The insulating value of the wool fleece is determined largely by the pockets of air trapped within the fabric. The amount of water vapor in the air within the wool fabric is a function of the temperature and the amount of water in the fibers, and affects the heat conductivity of this air. All else being equal, when the amount of activity or the ambient temperature increases the body perspires more, the heat conductivity of the air trapped in the wool fabric increases, and the insulating value of the wool decreases. Thus wool clothing not only keeps the skin dry and warm, but also adjusts its degree of insulation to some degree to suit the amount of activity and external temperature.