Air at a given temperature can hold only a certain amount of water vapour: the colder it is, the less it can hold. In cold weather, this saturation level is so low there is very little water vapour in the air, even if the relative humidity (the ratio of the amount in the air to the saturation value) is high.

The rate of evaporation from a moist surface depends on the difference between the concentration of vapour in the air right at the surface and the concentration in the bulk of the air. The former depends on the temperature of the moist surface: in the case of your skin, it is warm even in cold weather, whereas the vapour concentration in the bulk is low in cold weather. So evaporation takes place at a higher rate than in warm weather.

This is true even in a heated building, because heating the air that has come in from outside does not increase its water vapour content. Humidifiers can add water, but they consume a lot of energy. Large buildings sometimes have systems that can transfer water between incoming and outgoing air, which are more energy-efficient.

Though water evaporates faster from your skin in cold weather, your laundry dries more slowly. Because it is not heated by your body as your skin is, the level of water vapour right at the surface is quite low, so the rate of evaporation is low too. If you want your laundry to dry quickly, wear it.

Eric Kvaalen, La Courneuve, France

Firstly, cold air holds much less moisture, so is generally much drier than warmer air. When people talk about 'humidity' they usually mean 'relative humidity' (RH), so 50% RH at 5C is much drier than 50% RH at 25C.

The RH does tell us a lot about the airs ability to dry us, however...

We tend to carry a thin layer of warmed air around with us, close to our skin and in our cloths; this is fundamentally low moisture air, but it is heated by our bodies and is thus able to hold more water, which it takes... from our skin!

Next, we tend to spend time in heated buildings in winter, so again the dry winter air is heated, lowering its RH, and turbocharging its ability to dessicate your skin...

The phsical properties of the air are largely to blame but our own biology plays a part too: When we're cold the blood flow to extrainous parts of the body (such as the skin, hands and feet) is restricted. This will naturally take some of the life out of our skin as it is prevented from receiving replacment water and nutrition from the blood. The movement of our fingers will also feel sluggish and painful untill a few fist-clenches revive the blood flow. In extreme cold, parts of the skin (as well as fingers and toes) may be completely cut off and effectively die, something which no amount of hand-creme can cure.