One minute you're sweating buckets slogging up a talus field in the sun, the next you're chilled to the bone by an icy wind. But if you're wearing wool – nature's thermostat – its unique properties will help keep you comfortable in a variety of conditions.
In wet garments, you can lose heat up to 25 times faster than in dry clothes. This occurs because water conducts heat better than air. In cold conditions, evaporative cooling of wet fabrics, such as cotton, can quickly make you cold or lead to dangerous hypothermia. But, even when saturated, wool traps body-heated air within its fibres, making it a better choice for damp or rainy conditions.
Wool can absorb up to 35% of its weight in moisture and remain dry to the touch because the moisture is pulled inside the fibres. Wool will eventually feel wet because of water between the yarns, but only in extremely soggy conditions.
Wool fibres absorb and release water vapour to maintain equilibrium with the water content of the surrounding air. The fibres have a hydrophilic, water-absorbing interior, and a hydrophobic, water-repelling exterior. A thin protective membrane, like a wax covering, is bonded to the outside of each fibre. Even chemicals like dry-cleaning fluids won't remove it. The membrane is waterproof, but microscopic pores allow water vapour to pass through, like smoke through a screen door.