Down jackets are the preferred clothing for many people to keep warm in winter. The superior warmth of down jackets primarily comes from the unique structure of their filling-down.
Down is a mixture of feathers and down fibers, and the down content is a key factor in measuring the warmth of a down jacket. The down fibers have a hollow structure, containing still air. A large amount of air can be trapped between the down fibers and between the down fibers, providing excellent insulation.