Clothing provides a means of controlling heat exchange with the environment, particularly important in cold climates as changing weather and activity require a continuous conscious adjustment of clothing to maintain thermal comfort. The most important property of cold weather clothing is its thermal insulation, which can be measured according to ISO 15831. The insulation value of a cold weather ensemble can be used for prediction of its performance in terms of providing thermal comfort using ISO 11079. Cold weather clothing builds on the layering principle: the inner layer or underwear controls moisture and air motion over the skin; the second layer is mainly insulative; and the outer layer protects against ambient factors such as wind, rain and snow. Modern materials and functional design allow clothing to be developed for quite extreme exposures.