conditions any product was tested in.

The gradient across the skin, and why it matters

Water leaves the skin by diffusion, and the rate depends on the difference in water vapour pressure between the skin surface and the surrounding air. Skin sits at roughly 32 C. If its surface were fully saturated, the vapour pressure there would be about 47 hPa. It is not fully saturated, because the stratum corneum is a barrier and that is its job, but the direction of the gradient is fixed and only its size changes.

Transepidermal water loss is the term for that flow, and it is the measurement almost every claim about moisturisers rests on. A 40 percent increase in the gradient does not translate into a 40 percent increase in loss, because the barrier resists, but the direction is unambiguous and it is why the same skin that is fine in June is cracked in January.

Why the skin's own lipids are least helpful in winter

The unpleasant coincidence is that the barrier is at its weakest when it is asked to do the most. Two things happen in the cold.

The intercellular lipids of the stratum corneum are packed in ordered crystalline arrangements, and their packing becomes more rigid as temperature drops. A stiffer, less mobile lipid matrix is more prone to microfracture when the skin flexes, which is why cracks appear on knuckles, heels and the corners of the mouth rather than uniformly. Seasonal work has also found measurable differences in stratum corneum lipid composition between winter and summer, including changes in the ceramide fraction and reduced levels of natural moisturising factor, so the material properties genuinely differ rather than merely feeling different. The lipid classes involved are described under moisturisers and the skin barrier.

Sebum production also falls with skin temperature, and sebum is more viscous when cold. That matters most on the face, and least on the lips, which have no sebaceous glands to begin with and depend on whatever spreads onto them from the surrounding skin.

Wind, evaporative cooling and what windburn actually is

A layer of relatively humid, relatively still air sits against the skin. That boundary layer is a real part of the barrier: it reduces the local gradient and slows evaporation. Wind removes it, continuously, and replaces it with fresh dry air. The effect on water loss is large and immediate, and it also cools the tissue, which stiffens the surface lipids further and reduces local blood flow.

So windburn as a mechanical phenomenon exists: tight, rough, red skin after hours of exposure, particularly on the cheeks, nose and lips. But the term is used loosely, and a good deal of what gets called windburn on a mountain or a beach is ordinary sunburn on skin that also happened to be windblown. The tell is the pattern. Sunburn respects shade lines and follows sun exposure. Wind damage follows what was pointed into the wind. Both can be present at once.

Why an occlusive beats a humectant in dry cold

Humectants such as glycerin, urea and hyaluronic acid work by binding water. Where that water comes from depends on the air. In humid conditions they draw from the atmosphere and from deeper skin, and the net effect is good. In air at 15 percent relative humidity there is very little atmospheric water to draw on, and a humectant film applied alone can end up drawing water from below and then losing it to the air, which is the origin of the common complaint that a hydrating serum feels drying in winter.

An occlusive works differently. It does not depend on ambient humidity at all: it is a physical film that lowers the diffusion rate across the surface. That makes it the reliable choice when the air is very dry, which is the technical reason balms come into their own in winter and feel like too much in August. The most complete occlusion measured belongs to petrolatum, with lanolin close behind, and plant butter blends somewhat lower but often more pleasant to wear.

The two are not in competition. The sequence that works is humectant first onto damp skin, occlusive on top to hold it there. Applying a balm to bone-dry skin traps nothing, which is why the single most useful habit change in winter is to apply within a minute of washing, while the skin is still damp.

Conditions and the sensible response

The mountain case in particular

Ultraviolet intensity increases with altitude by roughly 10 to 12 percent per 1000 metres, and fresh snow reflects a large fraction of incident ultraviolet back upward, so exposure at 2500 metres on snow is far higher than the same hours at sea level. That reflected component reaches places sunscreen is usually skipped: under the nose, the underside of the chin, and the lower lip. Lips have very little melanin, so they are among the least protected surfaces on the body. A lip product with sun protection is the one product genuinely worth carrying on a ski day, and the regulatory reasons a home-made version cannot make an SPF claim are set out under SPF in lip balm.

Humidifiers, honestly

Raising indoor humidity attacks the cause rather than the symptom, and it is the intervention most likely to help lips, nose and eyes at the same time. Two caveats. Humidifiers need cleaning, because standing water grows organisms that then get aerosolised, and public health guidance on their use is mostly about maintenance rather than about how much moisture to add. And over-humidifying causes condensation on cold surfaces, which produces mould. Target 30 to 50 percent, buy a hygrometer rather than guessing, and empty and dry the tank daily.

A routine that follows the physics

1. Wash less aggressively. Cooler water, shorter contact, and a cleanser that does not leave the skin squeaking.
2. Apply while damp. Within about a minute of towelling, so there is water at the surface for the film to hold.
3. Humectant then occlusive. A water-based product first if you use one, then the balm on top. A balm alone on dry skin seals in less.
4. Match the amount to the area. Balms are concentrated and easy to over-apply on the face and under-apply on heels. See how much balm to apply.
5. Protect before exposure, not after. A film applied indoors before a walk works. One applied on numb lips in the wind mostly does not.
6. Deal with the environment. Humidity indoors, a scarf outdoors, sunscreen on snow.

For the specific areas that fail first, the reasoning behind each ingredient choice sits under ingredients for chapped lips, heavier body work under body balm, heels and cracked skin under foot and heel balm, and hands that get washed twenty times a day under gardener's hand salve.

One thing to stop doing

Stop licking. It is the single most common self-inflicted cause of winter lip damage, it feels like relief for about ten seconds, and the saliva then evaporates and takes surface water with it, along with digestive enzymes that irritate the vermilion border. That cycle, not the balm itself, is the real content of the belief that lip balm dries your lips. And if the balm you carry is always rock hard by the time you want it, that is a formulation problem with a known fix, covered under balm too hard or too soft.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.